In the aftermath of the accident, investigations focused on the amount of radiation released by the accident. According to the American Nuclear Society, using the official radiation emission figures, "The average radiation dose to people living within ten miles of the plant was eight millirem, and no more than 100 millirem to any single individual. Eight millirem is about equal to a chest X-ray, and 100 millirem is about a third of the average background level of radiation received by US residents in a year."

Based on these low emission figures, early scientific publications on the health effects of the fallout estimated one or two additional cancer deaths in the 10 mi (16 km) area around TMI. Disease rates in areas further than 10 miles from the plant were never examined. Local activism in the 1980s, based on anecdotal reports of negative health effects, led to scientific studies being commissioned. A variety of studies have been unable to conclude that the accident had substantial health effects.

The Radiation and Public Health Project cited calculations by Joseph Mangano, who has authored 19 medical journal articles and a book on Low Level Radiation and Immune Disease, that reported a spike in infant mortality in the downwind communities two years after the accident. Anecdotal evidence also records effects on the region's wildlife. For example, according to one anti-nuclear activist, Harvey Wasserman, the fallout caused "a plague of death and disease among the area's wild animals and farm livestock", including a sharp fall in the reproductive rate of the region's horses and cows, reflected in statistics from Pennsylvania's Department of Agriculture, though the Department denies a link with TMI.

The health effects of the 1979 Three Mile Island nuclear accident are widely, but not universally, agreed to be very low level. According to the official radiation release figures, average local radiation exposure was equivalent to a chest X-ray, and maximum local exposure equivalent to less than a year's background radiation. Local activism based on anecdotal reports of negative health effects led to scientific studies being commissioned. A variety of studies have been unable to conclude that the accident had substantial health effects, but a debate remains about some key data (such as the amount of radiation released, and where it went) and gaps in the literature.

Initial investigations

In the aftermath of the accident, investigations focused on the amount of radiation released by the accident. The official figures have been disputed by a number of insiders, who have suggested figures hundreds or thousands of times higher. According to the American Nuclear Society, using the relatively low official radiation emission figures, "The average radiation dose to people living within ten miles of the plant was eight millirem, and no more than 100 millirem to any single individual. Eight millirem is about equal to a chest X-ray, and 100 millirem is about a third of the average background level of radiation received by US residents in a year." To put this dose into context, while the average background radiation in the US is about 360 millirem per year, the Nuclear Regulatory Commission regulates all workers' of any US nuclear power plant exposure to radiation to a total of 5000 millirem per year. Based on these low emission figures, early scientific publications on the health effects of the fallout estimated one or two additional cancer deaths in the 10-mile area around TMI. Disease rates in areas further than 10 miles from the plant were never examined.

Local resident reports

The official figures are too low to account for the acute health effects reported by some local residents and documented in two books; such health effects require exposure to at least 100,000 millirems (100 rems) to the whole body - 1000 times more than the official estimates. The reported health effects are consistent with high doses of radiation, and comparable to the experiences of cancer patients undergoing radio-therapy,. but have many other potential causes. The effects included "metallic taste, erythema, nausea, vomiting, diarrhea, hair loss, deaths of pets and farm and wild animals, and damage to plants." Some local statistics showed dramatic one-year changes among the most vulnerable: "In Dauphin County, where the Three Mile Island plant is located, the 1979 death rate among infants under one year represented a 28 percent increase over that of 1978, and among infants under one month, the death rate increased by 54 percent." Physicist Ernest Sternglass, a specialist in low-level radiation, noted these statistics in the 1981 edition of his book Secret Fallout: low-level radiation from Hiroshima to Three-Mile Island. In their final 1981 report, however, the Pennsylvania Department of Health, examining death rates within the 10-mile area around TMI for the 6 months after the accident, said that the TMI-2 accident did not cause local deaths of infants or fetuses.

Scientific work continued in the 1980s, but focused heavily on the mental health effects due to stress, as the Kemeny Commission had concluded that this was the sole public health effect. A 1984 survey by a local psychologist of 450 local residents, documenting acute radiation health effects (as well as 19 cancers 1980-84 amongst the residents against an expected 2.6), ultimately led the TMI Public Health Fund reviewing the data and supporting a comprehensive epidemiological study by a team at Columbia University.

Columbia epidemiological study

In 1990-1 a Columbia University team, led by Maureen Hatch, carried out the first epidemiological study on local death rates before and after the accident, for the period 1975-1985, for the 10-mile area around TMI. Assigning fallout impact based on winds on the morning of March 28, 1979, the study found no link between fallout and cancer risk. The study found that cancer rates near the Three Mile Island plant peaked in 1982-3, but their mathematical model did not account for the observed increase in cancer rates, since they argued that latency periods for cancer are much longer than three years. The study concludes that stress may have been a factor (though no specific biological mechanism was identified), and speculated that changes in cancer screening were more important.

Wing review

Subsequently lawyers for 2000 residents asked epidemiologist Stephen Wing of the University of North Carolina at Chapel Hill, a specialist in nuclear radiation exposure, to re-examine the Columbia study. Wing was reluctant to get involved, later writing that "allegations of high radiation doses at TMI were considered by mainstream radiation scientists to be a product of radiation phobia or efforts to extort money from a blameless industry." Wing later noted that in order to obtain the relevant data, the Columbia study had to submit to what Wing called "a manipulation of research" in the form of a court order which prohibited "upper limit or worst case estimates of releases of radioactivity or population doses... [unless] such estimates would lead to a mathematical projection of less than 0.01 health effects." Wing found cancer rates raised within a 10-mile radius two years after the accident by 0.034% +/- 0.013%, 0.103% +/- 0.035%, and 0.139% +/- 0.073% for all cancer, lung cancer, and leukemia, respectively. An exchange of published responses between Wing and the Columbia team followed. Wing later noted a range of studies showing latency periods for cancer from radiation exposure between 1 and 5 years due to immune system suppression. Latencies between 1 and 9 years have been studied in a variety of contexts ranging from the Hiroshima survivors and the fallout from Chernobyl to therapeutic radiation; a 5-10 year latency is most common.

Further studies

On the recommendation of the Columbia team, the TMI Public Health Fund followed up its work with a longitudinal study. The 2000-3 University of Pittsburgh study compared post-TMI death rates in different parts of the local area, again using the wind direction on the morning of 28 March to assign fallout impact, even though, according to Joseph Mangano in the Bulletin of the Atomic Scientists, the areas of lowest fallout by this criterion had the highest mortality rates. In contrast to the Columbia study, which estimated exposure in 69 areas, the Pittsburgh study drew on the TMI Population Registry, compiled by the Pennsylvania Department of Health. This was based on radiation exposure information on 93% of the population living within five miles of the nuclear plant - nearly 36,000 people, gathered in door-to-door surveys shortly after the accident.^[19] The study found slight increases in cancer and mortality rates but "no consistent evidence" of causation by TMI. Wing et al. criticized the Pittsburgh study for making the same assumption as Columbia: that the official statistics on low doses of radiation were correct - leading to a study "in which the null hypothesis cannot be rejected due to a prior assumptions." Hatch et al. noted that their assumption had been backed up by dosimeter data, though Wing et al. noted the incompleteness of this data, particularly for releases early on.

In 2005 R. William Field, an epidemiologist at the University of Iowa, who first described radioactive contamination of the wild food chain from the accident suggested that some of the increased cancer rates noted around TMI were related to the area's very high levels of natural radon, noting that according to a 1994 EPA study, the Pennsylvania counties around TMI have the highest regional screening radon concentrations in the 38 states surveyed. The factor had also been considered by the Pittsburgh study and by the Columbia team, which had noted that "rates of childhood leukemia in the Three Mile Island area are low compared with national and regional rates." A 2006 study on the standard mortality rate in children in 34 counties downwind of TMI found an increase in the rate (for cancers other than leukemia) from 0.83 (1979–83) to 1.17 (1984–88), meaning a rise from below the national average to above it.

A 2008 study on thyroid cancer in the region found rates as expected in the county in which the reactor is located, and significantly higher than expected rates in two neighbouring counties beginning in 1990 and 1995 respectively. The research notes that "These findings, however, do not provide a causal link to the TMI accident." Mangano (2004) notes three large gaps in the literature: no study has focused on infant mortality data, or on data from outside the 10-mile zone, or on radioisotopes other than iodine, krypton, and xenon.

Three Mile Island Nuclear Accident Radioactive material release

Once the first line of containment is breached during a reactor plant accident, there is a possibility that the fuel or the fission products held inside can be released into the environment. Although the zirconium fuel cladding has been breached in other nuclear reactors without generating a release to the environment, at TMI-2 operators permitted fission products to leave the other containment barriers. This occured when the cladding was damaged while the PORV was still stuck open. Fission products were released into the reactor coolant. Since the PORV was stuck open and the loss of coolant accident was still in progress, primary coolant with fission products and/or fuel was released, and ultimately ended up in the auxiliary building. This auxiliary building was outside the containment boundary. This was evidence by the radiation alarms that eventually sounded. However, since very little of the fission products released were solids at room temperature, very little radiological contamination was reported in the environment. No significant level of radiation was attributed to the TMI-2 accident outside of the TMI-2 facility. Noble gases made up the bulk of the release of radioactive materials from TMI-2, with the next most abundant element being iodine.

Within hours of the accident the United States Environmental Protection Agency (EPA) began daily sampling of the environment at the three stations closest to the plant. By April 1, continuous monitoring at 11 stations was established and was expanded to 31 stations two days later. An inter-agency analysis concluded that the accident did not raise radioactivity far enough above background levels to cause even one additional cancer death among the people in the area. The EPA found no contamination in water, soil, sediment or plant samples.

Researchers at nearby Dickinson College, which had radiation monitoring equipment sensitive enough to detect Chinese atmospheric atomic weapons testing, collected soil samples from the area for the ensuing two weeks and detected no elevated levels of radioactivity, except after rainfalls (likely due to natural radon plate out, not the accident). Also, white-tailed deer tongues harvested over 50 mi (80 km) from the reactor subsequent to the accident were found to have significantly higher levels of Cs-137 than in deer in the counties immediately surrounding the power plant. Even then, the elevated levels were still below those seen in deer in other parts of the country during the height of atmospheric weapons testing. Had there been elevated releases of radioactivity, increased levels of Iodine-131 and Cesium-137 would have been expected to be detected in cattle and goat's milk samples. Yet elevated levels were not found.

A later scientific study noted that the official emission figures were consistent with available dosimeter data, though others have noted the incompleteness of this data, particularly for releases early on.

According to the official figures, as compiled by the 1979 Kemeny Commission from Metropolitan Edison and NRC data, a maximum of 480 petabecquerels (13 million curies) of radioactive noble gases (primarily xenon) were released by the event. However, these noble gases were considered relatively harmless, and only 481–629 GBq (13–17 curies) of thyroid cancer-causing iodine-131 were released. Total releases according to these figures were a relatively small proportion of the estimated 37 EBq (10 billion curies) in the reactor. It was later found that about half the core had melted, and the cladding around 90% of the fuel rods had failed, with five feet of the core gone, and around 20 tons of uranium flowing to the bottom head of the pressure vessel, forming a mass of corium. The reactor vessel, the second level of containment after the cladding, maintained integrity and contained the damaged fuel with nearly all of the radioactive isotopes in the core.

Anti-nuclear political groups disputed the Kemeny Commission's findings, claiming that independent measurements provided evidence of radiation levels up to five times higher than normal in locations hundreds of miles downwind from TMI. According to Randall Thompson, who claims to have been a health physics technician employed to monitor radioactive emissions at TMI after the accident, radiation releases were hundreds if not thousands of times higher. Some other insiders, including Arnie Gundersen, a former nuclear industry executive who is now an expert witness in nuclear safety issues, make the same claim; Gundersen offers evidence, based on pressure monitoring data, for a hydrogen explosion shortly before 2 p.m. on March 28, 1979, which would have provided the means for a high dose of radiation to occur. Gundersen cites affidavits from four reactor operators according to which the plant manager was aware of a dramatic pressure spike, after which the internal pressure dropped to outside pressure. Gundersen also notes that the control room shook and doors were blown off hinges. However official NRC reports refer merely to a "hydrogen burn." The Kemeny Commission referred to "a burn or an explosion that caused pressure to increase by 28 pounds per square inch in the containment building". The Washington Post reported that "At about 2 p.m., with pressure almost down to the point where the huge cooling pumps could be brought into play, a small hydrogen explosion jolted the reactor."

The Three Mile Island Nuclear Accident was a partial core meltdown in Unit 2 (a pressurized water reactor manufactured by Babcock & Wilcox) of the Three Mile Island Nuclear Generating Station in Dauphin County, Pennsylvania near Harrisburg, United States in 1979. The plant was owned and operated by General Public Utilities and the Metropolitan Edison Co. It is the most significant accident in the history of the American commercial nuclear power generating industry, resulting in the release of up to 481 PBq (13 million curies) of radioactive gases, but less than 740 GBq (20 curies) of the particularly dangerous iodine-131.

The Three Mile Island Nuclear Accident began at 4 a.m. on Wednesday, March 28, 1979, with failures in the non-nuclear secondary system, followed by a stuck-open pilot-operated relief valve (PORV) in the primary system, which allowed large amounts of nuclear reactor coolant to escape. The mechanical failures were compounded by the initial failure of plant operators to recognize the situation as a loss-of-coolant accident due to inadequate training and human factors, such as human-computer interaction design oversights relating to ambiguous control room indicators in the power plant's user interface. The scope and complexity of the accident became clear over the course of five days, as employees of Metropolitan Edison (Met Ed, the utility operating the plant), Pennsylvania state officials, and members of the U.S. Nuclear Regulatory Commission (NRC) tried to understand the problem, communicate the situation to the press and local community, decide whether the accident required an emergency evacuation, and ultimately end the crisis.

In the end, the reactor was brought under control, although full details of the accident were not discovered until much later, following extensive investigations by both a presidential commission and the NRC. The Kemeny Commission Report concluded that "there will either be no case of cancer or the number of cases will be so small that it will never be possible to detect them. The same conclusion applies to the other possible health effects." Several epidemiological studies in the years since the accident have supported the conclusion that radiation releases from the accident had no perceptible effect on cancer incidence in residents near the plant, though these findings have been contested by one team of researchers.

Public reaction to the event was probably influenced by The China Syndrome, a movie which had recently been released and which depicts an accident at a nuclear reactor. Communications from officials during the initial phases of the accident were felt to be confusing. The accident crystallized anti-nuclear safety concerns among activists and the general public, resulted in new regulations for the nuclear industry, and has been cited as a contributor to the decline of new reactor construction that was already underway in the 1970s.

Three Mile Island Nuclear Accident: Stuck valve

In the nighttime hours preceding the incident, the TMI-2 reactor was running at 97% of full power, while the companion TMI-1 reactor was shut down for refueling. The chain of events leading to the partial core meltdown began at 4 a.m. EST on March 28, 1979, in TMI-2's secondary loop, one of the three main water/steam loops in a pressurized water reactor. As a result of mechanical or electrical failure, the pumps in the condensate polishing system stopped running, followed immediately by the main feedwater pumps. This automatically triggered the turbine to shut down and the reactor to scram: control rods were inserted into the core to control the rate of fission. But the reactor continued to generate decay heat, and because steam was no longer being used by the turbine due to the turbine trip, the steam generators no longer removed that heat from the reactor.

Once the primary feedwater pump system failed, three auxiliary pumps activated automatically. However, because the valves had been closed for routine maintenance, the system was unable to pump any water. The closure of these valves was a violation of a key NRC rule, according to which the reactor must be shut down if all auxiliary feed pumps are closed for maintenance. This failure was later singled out by NRC officials as a key one, without which the course of events would have been very different. The pumps were activated manually eight minutes later, and manually deactivated between 1 and 2 hours later, as per procedure, due to excessive vibration in the pumps.

Due to the loss of heat removal from the primary loop and the failure of the auxiliary system to activate, the primary side pressure began to increase, triggering the pilot-operated relief valve (PORV) at the top of the pressurizer to open automatically. The PORV should have closed again when the excess pressure had been released and electric power to the solenoid of the pilot was automatically cut, but instead the main relief valve stuck open due to a mechanical fault. The open valve permitted coolant water to escape from the primary system, and was the principal mechanical cause of the crisis that followed.

Three Mile Island Nuclear Accident: Human factors – confusion over valve status

Critical human factors problems were revealed in the investigation about the user interface engineering of the reactor control system's user interface. A lamp in the control room, designed to illuminate when electric power was applied to the solenoid that operated the pilot valve of the PORV, went out, as intended, when the power was removed. This was incorrectly interpreted by the operators as meaning that the main relief valve was closed, when in reality it only indicated that power had been removed from the solenoid, not the actual position of the pilot valve or the main relief valve. Because this indicator was not designed to unambiguously indicate the actual position of the main relief valve, the operators did not correctly diagnose the problem for several hours.

The design of the PORV indicator light was fundamentally flawed, because it implied that the PORV was shut when it went dark. When everything was operating correctly this was true, and the operators became habituated to rely on it. However, when things went wrong and the main relief valve stuck open, the dark lamp was actually misleading the operators by implying that the valve was shut. This caused the operators considerable confusion, because the pressure, temperature and levels in the primary circuit, so far as they could observe them via their instruments, were not behaving as they would have done if the PORV was shut — which they were convinced it was. This confusion contributed to the severity of the accident: because the operators were unable to break out of a cycle of assumptions which conflicted with what their instruments were telling them. It was not until a fresh shift came in who did not have the mind-set of the first set of operators that the problem was correctly diagnosed. But by then, major damage had been done.

The operators had not been trained to understand the ambiguous nature of the PORV indicator and look for alternative confirmation that the main relief valve was closed. There was a temperature indicator downstream of the PORV in the tail pipe between the PORV and the pressurizer that could have told them the valve was stuck open, by showing that the temperature in the tail pipe remained high after the PORV should have, and was assumed to have, shut, but this temperature indicator was not part of the "safety grade" suite of indicators designed to be used after an incident, and the operators had not been trained to use it. Its location on the back of the desk also meant that it was effectively out of sight of the operators.

Three Mile Island Nuclear Accident: Consequences of stuck valve

As the pressure in the primary system continued to decrease, reactor coolant continued to flow, but it was boiling inside the core. First, small bubbles of steam formed and immediately collapsed, known as nucleate boiling. As the system pressure decreased further, steam pockets began to form in the reactor coolant. This departure from nucleate boiling caused steam voids in coolant channels, blocking the flow of liquid coolant and greatly increasing the fuel plate temperature. The steam voids also took up more volume than liquid water, causing the pressurizer water level to rise even though coolant was being lost through the open PORV. Because of the lack of a dedicated instrument to measure the level of water in the core, operators judged the level of water in the core solely by the level in the pressurizer. Since it was high, they assumed that the core was properly covered with coolant, unaware that because of steam forming in the reactor vessel, the indicator provided false readings. This was a key contributor to the initial failure to recognize the accident as a loss-of-coolant accident, and led operators to turn off the emergency core cooling pumps, which had automatically started after the initial pressure decrease, due to fears the system was being overfilled.

With the PORV still open, the quench tank that collected the discharge from the PORV overfilled, causing the containment building sump to fill and sound an alarm at 4:11 a.m. This alarm, along with higher than normal temperatures on the PORV discharge line and unusually high containment building temperatures and pressures, were clear indications that there was an ongoing loss-of-coolant accident, but these indications were initially ignored by operators. At 4:15, the quench tank relief diaphragm ruptured, and radioactive coolant began to leak out into the general containment building. This radioactive coolant was pumped from the containment building sump to an auxiliary building, outside the main containment, until the sump pumps were stopped at 4:39 a.m.

After almost 80 minutes of slow temperature rise, the primary loop pumps began to cavitate as steam, rather than water, began to pass through them. The pumps were shut down, and it was believed that natural circulation would continue the water movement. Steam in the system prevented flow through the core, and as the water stopped circulating it was converted to steam in increasing amounts. About 130 minutes after the first malfunction, the top of the reactor core was exposed and the intense heat caused a reaction to occur between the steam forming in the reactor core and the Zircaloy nuclear fuel rod cladding, yielding zirconium dioxide, hydrogen, and additional heat. This fiery reaction burned off the nuclear fuel rod cladding, the hot plume of reacting steam and zirconium damaged the fuel pellets which released more radioactivity to the reactor coolant and produced hydrogen gas that is believed to have caused a small explosion in the containment building later that afternoon.

At 6 a.m., there was a shift change in the control room. A new arrival noticed that the temperature in the PORV tail pipe and the holding tanks was excessive and used a backup valve — called a block valve — to shut off the coolant venting via the PORV, but around 32,000 US gal (120,000 L) of coolant had already leaked from the primary loop. It was not until 165 minutes after the start of the problem that radiation alarms activated as contaminated water reached detectors; by that time, the radiation levels in the primary coolant water were around 300 times expected levels, and the plant was seriously contaminated.

Three Mile Island Nuclear Accident: Emergency declared

At 6:56 a.m., a plant supervisor declared a site emergency, and less than half an hour later station manager Gary Miller announced a general emergency, defined as having the "potential for serious radiological consequences" to the general public. Metropolitan Edison notified the Pennsylvania Emergency Management Agency (PEMA), which in turn contacted state and local agencies, governor Richard L. Thornburgh and lieutenant governor William Scranton III, to whom Thornburgh assigned responsibility for collecting and reporting on information about the accident. The uncertainty of operators at the plant was reflected in fragmentary, ambiguous, or contradictory statements made by Met Ed to government agencies and to the press, particularly about the possibility and severity of off-site radiation releases. Scranton held a press conference in which he was reassuring, yet confusing, about this possibility, stating that though there had been a "small release of radiation,... no increase in normal radiation levels" had been detected. These were contradicted by another official, and by statements from Met Ed, who both claimed that no radiation had been released. In fact, readings from instruments at the plant and off-site detectors had detected radiation releases, albeit at levels that were unlikely to threaten public health as long as they were temporary, and providing that containment of the then highly contaminated reactor was maintained.

Angry that Met Ed had not informed them before conducting a steam venting from the plant and convinced that the company was downplaying the severity of the accident, state officials turned to the NRC. After receiving word of the accident from Met Ed, the NRC had activated its emergency response headquarters in Bethesda, Maryland and sent staff members to Three Mile Island. NRC chairman Joseph Hendrie and commissioner Victor Gilinsky initially viewed the accident, in the words of NRC historian Samuel Walker, as a "cause for concern but not alarm". Gilinsky briefed reporters and members of Congress on the situation and informed White House staff, and at 10 a.m. met with two other commissioners. However, the NRC faced the same problems in obtaining accurate information as the state, and was further hampered by being organizationally ill-prepared to deal with emergencies, as it lacked a clear command structure and the authority to tell the utility what to do, or to order an evacuation of the local area.

In a 2009 article, Gilinsky wrote that it took five weeks to learn that "the reactor operators had measured fuel temperatures near the melting point". He further wrote: "We didn't learn for years—until the reactor vessel was physically opened—that by the time the plant operator called the NRC at about 8 a.m., roughly one-half of the uranium fuel had already melted."

It was still not clear to the control room staff that the primary loop water levels were low and that over half the core was exposed. A group of workers took manual readings from the thermocouples and obtained a sample of primary loop water. Seven hours into the emergency, new water was pumped into the primary loop and the backup relief valve was opened to reduce pressure so that the loop could be filled with water. After 16 hours, the primary loop pumps were turned on once again, and the core temperature began to fall. A large part of the core had melted, and the system was still dangerously radioactive.

On the third day following the accident, a hydrogen bubble was discovered in the dome of the pressure vessel, and became the focus of concern. A hydrogen explosion might not only breach the pressure vessel, but, depending on its magnitude, might compromise the integrity of the containment vessel leading to large scale release of radiation. However, it was determined that there was no oxygen present in the pressure vessel, a prerequisite for hydrogen to burn or explode. Immediate steps were taken to reduce the hydrogen bubble, and by the following day it was significantly smaller. Over the next week, steam and hydrogen were removed from the reactor using a catalytic recombiner and, controversially, by venting straight to the atmosphere.

The Chernobyl Shelter Fund

The Chernobyl Shelter Fund was established in 1997 at the Denver 23rd G8 summit to finance the Shelter Implementation Plan (SIP). The plan calls for transforming the site into an ecologically safe condition by means of stabilization of the sarcophagus followed by construction of a New Safe Confinement (NSC). While the original cost estimate for the SIP was US$768 million, the 2006 estimate was $1.2 billion. The SIP is being managed by a consortium of Bechtel, Battelle, and Electricité de France, and conceptual design for the NSC consists of a movable arch, constructed away from the shelter to avoid high radiation, to be slid over the sarcophagus. The NSC is expected to be completed in 2013, and will be the largest movable structure ever built.

Dimensions:

• Span: 270 m (886 ft)
• Height: 100 m (330 ft)
• Length: 150 m (492 ft)

The United Nations Development Programme

The United Nations Development Programme has launched in 2003 a specific project called the Chernobyl Recovery and Development Programme (CRDP) for the recovery of the affected areas. The programme was initiated in February 2002 based on the recommendations in the report on Human Consequences of the Chernobyl Nuclear Accident. The main goal of the CRDP’s activities is supporting the Government of Ukraine in mitigating long-term social, economic, and ecological consequences of the Chernobyl catastrophe. CRDP works in the four most Chernobyl-affected areas in Ukraine: Kyivska, Zhytomyrska, Chernihivska and Rivnenska.

The International Project on the Health Effects of the Chernobyl Accident

The International Project on the Health Effects of the Chernobyl Accident (IPEHCA) was created and received US $20 million, mainly from Japan, in hopes of discovering the main cause of health problems due to ¹³¹I radiation. These funds were divided between Ukraine, Belarus, and Russia, the three main affected countries, for further investigation of health effects. As there was significant corruption in former Soviet countries, most of the foreign aid was given to Russia, and no positive outcome from this money has been demonstrated.

Chernobyl Nuclear Accident Effects: International spread of radioactivity

Four hundred times more radioactive material was released than had been by the atomic bombing of Hiroshima. However, compared to the total amount released by nuclear weapons testing during the 1950s and 1960s, the Chernobyl disaster released 100 to 1000 times less radioactivity. The fallout was detected over all of Europe except for the Iberian Peninsula.

The initial evidence that a major release of radioactive material was affecting other countries came not from Soviet sources, but from Sweden, where on the morning of 28 April workers at the Forsmark Nuclear Power Plant (approximately 1,100 km (680 mi) from the Chernobyl site) were found to have radioactive particles on their clothes. It was Sweden's search for the source of radioactivity, after they had determined there was no leak at the Swedish plant, that at noon on April 28 led to the first hint of a serious nuclear problem in the western Soviet Union. Hence the evacuation of Pripyat on April 27, 36 hours after the initial explosions, was silently completed before the disaster became known outside the Soviet Union. The rise in radiation levels had at that time already been measured in Finland, but a civil service strike delayed the response and publication.

Contamination from the Chernobyl accident was scattered irregularly depending on weather conditions. Reports from Soviet and Western scientists indicate that Belarus received about 60% of the contamination that fell on the former Soviet Union. However, the 2006 TORCH report stated that half of the volatile particles had landed outside Ukraine, Belarus, and Russia. A large area in Russia south of Bryansk was also contaminated, as were parts of northwestern Ukraine. Studies in surrounding countries indicate that over one million people could have been affected by radiation.

Recently published data from a long-term monitoring program (The Korma-Report) show a decrease in internal radiation exposure of the inhabitants of a region in Belarus close to Gomel. Resettlement may even be possible in prohibited areas provided that people comply with appropriate dietary rules.

In Western Europe, precautionary measures taken in response to the radiation included seemingly arbitrary regulations banning the importation of certain foods but not others. In France some officials stated that the Chernobyl accident had no adverse effects. Official figures in southern Bavaria in Germany indicated that some wild plant species contained substantial levels of caesium, which were believed to have been passed onto them by wild boars, a significant number of which had already contained radioactive particles above the allowed level, consuming them.

Chernobyl Nuclear Accident Effects: Radioactive release

Like many other releases of radioactivity into the environment, the Chernobyl release was controlled by the physical and chemical properties of the radioactive elements in the core. While the general population often perceives plutonium as a particularly dangerous nuclear fuel, its effects are almost eclipsed by those of its fission products. Particularly dangerous are highly radioactive compounds that accumulate in the food chain, such as some isotopes of iodine and strontium.

Two reports on the release of radioisotopes from the site were made available, one by the OSTI and a more detailed report by the OECD, both in 1998. At different times after the accident, different isotopes were responsible for the majority of the external dose. The dose that was calculated is that received from external gamma irradiation for a person standing in the open. The dose to a person in a shelter or the internal dose is harder to estimate.

The release of radioisotopes from the nuclear fuel was largely controlled by their boiling points, and the majority of the radioactivity present in the core was retained in the reactor.

• All of the noble gases, including krypton and xenon, contained within the reactor were released immediately into the atmosphere by the first steam explosion.
• About 1760 PBq of I-131, 55% of the radioactive iodine in the reactor, was released, as a mixture of vapor, solid particles, and organic iodine compounds.
• Caesium and tellurium were released in aerosol form.
• An early estimate for fuel material released to the environment was 3 ± 1.5%; this was later revised to 3.5 ± 0.5%. This corresponds to the atmospheric emission of 6 t of fragmented fuel.

Two sizes of particles were released: small particles of 0.3 to 1.5 micrometers (aerodynamic diameter) and large particles of 10 micrometers. The large particles contained about 80% to 90% of the released nonvolatile radioisotopes zirconium-95, niobium-95, lanthanum-140, cerium-144 and the transuranic elements, including neptunium, plutonium and the minor actinides, embedded in a uranium oxide matrix.

Chernobyl Nuclear Accident Effects to Health of plant workers and local people

In the aftermath of the accident, 237 people suffered from acute radiation sickness, of whom 31 died within the first three months. Most of these were fire and rescue workers trying to bring the accident under control, who were not fully aware of how dangerous exposure to the radiation in the smoke was. Whereas, the World Health Organization's report 2006 Report of the Chernobyl Forum Expert Group from the 237 emergency workers who were diagnosed with ARS, ARS was identified as the cause of death for 28 of these people within the first few months after the disaster. There were no further deaths identified in the general population affected by the disaster as being caused by ARS. Of the 72,000 Russian Emergency Workers being studied, 216 non cancer deaths are attributed to the disaster, between 1991 and 1998. The latency period for solid cancers caused by excess radiation exposure is 10 or more years, thus at the time of the WHO report being undertaken the rates of solid cancer deaths were no greater than the general population.Some 135,000 people were evacuated from the area, including 50,000 from Pripyat.

Chernobyl Nuclear Accident Effects of Residual radioactivity in the environment

Rivers, lakes and reservoirs

The Chernobyl nuclear power plant is located next to the Pripyat River, which feeds into the Dnipro River reservoir system, one of the largest surface water systems in Europe. The radioactive contamination of aquatic systems therefore became a major issue in the immediate aftermath of the accident. In the most affected areas of Ukraine, levels of radioactivity (particularly radioiodine: I-131, radiocaesium: Cs-137 and radiostrontium: Sr-90) in drinking water caused concern during the weeks and months after the accident. After this initial period, however, radioactivity in rivers and reservoirs was generally below guideline limits for safe drinking water.

Bio-accumulation of radioactivity in fish resulted in concentrations (both in western Europe and in the former Soviet Union) that in many cases were significantly above guideline maximum levels for consumption. Guideline maximum levels for radiocaesium in fish vary from country to country but are approximately 1,000 Bq/kg in the European Union. In the Kiev Reservoir in Ukraine, concentrations in fish were several thousand Bq/kg during the years after the accident. In small "closed" lakes in Belarus and the Bryansk region of Russia, concentrations in a number of fish species varied from 0.1 to 60 kBq/kg during the period 1990–92. The contamination of fish caused short-term concern in parts of the UK and Germany and in the long term (years rather than months) in the affected areas of Ukraine, Belarus, and Russia as well as in parts of Scandinavia.

Groundwater

Groundwater was not badly affected by the Chernobyl accident since radionuclides with short half-lives decayed away long before they could affect groundwater supplies, and longer-lived radionuclides such as radiocaesium and radiostrontium were adsorbed to surface soils before they could transfer to groundwater. However, significant transfers of radionuclides to groundwater have occurred from waste disposal sites in the 30 km (19 mi) exclusion zone around Chernobyl. Although there is a potential for transfer of radionuclides from these disposal sites off-site (i.e. out of the 30 km (19 mi) exclusion zone), the IAEA Chernobyl Report argues that this is not significant in comparison to current levels of washout of surface-deposited radioactivity.

Flora and fauna

After the disaster, four square kilometers of pine forest in the immediate vicinity of the reactor turned reddish-brown and died, earning the name of the "Red Forest". Some animals in the worst-hit areas also died or stopped reproducing. Most domestic animals were evacuated from the exclusion zone, but horses left on an island in the Pripyat River 6 km (4 mi) from the power plant died when their thyroid glands were destroyed by radiation doses of 150–200 Sv. Some cattle on the same island died and those that survived were stunted because of thyroid damage. The next generation appeared to be normal.

A robot sent into the reactor itself has returned with samples of black, melanin-rich radiotrophic fungi that are growing on the reactor's walls.

Causes of Chernobyl Accident: Operator error initially faulted

There were two official explanations of the accident: the first, subsequently acknowledged as erroneous, was published in August 1986 and effectively placed the blame on the power plant operators. To investigate the causes of the accident the IAEA created a group known as the International Nuclear Safety Advisory Group (INSAG), which in its report of 1986, INSAG-1, on the whole also supported this view, based on the data provided by the Soviets and the oral statements of specialists. In this view, the catastrophic accident was caused by gross violations of operating rules and regulations. "During preparation and testing of the turbine generator under run-down conditions using the auxiliary load, personnel disconnected a series of technical protection systems and breached the most important operational safety provisions for conducting a technical exercise." The operator error was probably due to their lack of knowledge of nuclear reactor physics and engineering, as well as lack of experience and training. According to these allegations, at the time of the accident the reactor was being operated with many key safety systems turned off, most notably the Emergency Core Cooling System (ECCS), LAR (Local Automatic control system), and AZ (emergency power reduction system). Personnel had an insufficiently detailed understanding of technical procedures involved with the nuclear reactor, and knowingly ignored regulations to speed test completion.

The developers of the reactor plant considered this combination of events to be impossible and therefore did not allow for the creation of emergency protection systems capable of preventing the combination of events that led to the crisis, namely the intentional disabling of emergency protection equipment plus the violation of operating procedures. Thus the primary cause of the accident was the extremely improbable combination of rule infringement plus the operational routine allowed by the power station staff.

In this analysis of the causes of the accident, deficiencies in the reactor design and in the operating regulations that made the accident possible were set aside and mentioned only casually. Serious critical observations covered only general questions and did not address the specific reasons for the accident. The following general picture arose from these observations. Several procedural irregularities also helped to make the accident possible. One was insufficient communication between the safety officers and the operators in charge of the experiment being run that night. The reactor operators disabled safety systems down to the generators, which the test was really about. The main process computer, SKALA, was running in such a way that the main control computer could not shut down the reactor or even reduce power. Normally the reactor would have started to insert all of the control rods. The computer would have also started the "Emergency Core Protection System" that introduces 24 control rods into the active zone within 2.5 seconds, which is still slow by 1986 standards. All control was transferred from the process computer to the human operators.

This view is reflected in numerous publications and also artistic works on the theme of the Chernobyl accident that appeared immediately after the accident, and for a long time remained dominant in the public consciousness and in popular publications.

Causes of Chernobyl Accident: Operating instructions and design deficiencies found

Human factors contributed to the conditions that led to the disaster. These included operating the reactor at a low power level—less than 700 MW—a level documented in the run-down test program, and operating with a small operational reactivity margin (ORM). Operating the reactor at this low power level was not forbidden by regulations, contradicting what Soviet experts asserted in 1986. However, regulations did forbid operating the reactor with a small margin of reactivity. However, "... post-accident studies have shown that the way in which the real role of the ORM is reflected in the Operating Procedures and design documentation for the RBMK-1000 is extremely contradictory," and furthermore, "ORM was not treated as an operational safety limit, violation of which could lead to an accident."

According to the INSAG-7 Report, the chief reasons for the accident lie in the peculiarities of physics and in the construction of the reactor. There are two such reasons:

• The reactor had a dangerously large positive void coefficient. The void coefficient is a measurement of how a reactor responds to increased steam formation in the water coolant. Most other reactor designs have a negative coefficient, i.e. they attempt to decrease heat output when the vapor phase in the reactor increases, because if the coolant contains steam bubbles, fewer neutrons are slowed down. Faster neutrons are less likely to split uranium atoms, so the reactor produces less power (a negative feed-back). Chernobyl's RBMK reactor, however, used solid graphite as a neutron moderator to slow down the neutrons, and the water in it, on the contrary, acts like a harmful neutron absorber. Thus neutrons are slowed down even if steam bubbles form in the water. Furthermore, because steam absorbs neutrons much less readily than water, increasing the intensity of vaporization means that more neutrons are able to split uranium atoms, increasing the reactor's power output. This makes the RBMK design very unstable at low power levels, and prone to suddenly increasing energy production to a dangerous level. This behavior is counter-intuitive, and this property of the reactor was unknown to the crew.

• A more significant flaw was in the design of the control rods that are inserted into the reactor to slow down the reaction. In the RBMK reactor design, the lower part of each control rod was made of graphite and was 1.3 meters shorter than necessary, and in the space beneath the rods were hollow channels filled with water. The upper part of the rod—the truly functional part that absorbs the neutrons and thereby halts the reaction—was made of boron carbide. With this design, when the rods are inserted into the reactor from the uppermost position, the graphite parts initially displace some coolant. This greatly increases the rate of the fission reaction, since graphite (in the RBMK) is a more potent neutron moderator (absorbs far fewer neutrons than the boiling light water). Thus for the first few seconds of control rod activation, reactor power output is increased, rather than reduced as desired. This behavior is counter-intuitive and was not known to the reactor operators.

• Other deficiencies besides these were noted in the RBMK-1000 reactor design, as were its non-compliance with accepted standards and with the requirements of nuclear reactor safety.

Both views were heavily lobbied by different groups, including the reactor's designers, power plant personnel, and the Soviet and Ukrainian governments. According to the IAEA's 1986 analysis, the main cause of the accident was the operators' actions. But according to the IAEA's 1993 revised analysis the main cause was the reactor's design. One reason there were such contradictory viewpoints and so much debate about the causes of the Chernobyl accident was that the primary data covering the disaster, as registered by the instruments and sensors, were not completely published in the official sources.

Once again, the human factor had to be considered as a major element in causing the accident. INSAG notes that both the operating regulations and staff handled the disabling of the reactor protection easily enough: witness the length of time for which the ECCS was out of service while the reactor was operated at half power. INSAG’s view is that it was the operating crew's deviation from the test program that was mostly to blame. “Most reprehensibly, unapproved changes in the test procedure were deliberately made on the spot, although the plant was known to be in a very different condition from that intended for the test.”

As in the previously released report INSAG-1, close attention is paid in report INSAG-7 to the inadequate (at the moment of the accident) “culture of safety” at all levels. Deficiency in the safety culture was inherent not only at the operational stage but also, and to no lesser extent, during activities at other stages in the lifetime of nuclear power plants (including design, engineering, construction, manufacture and regulation). The poor quality of operating procedures and instructions, and their conflicting character, put a heavy burden on the operating crew, including the Chief Engineer. “The accident can be said to have flowed from a deficient safety culture, not only at the Chernobyl plant, but throughout the Soviet design, operating and regulatory organizations for nuclear power that existed at that time.”

The radiation levels in the worst-hit areas of the reactor building have been estimated to be 5.6 roentgens per second (R/s) (1.4 milliamperes per kilogram), equivalent to more than 20,000 roentgens per hour. A lethal dose is around 500 roentgens (0.13 coulombs per kilogram) over 5 hours, so in some areas, unprotected workers received fatal doses within minutes. However, a dosimeter capable of measuring up to 1,000 R/s (0.3 A/kg) was inaccessible because of the explosion, and another one failed when turned on. All remaining dosimeters had limits of 0.001 R/s (0.3 µA/kg) and therefore read "off scale." Thus, the reactor crew could ascertain only that the radiation levels were somewhere above 0.001 R/s (3.6 R/h, or 0.3 µA/kg), while the true levels were much, much higher in some areas. Approximate radiation levels at different locations shortly after the explosion:

location	radiation (roentgens per hour)
vicinity of the reactor core	30,000
fuel fragments	15,000–20,000
debris heap at the place of circulation pumps	10,000
debris near the electrolyzers	5,000–15,000
water in the Level +25 feedwater room	5,000
level 0 of the turbine hall	500–15,000
area of the affected unit	1,000–1,500
water in Room 712	1,000
control room, shortly after explosion	3–5
Gidroelektromontazh depot	30
nearby concrete mixing unit	10–15

Because of the inaccurate low readings, the reactor crew chief Alexander Akimov assumed that the reactor was intact. The evidence of pieces of graphite and reactor fuel lying around the building was ignored, and the readings of another dosimeter brought in by 4:30 a.m. were dismissed under the assumption that the new dosimeter must have been defective. Akimov stayed with his crew in the reactor building until morning, trying to pump water into the reactor. None of them wore any protective gear. Most, including Akimov, died from radiation exposure within three weeks.

Plant layout

based on the image of the plant

level	objects
49.6	roof of the reactor building, gallery of the refueling mechanism
39.9	roof of the deaerator gallery
35.5	floor of the main reactor hall
31.6	upper side of the upper biological shield, floor of the space for pipes to steam separators
28.3	lower side of the turbine hall roof
24.0	deaerator floor, measurement and control instruments room
16.4	floor of the pipe aisle in the deaerator gallery
12.0	main floor of the turbine hall, floor of the main circulation pump motor compartments
10.0	control room, floor under the reactor lower biological shield, main circulation pumps
6.0	steam distribution corridor
2.2	upper pressure suppression pool
0.0	ground level; house switchgear, turbine hall level
-0.5	lower pressure suppression pool
-5.2, -4.2	other turbine hall levels
-6.5	basement floor of the turbine hall

At 1:23:04 a.m. the experiment began. The steam to the turbines was shut off, and a run down of the turbine generator began, together with four (of eight total) Main Circulating Pumps (MCP). The diesel generator started and sequentially picked up loads, which was complete by 01:23:43; during this period the power for these four MCPs was supplied by the coasting down turbine generator. As the momentum of the turbine generator that powered the water pumps decreased, the water flow rate decreased, leading to increased formation of steam voids (bubbles) in the core. Because of the positive void coefficient of the RBMK reactor at low reactor power levels, it was now primed to embark on a positive feedback loop, in which the formation of steam voids reduced the ability of the liquid water coolant to absorb neutrons, which in turn increased the reactor's power output. This caused yet more water to flash into steam, giving yet a further power increase. However, during almost the entire period of the experiment the automatic control system successfully counteracted this positive feedback, continuously inserting control rods into the reactor core to limit the power rise.

At 1:23:40, as recorded by the SKALA centralized control system, an emergency shutdown or scram of the reactor was initiated. The scram was started when the EPS-5 button (also known as the AZ-5 button) of the reactor emergency protection system was pressed thus fully inserting all control rods, including the manual control rods that had been incautiously withdrawn earlier. The reason the EPS-5 button was pressed is not known, whether it was done as an emergency measure or simply as a routine method of shutting down the reactor upon completion of the experiment. There is a view that the scram may have been ordered as a response to the unexpected rapid power increase, although there is no recorded data convincingly testifying to this. Some have suggested that the button was not pressed but rather that the signal was automatically produced by the emergency protection system; however, the SKALA clearly registered a manual scram signal. In spite of this, the question as to when or even whether the EPS-5 button was pressed was the subject of debate. There are assertions that the pressure was caused by the rapid power acceleration at the start, and allegations that the button was not pressed until the reactor began to self-destruct but others assert that it happened earlier and in calm conditions. For whatever reason the EPS-5 button was pressed, insertion of control rods into the reactor core began. The control rod insertion mechanism operated at a relatively slow speed (0.4 m/s) taking 18–20 seconds for the rods to travel the full approximately 7-meter core length (height). A bigger problem was a flawed graphite-tip control rod design, which initially displaced coolant before neutron-absorbing material was inserted and the reaction slowed. As a result, the scram actually increased the reaction rate in the lower half of the core.

A few seconds after the start of the scram, a massive power spike occurred, the core overheated, and seconds later resulted in the initial explosion. Some of the fuel rods fractured, blocking the control rod columns and causing the control rods to become stuck after being inserted only one-third of the way. Within three seconds the reactor output rose above 530 MW. The subsequent course of events was not registered by instruments: it is known only as a result of mathematical simulation. First a great rise in power caused an increase in fuel temperature and massive steam buildup with rapid increase in steam pressure. This destroyed fuel elements and ruptured the channels in which these elements were located. Then according to some estimations, the reactor jumped to around 30 GW thermal, ten times the normal operational output. It was not possible to reconstruct the precise sequence of the processes that led to the destruction of the reactor and the power unit building. There is a general understanding that it was steam from the wrecked channels entering the reactor inner structure that caused the destruction of the reactor casing, tearing off and lifting by force the 2,000 ton upper plate (to which the entire reactor assembly is fastened). Apparently this was the first explosion that many heard. This was a steam explosion like the explosion of a steam boiler from the excess pressure of vapor. This ruptured further fuel channels—as a result the remaining coolant flashed to steam and escaped the reactor core. The total water loss combined with a high positive void coefficient to increase the reactor power.

A second, more powerful explosion occurred about two or three seconds after the first; evidence indicates that the second explosion resulted from a nuclear excursion. The nuclear excursion dispersed the core and effectively terminated that phase of the event. However, the graphite fire continued, greatly contributing to the spread of radioactive material and the contamination of outlying areas. There were initially several hypotheses about the nature of the second explosion. One view was that "the second explosion was caused by the hydrogen which had been produced either by the overheated steam-zirconium reaction or by the reaction of red-hot graphite with steam that produce hydrogen and carbon monoxide." Another hypothesis posits that the second explosion was a thermal explosion of the reactor as a result of the uncontrollable escape of fast neutrons caused by the complete water loss in the reactor core. A third hypothesis was that the explosion was caused, exceptionally, by steam. According to this version, the flow of steam and the steam pressure caused all the destruction following the ejection from the shaft of a substantial part of the graphite and fuel.

According to observers outside Unit 4, burning lumps of material and sparks shot into the air above the reactor. Some of them fell on to the roof of the machine hall and started a fire. About 25 per cent of the red-hot graphite blocks and overheated material from the fuel channels was ejected. ... Parts of the graphite blocks and fuel channels were out of the reactor building. ... As a result of the damage to the building an airflow through the core was established by the high temperature of the core. The air ignited the hot graphite and started a graphite fire.

However, the ratio of xenon radioisotopes released during the event provides compelling evidence that the second explosion was a nuclear power transient. This nuclear transient released ~0.01 kiloton of TNT equivalent (40 GJ) of energy; the analysis indicates that the nuclear excursion was limited to a small portion of the core.

Contrary to safety regulations, a combustible material (bitumen) had been used in the construction of the roof of the reactor building and the turbine hall. Ejected material ignited at least five fires on the roof of the (still operating) adjacent reactor 3. It was imperative to put those fires out and protect the cooling systems of reactor 3. Inside reactor 3, the chief of the night shift, Yuri Bagdasarov, wanted to shut down the reactor immediately, but chief engineer Nikolai Fomin would not allow this. The operators were given respirators and potassium iodide tablets and told to continue working. At 05:00, however, Bagdasarov made his own decision to shut down the reactor, leaving only those operators there who had to work the emergency cooling systems.

The Fukushima I nuclear reactor accidents are a series of ongoing events at the Fukushima I Nuclear Power Plant, following the 11 March 2011 Sendai earthquake and tsunami. As of 13 March, other incidents are ongoing at the Fukushima II plant 11.5 kilometres (7.1 mi) to the south.

On 11 March 2011, the Japanese government declared a "nuclear power emergency" due to a loss of coolant and evacuated thousands of residents living close to Fukushima I. The next day, while evidence for partial meltdown of the fuel rods in Unit 1 was growing, a hydrogen explosion destroyed the upper story of the building housing Reactor Unit 1 and injured four workers, but the container of the reactor remained intact.

On 13 March 2011, that a partial meltdown at Unit 3 has occurred appeared also possible. As of 1pm 13 March, JST, both reactors 1 and 3 had been vented and were being filled with water and boric acid to both cool and inhibit further nuclear reactions. Unit 2 was reported to have lower than normal water level but to be stable, although pressure inside the containment vessel is high. According to a Reuters report of 3:05pm EDT of 13 March 2011, all three reactors were being cooled with seawater.

On 13 March 2011, the Japan Atomic Energy Agency announced that it was rating the Fukushima accidents at 4 (accident with local consequences) on the International Nuclear Event Scale (INES). 170,000–200,000 people were evacuated after officials voiced the possibility of a meltdown.

On 14 March, the reactor building for Unit 3 exploded injuring eleven people. There was no release of radioactive material beyond that already being vented but blast damage affected water supply to Unit 2. The president of the French nuclear safety authority, Autorité de sûreté nucléaire (ASN), said that the accident should be rated as a 5 or even a 6 on INES. TEPCO shares dropped 24% in this first day of trading after the tsunami.

On 15 March, problems with the vents on Unit 2 apparently meant that pressure in its containment vessel had prevented adding water, to the extent that Unit 2 was in the most severe condition of the three reactors. An explosion in Unit 2 occurred at 06:14 JST in the "pressure suppression room", causing some damage to the reactor’s containment system. A fire broke out at Unit 4 involving spent fuel rods from the reactor, which are normally kept in the water-filled spent fuel pool to prevent overheating. Radiation levels at the plant rose significantly but have since fallen back.

16 March: at 5:45 a.m. JST, Kyodo News reported that a worker spotted new flames on the fourth story of Unit 4, where the spent fuel pool is located. This cast into doubt the earlier hope that the Tuesday blaze in the Unit 4 housing was caused by lubricating oil pumps; instead TEPCO officials acknowledged it was possible the spent fuel rods are uncovered and overheating, remarking that "the possibility of a re-criticality is not zero." By midday, NHK TV was reporting white smoke rising from the Fukushima I plant, which officials suggested was likely coming from Reactor 3. Shortly afterwards, reports surfaced that all but a small group of remaining workers at the plant had been placed on standby because of the dangerously rising levels of radioactivity up to 1000 mSv/h. Later reports stated that TEPCO had temporarily suspended operations at the facility due to radiation spikes and had pulled all their employees out. A TEPCO press release stated that workers had been withdrawn at 06:00 JST because of abnormal noises coming from one of the reactor pressure suppression chambers. Late in the evening, Reuters reported that water was being poured into reactors 5 and 6.

17 March: During the morning, Self-Defense Force helicopters dropped four containers of water on the spent fuel pools of Units 3 and 4. In the afternoon it was reported that the Unit 4 spent fuel pool is full with water and none of the fuel rods are exposed. Construction work was started to supply a working external electrical power source to all six units of Fukushima I.

18 March: Pursuant to a request from Tokyo Governor Shintaro Ishihara, Tokyo Fire Department dispatched thirty fire engines with 139 fire-fighters and trained rescue team at approximately 03:00 JST. These include a fire truck with a 22 m water tower; all units will join Japan Defense Forces fire equipment which is already deployed. JDF anticipated utilizing TFD equipment to address low water which was confirmed to exist in unit 4 and also an emergent concern with unit 3, which appeared to be more problematic than previously believed. W inds were forecast to shift to the northeast, which would continue to be toward the sea.For the second consecutive day, high radiation levels have been detected in an area 30 kilometers (18.6 miles) northwest of the damaged Fukushima Daiichi nuclear plant. The reading was 150 microsieverts per hour. Human exposure to that level of radiation for six to seven hours would result in absorption of what is considered safe in a year.

Fukushima 1 Reactor unit 1

Cooling problems at unit 1

On 11 March 2011 at 16:36 JST, a nuclear emergency situation (Article 15 of the Japanese law on Special Measures Concerning Nuclear Emergency Preparedness) was declared when "the status of reactor water coolant injection could not be confirmed for the emergency core cooling systems of Units 1 and 2". The alert was cleared "when the reactor water level monitoring function was restored for Unit 1." However, it was reinstated at 17:07 JST. Potentially radioactive steam was released from the primary circuit into the secondary containment area to reduce mounting pressure.

In the early hours of 12 March TEPCO reported that radiation levels were rising in the turbine building for Reactor Unit 1 and that it was considering venting hot gas from the Unit 1 reactor vessel into the atmosphere, which could result in the release of radiation. Chief Cabinet Secretary Yukio Edano stated later in the morning that the amount of potential radiation would be small and that the prevailing winds are blowing out to sea. At 02:00 JST, the pressure inside the reactor containment was reported to be 600 kPa (6 bar or 87 psi), 200 kPa higher than under normal conditions. At 05:30 JST the pressure inside Reactor 1 was reported to be 2.1 times the "design capacity", 820 kPa. Rising heat within the containment area would have led to increasing pressure, with both cooling water pumps and ventilation fans for driving air through heat exchangers within containment dependant on electricity.

In a press release at 07:00 JST 12 March, TEPCO stated, "Measurement of radioactive material (iodine, etc.) by monitoring car indicates increasing value compared to normal level. One of the monitoring posts is also indicating higher than normal level." The gamma ray radiation recorded on the main gate was increased from 69 nanogray/hour (nGy/h) (04:00 JST, 12 March) to 866 nGy/h 40 minutes later and reached the peak of 385.5 μSv/hour (1μSv = 0.1 mrem, 1 μGy = 1000 nGy) at 10:30 JST. At 13:30 JST, radioactive caesium-137 and iodine-131 was detected near reactor 1, which indicates that some of the core was exposed to air due to a partial-meltdown or other damage of the nuclear fuel. The NHK website reported that cooling water had lowered so much that parts of the nuclear fuel rods were exposed. Radiation levels at the site boundary exceeded the regulatory limits. Kyodo News Service later reported that partial melting may have occurred. On 14 March 2011, Kyodo News reported the radiation levels have continued to increase on the premises, measuring at two different locations at 2:20 AM an intensity of 751 μSv/hour on one location and at 2:40 AM an intensity of 650 μSv/hour at another location on the premises.

Government statements on possibility of meltdown

In a press conference, the chief spokesman of the Japanese nuclear authorities was translated into English as having said that a nuclear meltdown may be a possibility at Unit 1. Toshihiro Bannai, director of the international affairs office of Japan's Nuclear and Industrial Safety, in a telephone interview with CNN, stated that a meltdown was possible. However, the Japanese prime minister soon indicated that a nuclear meltdown was not in progress and emphasized that the containment of Unit 1 was still intact. After the statement, the government added that the claim of a meltdown had been mistranslated. The temperature inside the reactor was not reported, but Japanese regulators said it was not dropping as quickly as they wanted. The chief spokesman of the Japanese government, Yukio Edano, confirmed that there was a "significant chance" that radioactive fuel rods had partially melted in Unit 1. "I am trying to be careful with words... This is not a situation where the whole core suffers a meltdown."

Explosion of reactor building

At 15:36 JST on 12 March 2011 there was an explosion at Unit 1. Four workers were injured, and the upper shell of the reactor building was blown away leaving in place its steel frame. This building has been designed as a secondary containment of radioactive materials, but not to withstand the high pressure of an explosion: in the Fukushima I reactors the primary containment consists of "drywell" and "wetwell" concrete structures immediately surrounding the reactor pressure vessel.

Experts soon agreed that the cause was a hydrogen explosion. Almost certainly the hydrogen was formed inside the reactor vessel because of falling water levels, and this hydrogen then leaked into the containment building. Safety devices should ignite the hydrogen before explosive concentrations are reached but apparently these systems failed.

Officials indicated that the container of the reactor had remained intact and there had been no large leaks of radioactive material, although an increase in radiation levels was confirmed following the explosion. ABC news reported that according to the Fukushima prefectural government, the hourly radiation from the plant reached 1,015 µSv. Two independent nuclear experts cited design differences between the Chernobyl Nuclear Power Plant and the Fukushima I Nuclear Power Plant, one of them saying he did not believe that a Chernobyl-style disaster will occur.

At 20:05 on 12 March 2011, according to the nuclear regulation act and to the directives of the Prime Minister, the Japanese government ordered seawater to be used in Unit 1 in an effort to cool down the degraded reactor core. At 21:00 JST TEPCO announced that they planned to cool the leaking reactor with seawater (which started at 20:20 JST), then using boric acid to act as a neutron absorber to prevent a criticality accident. The water would take five to ten hours to fill the reactor core, after which it would need to stay for cooling for around ten days. At 23:00 JST TEPCO announced that due to the quake at 22:15 the filling of the reactor had been temporarily stopped but has been resumed after a short while. Filling the reactor with seawater will contaminate the reactor with impure water, a substance not usually allowed in reactors, meaning the reactor will likely be decommissioned, since it is not cost effective to decontaminate.

Seawater used for cooling

At 20:05 on 12 March 2011, according to the nuclear regulation act and to the directives of the Prime Minister, the Japanese government ordered seawater to be used in Unit 1 in an effort to cool down the degraded reactor core. At 21:00 JST TEPCO announced that they planned to cool the leaking reactor with seawater (which started at 20:20 JST), then using boric acid to act as a neutron absorber to prevent a criticality accident. The water would take five to ten hours to fill the reactor core, after which it would need to stay for cooling for around ten days. At 23:00 JST TEPCO announced that due to the quake at 22:15 the filling of the reactor had been temporarily stopped but has been resumed after a short while. Filling the reactor with seawater will contaminate the reactor with impure water, a substance not usually allowed in reactors, meaning the reactor will likely be decommissioned, since it is not cost effective to decontaminate.

NISA reported that injection of sea water into the primary containment vessel through the fire extinguisher system commenced at 11:55 on 13 March. At 01:10 on 14 March injection of sea water was halted because all available water in the plant pools had run out (similarly, feed to unit 3 was halted). Water supply was restored at 03:20. Radiation levels around the plant were measured at around 0.03 µSv/h at 05:00 and 15:00 on 14 March.

Fukushima 1 Reactor unit 2

Unit two was operational during the earthquake and experienced the same cooling procedures directly after the earthquake (power supply by Diesel engine, which failed after circa 1 hour), and stable water levels were reported. Power was achieved by mobile power units, while preparations were made to perform pressure venting. According to a Reuters report of 3:05pm EDT of 13 March 2011, this reactor was also being cooled with seawater.

Cooling problems at Fukushima reactor unit 2

On Mar 14, at 15:29 JST the Jiji news agencies reported that the cooling functions at reactor unit 2 had stopped and that the cooling water levels were falling. This was caused when fuel for pumps ran out. Jiji news agencies later reported that nuclear fuel rods at reactor unit 2 were fully exposed and there was a risk of a full meltdown at reactor unit 2. Jiji later reported that according to TEPCO, a meltdown cannot be ruled out.

At 22:29 JST, NHK reported that workers had succeeded in refilling half the reactor with water. However, at that time, part of the rods were still exposed, and technicians could not rule out the possibility that fuel rods had melted. Work was in hand to demolish parts of the walls of reactor building 2 to allow the escape of hydrogen and hopefully prevent another explosion. At 21:37 JST the measured radiation levels at the gate of the plant had reached the a maximum of 3130 μSv per hour, which was enough to reach the annual limit for non-nuclear workers in twenty minutes, but had fallen back to 326 μSv/hr by 22:35.

It was believed that around 23:00 JST the 4m long fuel rods in the reactor were fully exposed for the second time. At 00:30 JST of 15 March, NHK ran a live press conference with TEPCO stating that the water level had sunk under the rods once again and pressure in the vessel was raised. The utility said that the hydrogen explosion at unit 3 may have caused a glitch in the cooling system of unit 2: Four out of five water pumps being used to cool unit 2 reactor had failed after the explosion at unit 3. In addition, the last pump had briefly stopped working. To replenish the water, the contained pressure would have to be lowered first by opening a valve of the vessel. Due to an accident the unit's air flow gauge was turned off. With the gauge turned off, flow of water into the reactor was blocked, leading to full exposure of the rods.

As of 04:11 JST (March 15), water was being pumped into the reactor of unit 2 again.

At 01:38 CET (10:38 JST, March 15), water level was reported to be at 1.20 meters and rising.

Explosion in reactor unit 2 building

An explosion was heard after 6:10 JST on 15 March in unit 2, possibly damaging the pressure-suppression system, which is at the bottom part of the container. The radiation level was reported to exceed the legal limit and the plant's operator started to evacuate workers from the plant. Soon after, Kyodo News reported that radiation had risen to 8,217 μSv per hour around two hours after the explosion—about eight times what one usually is exposed to within a whole year—and again down to 2,400 μSv, shortly after. Three hours after the explosion the radiation has risen to 11,900 μSv per hour.

Japanese nuclear authorities initially said that the containment vessel had not been damaged as a result of the explosion. Later reports indicated that the containment vessel had, in fact, been damaged in the explosion, that radiation levels had spiked, and that workers were being evacuated. If all workers leave the plant, the nuclear fuel in the reactors is likely to melt down, prompting a release of radioactive material. The suppression pool beneath the reactor may have cracked.

Fukushima 1 Reactor unit 3

Unlike the other five reactor units, reactor 3 runs on mixed uranium and plutonium oxide, or MOX fuel, making it potentially more dangerous in an incident due to the neutronic effects of plutonium on the reactor and the carcinogenic effects in the event of release to the environment.

Cooling problems at unit 3

Early on 13 March 2011, an official of the Japan Nuclear and Industrial Safety Agency told a news conference that the emergency cooling system of Unit 3 had failed, spurring an urgent search for a means to supply cooling water to the reactor vessel in order to prevent a meltdown of its reactor core. At 05:38 there was no means of adding coolant to the reactor due to loss of power. Work to restore power and vent pressure continued. At one point, the top three meters of mixed oxide (MOX) fuel rods were exposed to the air.

At 07:30 JST, TEPCO prepared to release radioactive steam, indicating that "the amount of radiation to be released would be small and not of a level that would affect human health" and manual venting took place at 08:41 and 09:20. At 09:25 JST on 13 March 2011, operators began injecting water containing boric acid into the reactor via a fire pump. When water levels continued to fall and pressure to rise, the injected water was switched to sea water at 13:12. By 15:00 it was noted that despite adding water the level in the reactor did not rise and radiation had increased. A rise was eventually recorded but the level stuck at 2m below the top of reactor core. Other readings suggested that this could not be the case and the gauge was malfunctioning.

At 12:33 JST on 13 March 2011, the chief spokesman of the Japanese government, Yukio Edano, was reported to have confirmed that there was a “significant chance” that radioactive fuel rods had partially melted in unit 3 just as in unit 1, or that "it was 'highly possible' a partial meltdown was underway". “I am trying to be careful with words ... This is not a situation where the whole core suffers a meltdown”. He added that hydrogen was building up inside the outer building of unit 3 just as it had in unit 1, threatening the same kind of explosion. Soon after, Edano disclaimed that a meltdown was in progress. He stated that there is no “significant chance” that radioactive fuel rods had partially melted and he emphasized that there is no danger for the health of the population. He indicated that increased radiation had been measured inside the reactor.

Fukushima I Explosion of reactor unit 3 building

At 11:15 JST on 14 March 2011, a building surrounding Reactor 3 of Fukushima 1 exploded as well, presumably due to the ignition of built up hydrogen gas. There is no health risk reported, though 600 people have been ordered to stay indoors. Within minutes, it was reported that as with reactor 1, the outer reactor building was blown apart, but the inner containment vessel was not breached. TEPCO stated that one worker was injured and seven missing.

Hydrogen Blast

A hydrogen explosion occurred Monday morning at the quake-hit Fukushima No. 1 nuclear power plant’s troubled No. 3 reactor, the government’s nuclear safety agency said.

The 11:01 a.m. incident came after a hydrogen explosion hit the No. 1 reactor at the same plant Saturday, and prompted the Nuclear and Industrial Safety Agency to urge residents within a 20-kilometer radius to take shelter inside buildings.

It also followed a report by Tokyo Electric Power Co., the plant’s operator, to the government earlier in the day that the radiation level at the plant had again exceeded the legal limit and pressure in the container of the No. 3 reactor had increased.

The Fukushima No. 1 nuclear plant has been shut down since a magnitude 9.0 quake struck northeastern and eastern Japan on Friday, but some of its reactors have lost their cooling functions, leading to brief rises in the radiation level over the weekend.

On Monday, radiation at the plant’s premises rose over the benchmark limit of 500 micro sievert per hour at two locations, measuring 751 micro sievert at the first location at 2:20 a.m. and 650 at the second at 2:40 a.m., according to the report.

Fukushima 1 Reactor unit 4

Although the reactor unit 4 was not functioning at the time of the earthquake, fire was observed coming from the reactor on 15 March. Authorities believe that the used spent fuel is the cause of this fire, or that fallout from the explosions at units 1-3 supplied the ignition source.

At the time of the earthquake unit 4 had been shut down for a scheduled periodic inspection since 30 November 2010. All fuel rods had been transferred in December 2010 from the reactor to the spent fuel pool on the top floor of the reactor building where they were held in racks containing boron to damp down any nuclear reaction. These recently active fuel rods were hotter and required more cooling than the spent fuel in units 5 and 6. At 04:00 JST on Monday 14 March water in the pool had reached a temperature of 84°C compared to a normal value of 40-50°C.At approximately 06:00 JST on 15 March, a loud explosion was heard within the power station, and later it was confirmed that the 4th floor rooftop area of the Unit 4 reactor building had sustained damage. At 09:40 JST on 15 March 2011, the Unit 4 spent fuel pool caught fire, likely releasing radioactive contamination from the fuel stored there. TEPCO said workers extinguished the fire by 12:00. As radiation levels rose, some of the employees still at the plant were evacuated. The reason for the fire seems to have been a hydrogen explosion.

On the morning of 15 March 2011 (JST), Secretary Edano announced that according to the Tokyo Electric Power Company, radiation dose equivalent rates measured from the reactor unit 4 reached 100 mSv per hour. Government speaker Edano has stated that there was no continued release of radiation. The dose after which the symptoms of acute radiation poisoning typically appear is approximately 1000 mSv, or 1 Sv, received over one day. An exposed worker would be expected to begin experiencing radiation sickness soon after receiving a 100 mSv/h dose rate for 10 hours of a day, or a 400 mSv/h dose rate for 2.5 hours of a day.

Japan's nuclear safety agency reported two holes, each 8 meters square (64 m² or 689 sq. feet -- not 8 sq. meters each) in a wall of the outer building of the number 4 reactor after an explosion there. Further, at 17:48 JST it was reported that water in the spent fuel pool might be boiling.

As of 15 March 2011 21:13 JST, radiation inside unit 4 had increased so much inside the control room that employees could not stay there permanently any more. Seventy staff remained on site but 800 had been evacuated.^[158] By 22:30 JST, TEPCO was reported to be unable to pour water into No. 4 reactor's storage pool for spent fuel. At around 22:50 JST, it was reported that TEPCO was considering using helicopters to drop water on the spent fuel storage pool. However, TEPCO soon dismissed the option of helicopters because of concerns over safety and effectiveness. Chinook helicopters have been used in an attempt to dump water but have a maximum payload of around 10 tonnes TEPCO went on to consider the use of high-pressure fire hoses instead.

A fire was discovered at 05:45 JST on 16 March in the north west corner of the reactor building by a worker taking batteries to the central control room of unit 4. This was reported to the authorities, but on further inspection at 06:15 no fire was found. Other reports stated that the fire was under control. At 11:57 JST, TEPCO released a photograph of No.4 reactor showing that "a large portion of the building's outer wall has collapsed." Technicians reportedly considered spraying boric acid on the building from a helicopter.

Possibility of criticality in the spent fuel pool

This new fire cast into doubt the earlier hope that the Tuesday blaze in the Unit 4 housing was caused by lubricating oil pumps; instead at approximately 14:30, TEPCO announced its belief that the storage pool may have begun boiling, raising the possibility that exposed rods would reach criticality. BBC commented that criticality would not mean a nuclear bomb-like explosion; however, a sustained release of radioactive materials would be a possible scenario.

Around 20:00 JST on 16 March it was planned to use a police water cannon to spray water on unit 4.

Iouli Andreev, former director of the Soviet Spetsatom clean-up agency involved in the Chernobyl clean-up, as well as Laurence Williams, professor of nuclear safety at the University of Central Lancashire, speculate that the Fukushima management could have been engaged in an unsafe industry practice of re-racking spent rods in the pool well beyond its rated capacity, in effect heightening danger of melting and pool boil-off.

On 16 March the chairman of United States Nuclear Regulatory Commission, Gregory B. Jaczko, said in Congressional testimony that the NRC believes all of the water in the spent fuel pool has boiled dry. Japanese nuclear authorities and TEPCO contradicted this report, but later in the day Jaczko stood by his claim saying it had been confirmed by sources in Japan. At 1PM TEPCO observed via helicopter the pool had not boiled off, nor were any fuel rods exposed.

Fukushima 1 Reactor unit 5 and 6

Both reactors were off line at the time the earthquake struck (reactor 5 had been shut down on 3 January 2011 and reactor 6 on 14 August 2010). Although an IAEA report indicated that the fuel rods are still in the reactor vessels of both units and not in the spent fuel pools as in Unit 4, Kyodo News said that there were rods in the pools, but only one-third as many in the pools as compared to Unit 4.

Government spokesman Edano stated on 15 March that reactors 5 and 6 were being closely monitored, as cooling processes were not functioning well. At 21:00 on 15 March water levels in unit 5 were reported to be 2 m above fuel rods, but were falling at a rate of 8 cm per hour. Unit 6 was reported to have operational diesel generated power and this was to be used to power pumps in unit 5 to supply more water.

The removal of roof panels from reactor buildings 5 and 6 was being considered in order to allow any hydrogen build-up to escape. The BBC later reported that units 5 and 6 were believed to be heating up. At 18:31 on 16 March, TEPCO was reported to be pouring water into both reactors.

Radioactive levels and radioactive contamination

Radiation levels at the stricken Fukushima I power plant have varied up to 1,000 mSv/h(millisievert per hour), which is a level that can cause radiation sickness. The level of radiation within the 20 km exclusion zone surrounding the power plant is such that people have been advised to evacuate, and people within the 20-30km zone are being advised to stay indoors. The radiation is believed to come from short-lived isotopes (noble gases and nitrogen) that escape mixed with steam through venting, or with the hydrogen explosions.

Chief Cabinet Secretary, Yukio Edano, said that on 15 March 2011 radiation rates had been measured as high as 30 mSv/h between the Units 2 and 3, as high as 400 mSv/h near Unit 3 between it and Unit 4, and 100 mSv/h near Unit 4. He indicated that "There is no doubt that unlike in the past, the figures are the level at which human health can be affected," Prime Minister Naoto Kan urged people living between 20 and 30 kilometers of the plant to stay indoors, "The danger of further radiation leaks (from the plant) is increasing," Kan warned the public at a press conference, while asking people to "act calmly".

A spokesman for Japan's nuclear safety agency said TEPCO had told it that radiation levels in Ibaraki, between Fukushima and Tokyo, had risen. "The level does not pose health risks," the spokesman said. The Tokyo metropolitan government said it has detected radioactive material, such as iodine and cesium, up to 40 times normal levels in Saitama, near Tokyo. Radiation levels in Tokyo were at one point measured at 0.8 μSv/hour although they were later measured at "about twice the normal level". Later, on 15 March 2011, Edano reported that radiation levels were lower. A changed wind direction dispersed radiation away from the land and back over the Pacific Ocean. Thousands of Tokyo residents are reported to have left for cities further south, although Edano insisted that levels in Greater Tokyo were not hazardous.

On 16 March power plant staff were briefly evacuated after smoke rose above the plant and radiation levels surged to 1,000 mSv/h before coming down to 800–600 mSv/h, and staff returned. Japan's defence ministry criticized the nuclear safety agency and TEPCO after some of its troops were possibly exposed to radiation when working on the site. The Japan's ministry of science measured radiation levels of up to 0.33 millisieverts per hour 20 kilometers northwest of the power plant.

International commentators were divided in their analysis of the scale of the danger, with French Foreign Minister, Alain Juppe, saying that the threat was "extremely high" while others said it was too early to make comparisons to the 1986 Chernobyl disaster.

The United Nations are predicting that a radiation plume from the stricken Japanese reactors will reach the USA by Friday March 18. Health and nuclear experts emphasize that radiation in the plume will be diluted and, at worst, would have extremely minor health consequences in the United States.

Government reaction

The Prime Minister of Japan, Naoto Kan, visited the plant for a briefing on 12 March 2011.

At 01:17 JST on Sunday 13 March 2011, the Japan Atomic Energy Agency announced that it was rating the Fukushima accidents at 4 (accident with local consequences) on the 0–7 International Nuclear Event Scale (INES), below the Three Mile Island accident in seriousness which was at 5, a rating that would make the severity of the Fukushima event comparable to Sellafield accidents between 1955 and 1979 that were also at 4.

On the morning of 15 March, the evacuation area was again extended. Prime Minister Naoto Kan issued instructions that any remaining people within a 20km (12 mile) zone around the plant must leave, and urged that those living between 20km and 30km from the site should stay indoors. Prime Minister Naoto Kan also issued instructions that140.000 peoples in fukushima to seal up themself and 800 workers must be evacuated.

Evacuations

After the declaration of a nuclear emergency by the Government at 19:03 on 11 March, the Fukushima prefecture ordered the evacuation of an estimated 1,864 people within a distance of 2km from the plant. This was extended to 3 kilometres (1.9 mi) and 5,800 people at 21:23 by a directive to the local governor from the Prime Minister, together with instructions for residents within 10 kilometres (6.2 mi) of the plant to stay indoors. The evacuation was expanded to a 10 kilometres (6.2 mi) radius at 5:44 on 12 March, and then to 20 kilometres (12 mi) at 18:25, shortly before ordering use of sea water for emergency cooling.

Evacuations were also ordered around the nearby Fukushima II (Daini) plant. Residents within 3 kilometres (1.9 mi) were ordered to evacuate at 7:45 on 12 March, again with instructions for those within 10km to stay indoors. Evacuation was extended to 10km by 17:39. BBC correspondent Nick Ravenscroft was stopped 60 kilometres (37 mi) from the plants by police. Over 50,000 people were evacuated during 12 March. The figure increased to 170,000–200,000 people on 13 March, after officials voiced the possibility of a meltdown.

On the morning of 15 March, the evacuation area was again extended. Prime Minister Naoto Kan issued instructions that any remaining people within a 20km (12 mile) zone around the plant must leave, and urged that those living between 20km and 30km from the site should stay indoors.

Fukushima I Nuclear Accidents Effect on employees and residents

The Guardian reported at 17:35 JST on 12 March that NHK advised residents of the Fukushima area "to stay inside, close doors and windows and turn off air conditioning. They were also advised to cover their mouths with masks, towels or handkerchiefs" as well as not to drink tap water. Air traffic has been restricted in a 20-kilometre (12 mi) radius around the plant, according to a NOTAM. The BBC has reported as of 22:49 JST (13:49 GMT) "A team from the National Institute of Radiological Sciences has been dispatched to Fukushima as a precaution, reports NHK. It was reportedly made up of doctors, nurses and other individuals with expertise in dealing with radiation exposure, and had been taken by helicopter to a base 5 km from the nuclear plant."

The IAEA stated on 13 March that four workers had been injured by the explosion at the Unit 1 reactor, and that three injuries were reported in other incidents at the site. They also reported one worker was exposed to higher-than-normal radiation levels but that fell below their guidance for emergency situations.

At 22:53 JST Tokyo Broadcasting System (TBS), quoting Fukushima representatives, has reported that there was an evacuation of 30 staff members and 60 patients due to the explosion. From those evacuees three patients received a checkup for radiation exposure by the hospital staff at Futaba, a town 5.6 km (3.5 miles) from the power plant. All three patients required decontamination, but about 90 other evacuees may also require decontamination.

Effects on human health

Normal background radiation varies from place to place but delivers a dose equivalent in the vicinity of 2.4 mSv/year annually, or about 0.3 µSv/h. The international limit for radiation exposure for nuclear workers is 20 mSv per year, averaged over five years, with a limit of 50 mSv in any one year, however for workers performing emergency services EPA guidance on dose limits is 100 mSv when "protecting valuable property" and 250 mSv when the activity is "life saving or protection of large populations." A 250 mSv dose is estimated to increase one's lifetime risk of developing fatal cancer from about 20% to about 21%, and chronic exposure of 100 mSv per year is the "lowest level at which any increase in cancer is clearly evident," according to the World Nuclear Association. Symptoms of radiation poisoning typically emerge with a 1000 mSv total dose over a day.

Radiation dose rates at one location between reactor Units 3 and 4 was measured at 400 mSv/h at 10:22 JST, 13 March 2011, causing experts to urge rapid rotation of emergency crews as a method of limiting exposure to radiation. Prior to the accident, the maximum permissible dose for Japanese nuclear workers was 100 mSv in any one year, but on 15 March 2011, the Japanese Health and Labor Ministry increased that annual limit to 250 mSv, for emergency situations. The general population faced separate risks from chronic exposure to lower-level contaminants released into the environment.