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.

The International Nuclear and Radiological Event Scale (INES) was introduced in 1990 by the International Atomic Energy Agency (IAEA) in order to enable prompt communication of safety significance information in case of nuclear accidents.

The scale is intended to be logarithmic, similar to the Richter scale that is used to describe the comparative magnitude of earthquakes. Each increasing level represents an accident approximately ten times more severe than the previous level. Compared to earthquakes, where the event intensity can be quantitatively evaluated, the level of severity of a man-made disaster, such as a nuclear accident, is more subject to interpretation. Because of the difficulty of interpreting, the INES level of an incident is assigned well after the incident occurs. Therefore, the scale has a very limited ability to assist in disaster-aid deployment.

Commonly, the organisation where the nuclear incident occurs assigns a first provisional INES rating to an incident, after it is being reviewed and possibly revised by the designated national radiation authority.

A number of criteria and indicators are defined to assure coherent reporting of nuclear events by different official authorities. There are 7 levels on the INES scale; 3 incident-levels and 4 accident-levels. There is also a level 0.

The level on the scale is determined by the highest of three scores: off-site effects, on-site effects, and defence in depth degradation.

Level 7: Major accident

Impact on People and Environment

Major release of radio­active ­material with widespread health and environmental effects r­equiring implementation of planned and extended ­countermeasures

The only accident:

• Chernobyl disaster, 26 April 1986. A power surge during a test procedure resulted in a criticality accident, leading to a powerful steam explosion and fire that released a significant fraction of core material into the environment, resulting in a death toll of 56 as well as estimated 4 000 additional cancer fatalities among people exposed to elevated doses of radiation and a permanent loss of large areas of habitable land. The disaster is the only Level 7 Event that has ever occurred.

Level 6: Serious accident

Impact on People and Environment

Significant release of radioactive material likely to require implementation of planned countermeasures.

Example

• Kyshtym disaster at Mayak, Soviet Union, 29 September 1957. A failed cooling system at a military nuclear waste reprocessing facility caused a steam explosion that released 70–80 tons of highly radioactive material into the environment. Impact on local population is not fully known.

Level 5: Accident with wider consequences

Impact on People and Environment

Limited release of radioactive ­material likely to require i­mplementation of some planned­ countermeasures.

Several deaths from ­radiation.

Example

• Windscale fire (United Kingdom), 10 October 1957. Annealing of graphite moderator at a military air-cooled reactor caused the graphite and the metallic uranium fuel to catch fire, releasing radioactive pile material as dust into the environment.

Impact on Radiological Barriers and Control

Severe damage to reactor core.

Release of large quantities of radioactive material within an installation with a high probability of significant public exposure. This could arise from a major criticality accident or fire.

Example: Three Mile Island accident (Harrisburg, United States), 28 March 1979. A combination of design and operator errors caused a gradual loss of coolant, leading to a partial meltdown. Radioactive gases were released into the atmosphere.

Other examples:

• First Chalk River Accident Chalk River, Ontario, Canada, 12 December 1952. Reactor core damaged.
• Goiânia accident (Brazil), 13 September 1987. An unsecured caesium chloride radiation source left in an abandoned hospital was recovered by scavenger thieves unaware of its nature and sold at a scrapyard. 249 people were contaminated and 4 died.

Level 4: Accident with local consequences

Impact on People and the Environment

Minor release of radioactive material unlikely to result in implementation of planned countermeasures other than local food controls.

At least one death from radiation.

Impact on Radiological Barriers and Control

Fuel melt or damage to fuel ­resulting in more than 0.1% release of core inventory.

Release of significant quantities of radioactive material within an installation with a high ­probability of significant public exposure.

Examples:

• Sellafield (United Kingdom) – 5 incidents 1955 to 1979
• SL-1 Experimental Power Station (United States) – 1961, reactor reached prompt criticality, killing three operators.
• Saint-Laurent Nuclear Power Plant (France) – 1980, partial core meltdown.
• Buenos Aires (Argentina) – 1983, criticality accident during fuel rod rearrangement killed one operator and injured 2 others.
• Jaslovské Bohunice (Czechoslovakia) – 1977, contamination of reactor building.
• Tokaimura nuclear accident (Japan) – 1999, three inexperienced operators at a reprocessing facility caused a criticality accident; two of them died.
• Fukushima I Nuclear Power Plant (Japan) – 2011, reactor shutdown after the 2011 Sendai earthquake and tsunami, failure of emergency cooling caused an explosion.

Level 3: Serious incident

Impact on People and Environment

Exposure in excess of ten times the statutory annual limit for workers.

Non-lethal deterministic health effect (e.g., burns) from radiation.

Impact on Radiological Barriers and Control

Exposure rates of more than 1 Sv/h in an operating area.

Severe contamination in an area not expected by design, with a low probability of ­significant public exposure.

Impact on Defence-in-Depth

Near accident at a nuclear power plant with no safety provisions remaining.

Lost or stolen highly radioactive sealed source.

Misdelivered highly radioactive sealed source without adequate procedures in place to handle it.

Examples:

• THORP plant Sellafield (United Kingdom) – 2005.
• Paks Nuclear Power Plant (fuel rod damage in cleaning tank) (Hungary) – 2003.
• Vandellos Nuclear Power Plant, Spain (A fire destroyed many control systems; the reactor was shut down) – 1989.

Level 2: Incident

Impact on People and Environment

Exposure of a member of the public in excess of 10 mSv.

Exposure of a worker in excess of the statutory annual limits.

Impact on Radiological Barriers and Control

Radiation levels in an operating area of more than 50 mSv/h.

Significant contamination within the facility into an area not expected by design.

Impact on Defence-in-Depth

Significant failures in safety ­provisions but with no actual ­consequences.

Found highly radioactive sealed orphan source, device or transport package with safety provisions intact.

Inadequate packaging of a highly radioactive sealed source.

Examples:

• Ascó Nuclear Power Plant, (Catalonia, Spain) April 2008; radioactive contamination
• Forsmark Nuclear Power Plant (Sweden); backup generator failure.

Level 1: Anomaly

Impact on Defence-in-Depth

Overexposure of a member of the public in excess of statutory ­annual limits.

Minor problems with safety components with significant defence-in-depth remaining.

Low activity lost or stolen radioactive source, device or transport package.

(Arrangements for reporting minor events to the public differ from country to country. It is difficult to ensure precise consistency in rating events between INES Level-1 and Below scale/Level-0)

Examples:

• Gravelines (Nord, France), 8 August 2009; during the annual fuel bundle exchange in reactor #1, a fuel bundle snagged on to the internal structure. Operations were stopped, the reactor building was evacuated and isolated in accordance with operating procedures.
• TNPC (Drôme, France), July 2008; leak of 6,000 litres (1,300 imp gal; 1,600 US gal) of water containing 75 kilograms (170 lb) of Uranium into the environment.

Level 0: Deviation

No safety significance.

Examples:

• 4 June 2008: Krško, Slovenia: Leakage from the primary cooling circuit
• 17 December 2006, Atucha, Argentina: Reactor shutdown due to Tritium increase in reactor compartment
• 13 February 2006: Fire in Nuclear Waste Volume Reduction Facilities of the Japanese Atomic Energy Agency (JAEA) in Tokaimura

Out of Scale

There are also events of no safety relevance, characterized as "out of scale".

Examples:

• 17 November 2002, Natural Uranium Oxide Fuel Plant at the Nuclear Fuel Complex in Hyderabad, India: A chemical explosion at a fuel fabrication facility
• 4 November 1999: H.B. Robinson, United States: A Tornado sighting within the protected area of the NPP.
• 15 April 1999: San Onofre, United States: Discovery of suspicious item in nuclear power plant

Radiation poisoning was a major concern after the Nuclear power plant reactor accident. Radiation poisoning, radiation sickness or a creeping dose, is a form of damage to organ tissue caused by excessive exposure to ionizing radiation. The term is generally used to refer to acute problems caused by a large dosage of radiation in a short period, though this also has occurred with long term exposure. The clinical name for radiation sickness is acute radiation syndrome (ARS) as described by the CDC. A short exposure can result in acute radiation syndrome; chronic radiation syndrome requires a prolonged high level of exposure.

Radiation exposure can also increase the probability of developing some other diseases, mainly cancer, tumours, and genetic damage. These are referred to as the stochastic effects of radiation, and are not included in the term radiation sickness.

The use of radionuclides in science and industry is strictly regulated in most countries. In the event of an accidental or deliberate release of radioactive material, either evacuation or sheltering in place are the recommended measures.

Signs and symptoms

Radiation sickness is generally associated with acute (a single large) exposure. Nausea and vomiting are usually the main symptoms. The amount of time between exposure to radiation and the onset of the initial symptoms may be an indicator of how much radiation was absorbed, as symptoms appear sooner with higher doses of exposure. The symptoms of radiation sickness become more serious (and the chance of survival decreases) as the dosage of radiation increases. A few symptom-free days may pass between the appearance of the initial symptoms and the onset of symptoms of more severe illness associated with higher doses of radiation. Nausea and vomiting generally occur within 24–48 hours after exposure to mild (1–2 Sv) doses of radiation. Radiation damage to the intestinal tract lining will cause nausea, bloody vomiting and diarrhea. This occurs when the victim's exposure is 200 rems (1 Sv = 100 rems) or more. The radiation will begin to destroy the cells in the body that divide rapidly. These including blood, GI tract, reproductive and hair cells, and harms the DNA and RNA of surviving cells. Headache, fatigue, and weakness are also seen with mild exposure. Moderate (2–3.5 Sv of radiation) exposure is associated with nausea and vomiting beginning within 12–24 hours after exposure. In addition to the symptoms of mild exposure, fever, hair loss, infections, bloody vomit and stools, and poor wound healing are seen with moderate exposure. Nausea and vomiting occur in less than 1 hour after exposure to severe (3.5–5.5 Sv) doses of radiation, followed by diarrhea and high fever in addition to the symptoms of lower levels of exposure. Very severe (5.5–8 Sv of radiation) exposure is followed by the onset of nausea and vomiting in less than 30 minutes followed by the appearance of dizziness, disorientation, and low blood pressure in addition to the symptoms of lower levels of exposure. Severe exposure is fatal about 50% of the time. See criticality accident for a number of incidents in which humans have been accidentally exposed to such levels of radiation.

Longer term exposure to radiation, at doses less than that which produces serious radiation sickness, can induce cancer as cell-cycle genes are mutated. The probability cancer will develop is a function of radiation dose. In radiation-induced cancer the disease, the speed at which the condition advances, the prognosis, the degree of pain, and every other feature of the disease are not functions of the radiation dose to which the person is exposed.

Since tumors grow by abnormally rapid cell division, the ability of radiation to disturb cell division is also used to treat cancer (radiotherapy), and low levels of ionizing radiation have been claimed to lower one's risk of cancer.

Cutaneous radiation syndrome

The concept of cutaneous radiation syndrome (CRS) was introduced in recent years to describe the complex pathological syndrome that results from acute radiation exposure to the skin.

Acute radiation syndrome (ARS) usually will be accompanied by some skin damage. It is also possible to receive a damaging dose to the skin without symptoms of ARS, especially with acute exposures to beta radiation or X-rays. Sometimes this occurs when radioactive materials contaminate skin or clothes.

When the basal cell layer of the skin is damaged by radiation, inflammation, erythema, and dry or moist desquamation can occur. Also, hair follicles may be damaged, causing hair loss. Within a few hours after irradiation, a transient and inconsistent erythema (associated with itching) can occur. Then, a latent phase may occur and last from a few days up to several weeks, when intense reddening, blistering, and ulceration of the irradiated site are visible. In most cases, healing occurs by regenerative means; however, very large skin doses can cause permanent hair loss, damaged sebaceous and sweat glands, atrophy, fibrosis, decreased or increased skin pigmentation, and ulceration or necrosis of the exposed tissue.

Exposure levels

A gray (Gy) is a unit of radiation dose absorbed by matter. To gauge biological effects the dose is multiplied by a 'quality factor' which is dependent on the type of ionising radiation. Such measurement of biological effect is called "dose equivalent" and is measured in sievert (Sv). For electron and photon radiation (e.g. gamma), 1 Gy = 1 Sv.

The corresponding non-SI units are the rad (radiation absorbed dose; 1 rad = 0.01 Gy), and rem (roentgen equivalent mammal/man; 1 rem=0.01 Sv).

Annual limit on intake (ALI) is the derived limit for the amount of radioactive material taken into the body of an adult worker by inhalation or ingestion in a year. ALI is the intake of a given radionuclide in a year that would result in:

• a committed effective dose equivalent of 0.05 Sv (5 rems) for a "reference human body", or
• a committed dose equivalent of 0.5 Sv (50 rems) to any individual organ or tissue,

Prevention

The best prevention for radiation sickness is to minimize the dose suffered by the human, or to reduce the dose rate.

Distance

Increasing distance from the radiation source reduces the dose according to the inverse-square law for a point source. Distance can sometimes be effectively increased by means as simple as handling a source with forceps rather than fingers.

Time

The longer that humans are subjected to radiation the larger the dose will be. The advice in the nuclear war manual entitled "Nuclear War Survival Skills" published by Cresson Kearny in the U.S. was that if one needed to leave the shelter then this should be done as rapidly as possible to minimize exposure.

In chapter 12 he states that "Quickly putting or dumping wastes outside is not hazardous once fallout is no longer being deposited. For example, assume the shelter is in an area of heavy fallout and the dose rate outside is 400 R/hr enough to give a potentially fatal dose in about an hour to a person exposed in the open. If a person needs to be exposed for only 10 seconds to dump a bucket, in this 1/360th of an hour he will receive a dose of only about 1 R. Under war conditions, an additional 1-R dose is of little concern."

In peacetime, radiation workers are taught to work as quickly as possible when performing a task which exposes them to radiation. For instance, the recovery of a lost radiography source should be done as quickly as possible.

Reduction of incorporation into the human body

Potassium iodide (KI), administered orally immediately after exposure, may be used to protect the thyroid from ingested radioactive iodine in the event of an accident or attack at a nuclear power plant, or the detonation of a nuclear explosive. KI would not be effective against a dirty bomb unless the bomb happened to contain radioactive iodine, and even then it would only help to prevent thyroid cancer.

Fractionation of dose

Devair Alves Ferreira received a large dose during the Goiânia accident of 7.0 Gy. He lived, while his wife received a dose of 5.7 Gy and died. The most likely explanation is that his dose was fractionated into many smaller doses which were absorbed over a length of time, while his wife stayed in the house more and was subjected to continuous irradiation without a break, giving her body less time to repair some of the damage done by the radiation. In the same way, some of the people who worked in the basement of the wrecked Chernobyl plant received doses of 10 Gy, but in small fractions, so the acute effects were avoided.

It has been found in radiation biology experiments that if a group of cells is irradiated, then as the dose increases, the number of cells which survive decreases. It has also been found that if a population of cells is irradiated, then set aside for a length of time before being irradiated again, the radiation causes less cell death. The human body contains many types of cells and a human can be killed by the loss of a single type of cells in a vital organ. For many short term radiation deaths (3 days to 30 days), the loss of two important types of cells that are constantly being regenerated causes death. The loss of cells forming blood cells (bone marrow) and the cells in the digestive system (microvilli which form part of the wall of the intestines is fatal.

In the graph below, dose/survival curves for a hypothetical group of cells have been drawn, with and without a rest time for the cells to recover. Other than the recovery time partway through the irradiation, the cells would have been treated identically.

Treatment

Treatment reversing the effects of radiation is currently not possible. Anaesthetics and antiemetics are administered to counter the symptoms of exposure, as well as antibiotics for countering secondary infections caused by the resulting immune system deficiency.

There are also a number of substances used to mitigate the prolonged effects of radiation poisoning, by eliminating the remaining radioactive materials, post exposure.

Whole body vs. part of body exposure

In the case of a person who has had only part of their body irradiated then the treatment is easier, as the human body can tolerate very large exposures to the non-vital parts such as hands and feet, without having a global effect on the entire body. For instance, if the hands get a 100 Gy dose which results in the body receiving a dose (averaged over the entire body) of less than 1 Gy then the hands may be lost but radiation poisoning may not occur. The resulting injury would be described as localized radiation burn.

As described below, one of the primary dangers of whole-body exposure is immunodeficiency caused by the destruction of bone marrow and consequent shortage of white blood cells. It is treated by maintaining a sterile environment, bone marrow transplants (see hematopoietic stem cell transplantation), and blood transfusions.

Experimental treatments

Neumune, an androstenediol, was introduced as a radiation countermeasure by the US Armed Forces Radiobiology Research Institute, and was under joint development with Hollis-Eden Pharmaceuticals until March, 2007. Neumune is in Investigational New Drug (IND) status and Phase I trials have been performed.

Some work has been published in which Cordyceps sinensis, a Chinese herbal medicine, has been used to protect the bone marrow and digestive systems of mice from whole body irradation.

Recent lab studies conducted with bisphosphonate compounds have shown promise of mitigating radiation exposure effects.

The conditions to run the test were established prior to the day shift of 25 April 1986. The day shift workers had been instructed in advance and were familiar with procedures. A special team of electrical engineers was present to test the new voltage regulating system. As planned, on 25 April a gradual reduction in the output of the power unit begun at 01:06 a.m., and by the beginning of the day shift the power level had reached 50% of its nominal 3200 MW thermal. At this point, another regional power station unexpectedly went off-line, and the Kiev electrical grid controller requested that the further reduction of Chernobyl's output be postponed, as power was needed to satisfy the peak evening demand. The Chernobyl plant director agreed and postponed the test.

At 11:04 p.m., the Kiev grid controller allowed the reactor shut-down to resume. This delay had some serious consequences: the day shift had long since departed, the evening shift was also preparing to leave, and the night shift would not take over until midnight, well into the job. According to plan, the test should have been finalized during the day shift, and the night shift would only have had to maintain decay heat cooling systems in an otherwise shut-down plant; the night shift had very limited time to prepare for and carry out the experiment. Further rapid reduction in the power level from 50% was actually executed during the shift change-over. Alexander Akimov was chief of the night shift, and Leonid Toptunov was the operator responsible for the reactor's operational regimen, including the movement of the control rods. Toptunov was a young engineer who had worked independently as a senior engineer for approximately three months.

The test plan called for the power output of reactor 4 to be gradually reduced to 700–1000 MW thermal. The level established in the test program (700 MW) was achieved at 00:05 on April 26; however, because of the natural production in the core of a neutron absorber, xenon-135, reactor power continued to decrease, even without further operator action. And as the power reached approximately 500 MW, Toptunov committed an error, inserting the control rods too far, bringing the reactor to a near-shutdown state. The exact circumstances are hard to know, as both Akimov and Toptunov died from radiation sickness.

The reactor power dropped to 30 MW thermal (or less)—an almost completely shutdown power level that was approximately 5 percent of the minimum initial power level established as safe for the test. Control-room personnel therefore made the decision to restore the power and extracted the reactor control rods, though several minutes elapsed between their extraction and the point that the power output began to increase and subsequently stabilize at 160–200 MW (thermal). In this case the majority of control rods were withdrawn to their upper limits, but the low value of the operational reactivity margin restricted any further rise of reactor power. The rapid reduction in the power during the initial shutdown, and the subsequent operation at a level of less than 200 MW led to increased poisoning of the reactor core by the accumulation of xenon-135. This made it necessary to extract additional control rods from the reactor core in order to counteract the poisoning.

The operation of the reactor at the low power level with a small reactivity margin was accompanied by unstable core temperature and coolant flow, and possibly by instability of neutron flux. The control room received repeated emergency signals of the levels in the steam/water separator drums, of relief valves opened to relieve excess steam into a turbine condenser, of large excursions or variations in the flow rate of feed water, and from the neutron power controller. In the period between 00:35 and 00:45, it seems emergency alarm signals concerning thermal-hydraulic parameters were ignored, apparently to preserve the reactor power level. Emergency signals from the Reactor Emergency Protection System (EPS-5) triggered a trip which turned off both turbine-generators.

After a period, a more or less stable state at a power level of 200 MW was achieved, and preparation for the experiment continued. As part of the test plan, at 1:05 a.m. on 26 April extra water pumps were activated, increasing the water flow. The increased coolant flow rate through the reactor produced an increase in the inlet coolant temperature of the reactor core, which now more closely approached the nucleate boiling temperature of water, reducing the safety margin. The flow exceeded the allowed limit at 1:19 a.m. At the same time the extra water flow lowered the overall core temperature and reduced the existing steam voids in the core. Since water also absorbs neutrons (and the higher density of liquid water makes it a better absorber than steam), turning on additional pumps decreased the reactor power still further. This prompted the operators to remove the manual control rods further to maintain power.

All these actions led to an extremely unstable reactor configuration. Nearly all of the control rods were removed, which would limit the value of the safety rods when initially inserted in a scram condition. Further, the reactor coolant had reduced boiling, but had limited margin to boiling, so any power excursion would produce boiling, reducing neutron absorption by the water. The reactor was in an unstable configuration that was clearly outside the safe operating envelope established by the designers.

The Chernobyl nuclear accident that occurred on 26 April 1986, at the Chernobyl Nuclear Power Plant in Ukraine (the Ukrainian Soviet Socialist Republic then, part of the Soviet Union). It is considered the worst nuclear power plant accident in history and is the only level 7 event on the International Nuclear Event Scale.

The Chernobyl nuclear accident began on 26 April 1986, at reactor number four at the Chernobyl plant, near the town of Pripyat in the Ukrainian Soviet Socialist Republic, during a systems test. A sudden power output surge took place, and when an attempt was made for emergency shutdown, a more extreme spike in power output occurred which led to a reactor vessel rupture and a series of explosions. This event exposed the graphite moderator components of the reactor to air and they ignited; the resulting fire sent a plume of radioactive fallout into the atmosphere and over an extensive geographical area, including Pripyat. The plume drifted over large parts of the western Soviet Union. Large areas in Ukraine, Belarus, and Russia had to be evacuated, with over 336,000 people resettled. According to official post-Soviet data, about 60% of the fallout landed in Belarus.

Despite the accident, Ukraine continued to operate the remaining reactors at Chernobyl for many years. The last reactor at the site was closed down in 2000, 14 years after the accident.

The accident raised concerns about the safety of the Soviet nuclear power industry as well as nuclear power in general, slowing its expansion for a number of years while forcing the Soviet government to become less secretive about its procedures.

Russia, Ukraine, and Belarus have been burdened with the continuing and substantial decontamination and health care costs of the Chernobyl accident. Fifty deaths, all among the reactor staff and emergency workers, are directly attributed to the accident. It is estimated that there may ultimately be a total of 4,000 deaths attributable to the accident, due to increased cancer risk.

On 26 April 1986, at 01:23 a.m. (UTC+3), reactor four suffered a catastrophic power increase, leading to explosions in the core. This dispersed large quantities of radioactive fuel and core materials into the atmosphere and ignited the combustible graphite moderator. The burning graphite moderator increased the emission of radioactive particles, carried by the smoke, as the reactor had not been contained by any kind of hard containment vessel (unlike all Western plants). The accident occurred during an experiment scheduled to test a potential safety emergency core cooling feature, which took place during the normal shutdown procedure.

Nuclear power reactors require cooling, typically provided by coolant flow, to remove decay heat, even when not actively generating power. Pressurized Water Reactors use water flow at high pressure to move waste heat. Once the reactor is scrammed, the core still generates a significant amount of residual heat, which is initially about seven percent of the total thermal output of the plant. If not removed by coolant systems, the heat could lead to core damage.

Following an emergency shutdown (scram), reactor cooling is still required to keep the temperature in the reactor core low enough to avoid fuel damage. The reactor consisted of about 1,600 individual fuel channels, and each operational channel required a flow of 28 metric tons (28,000 liters (7,400 USgal)) of water per hour. There had been concerns that in the event of a power grid failure, external power would not have been immediately available to run the plant's cooling water pumps. Chernobyl's reactors had three backup diesel generators. Each generator required 15 seconds to start up but took 60–75 seconds to attain full speed and reach the capacity of 5.5 MW required to run one main cooling water pump.

This one-minute power gap was considered unacceptable, and it had been suggested that the mechanical energy (rotational momentum) of the steam turbine could be used to generate electricity to run the main cooling water pumps while the turbine was still spinning down. In theory, analyses indicated that this residual momentum had the potential to provide power for 45 seconds, which would bridge the power gap between the onset of the external power failure and the full availability of electric power from the emergency diesel generators. This capability still needed to be confirmed experimentally, and previous tests had ended unsuccessfully. An initial test carried out in 1982 showed that the excitation voltage of the turbine-generator was insufficient; it did not maintain the desired magnetic field during the spin-down. The system was modified, and in 1984 the test was repeated, but again proved unsuccessful. In 1985 the tests were attempted a third time, but also yielded negative results. The test procedure was to be repeated again in 1986, and scheduled to take place during the maintenance shutdown of Reactor Four.

The test focused on the switching sequences of the electrical supplies for the reactor. Since the test procedure was to begin when the reactor was scrammed automatically at the very beginning of the experiment, it was not anticipated to have any detrimental effect on the safety of the reactor; so the test program was not formally coordinated with either the chief designer of the reactor (NIKIET) or the scientific manager. Instead, it was approved only by the director of the plant (and even this approval was not consistent with established procedures). According to the test parameters, at the start of the experiment the thermal output of the reactor should have been no lower than 700 MW. If test conditions had been as planned the procedure would almost certainly have been carried out safely; the eventual disaster resulted from attempts to boost the reactor output once the experiment had been started, which was inconsistent with approved procedure.

The Chernobyl power plant had been in operation for two years without the capability to ride through the first 60–75 seconds of a total loss of electric power—an important safety feature. The station managers presumably wished to correct this at the first opportunity; which may explain why they continued the test even when serious problems arose, and why the requisite approval for the test was not sought from the Soviet nuclear oversight regulator (even though there was a representative at the complex of 4 reactors).

The experimental procedure was intended to run as follows:

• the reactor was to be running at a low power, between >700 MW & 800 MW
• the steam turbine was to be run up to full speed
• when these conditions were achieved, the steam supply was to be closed off
• the turbines would be allowed to freewheel down
• generator performance was to be recorded to determine whether it could provide the bridging power for coolant pumps

Chernobyl Conditions prior to the Accident
Chernobyl Experiment and Explosion
Radiation Levels of Chernobyl Nuclear Accident
Causes of Chernobyl Nuclear Accident
Chernobyl Nuclear Accident Effects
Chernobyl Recovery Process