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.