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It began at 4 a. It is the most significant accident in U. The accident began with failures in the non-nuclear secondary system [7] followed by a stuck-open pilot-operated relief valve PORV in the primary system [8] that 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 LOCA. TMI training and procedures left operators and management ill-prepared for the deteriorating situation.

During the event these inadequacies were compounded by design flaws, including inconveniently arranged instruments and controls , the use of multiple similar alarms, and a failure of the equipment to clearly indicate coolant inventory level or the position of the stuck-open PORV. The accident crystallized anti-nuclear safety concerns among activists and the general public, and resulted in new regulations for the nuclear industry. It has been cited as a contributor to the decline of a new reactor construction program, a slowdown that was already underway in the s.

Anti-nuclear movement activists expressed worries about regional health effects from the accident. However, due to their nature, such studies cannot conclusively establish whether or not there was a causal connection linking the accident with these cancers.

The initial cause of the accident happened 11 hours earlier, during an attempt by operators to fix a blockage in one of the eight condensate polishers , the sophisticated filters cleaning the secondary loop water. These filters are designed to stop minerals and impurities in the water from accumulating in the steam generators and to decrease corrosion rates on the secondary side.

Blockages are common with these resin filters and are usually fixed easily, but in this case, the usual method of forcing the stuck resin out with compressed air did not succeed.

The operators decided to blow the compressed air into the water and let the force of the water clear the resin. When they forced the resin out, a small amount of water forced its way past a stuck-open check valve and found its way into an instrument air line.

This would eventually cause the feedwater pumps , condensate booster pumps, and condensate pumps to turn off around a. Given that the steam generators were no longer receiving feedwater, heat transfer from the reactor coolant system [23] RCS was greatly reduced, and RCS temperature rose. The rapidly heating coolant expanded and surged into the pressurizer, [24] [25] [26] compressing the steam bubble at the top. When RCS pressure rose to 2, psi RCS pressure continued to rise, reaching the reactor protection system RPS high-pressure trip setpoint of 2, psi The reactor automatically tripped , its control rods falling into the core under gravity, halting the nuclear chain reaction and stopping the heat generated by fission.

Because steam was no longer being used by the turbine and feed was not being supplied to the steam generators, heat removal from the reactor's primary water loop was limited to steaming the small amount of water remaining in the secondary side of the steam generators to the condenser using turbine bypass valves.

When the feedwater pumps tripped, three emergency feedwater pumps started automatically. An operator noted that the pumps were running, but did not notice that a block valve was closed in each of the two emergency feedwater lines, blocking emergency feed flow to both steam generators.

The valve position lights for one block valve were covered by a yellow maintenance tag. The reason why the operator missed the lights for the second valve is not known, although one theory is that his own large belly hid it from his view.

The closure of these valves was a violation of a key Nuclear Regulatory Commission NRC rule, according to which the reactor must be shut down if all auxiliary feed pumps are closed for maintenance. This was later singled out by NRC officials as a key failure. After the reactor tripped, secondary system steam valves operated to reduce steam generator temperature and pressure, cooling the RCS and lowering RCS temperature, as designed, resulting in a contraction of the primary coolant.

With the coolant contraction and loss of coolant through the open PORV, RCS pressure dropped as did pressurizer level after peaking fifteen seconds after the turbine trip. Also, fifteen seconds after the turbine trip, coolant pressure had dropped to 2, psi Electric power to the PORV's solenoid was automatically cut, but the relief valve was stuck open with coolant water continuing to be released.

In post-accident investigations, the indication for the PORV was one of many design flaws identified in the operators' controls, instruments and alarms. As a result, they did not correctly diagnose the problem for several hours. The operators had not been trained to understand the ambiguous nature of the pilot-operated relief valve indicator and to look for alternative confirmation that the main relief valve was closed. A downstream temperature indicator, the sensor for which was located in the tail pipe between the pilot-operated relief valve and the pressurizer relief tank, could have hinted at a stuck valve had operators noticed its higher-than-normal reading.

It was not, however, part of the "safety grade" suite of indicators designed to be used after an incident, and personnel had not been trained to use it. Its location behind the seven-foot-high instrument panel also meant that it was effectively out of sight.

Less than a minute after the beginning of the event, the water level in the pressurizer began to rise, even though RCS pressure was falling.

The operators' training and plant procedures did not cover a situation where the two parameters went in opposite directions. The water level in the pressurizer was rising because the steam in the space at the top of the pressurizer was being vented off through the stuck-open PORV, lowering the pressure in the pressurizer because of the lost inventory.

The lowering of pressure in the pressurizer made water from the coolant loop surge in and created a steam bubble in the reactor pressure vessel head, aided by the decay heat from the fuel. Indications of high water levels in the pressurizer contributed to confusion, as operators were concerned about the primary loop "going solid", i. This confusion was a key contributor to the initial failure to recognize the accident as a LOCA [43] and led operators to turn off the emergency core cooling pumps, which had automatically started after the pilot-operated relief valve stuck and core coolant loss began, due to fears the system was being overfilled.

With the pilot-operated relief valve still open, the pressurizer relief tank that collected the discharge from the pilot-operated relief valve overfilled, causing the containment building sump to fill and sound an alarm at a. This alarm, along with higher than normal temperatures on the pilot-operated relief valve 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.

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 a. At about a. 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.

Soon after a. This reaction melted the nuclear fuel rod cladding and damaged the fuel pellets, which released radioactive isotopes 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 a. Thornburgh and Lieutenant Governor William Scranton III , to whom Thornburgh assigned responsibility for collecting and reporting on information about the accident.

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 These were contradicted by another official, and by statements from Met Ed, who both claimed that no radioactivity had been released. 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.

NRC chairman Joseph Hendrie and commissioner Victor Gilinsky [57] initially viewed the accident as a "cause for concern but not alarm". 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 did not have the authority either to tell the utility what to do or to order an evacuation of the local area.

In a article, Gilinsky wrote that it took five weeks to learn that "the reactor operators had measured fuel temperatures near the melting point". It was still not clear to the control room staff that the primary loop water levels were low and that over half of 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 [ clarification needed ] 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 building leading to a large-scale release of radioactive material. 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 release occurred when the cladding was damaged while the pilot-operated relief valve was still stuck open. Fission products were released into the reactor coolant. The auxiliary building was outside the containment boundary. This was evidenced 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. According to the Rogovin report, the vast majority of the radioisotopes released were noble gases xenon and krypton resulting in an average dose of 1.

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.

Continuous monitoring at 11 stations was not established until April 1, and was expanded to 31 stations on April 3. 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, but measures of beta radiation were not included, because 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.

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. According to the official figures, as compiled by the Kemeny Commission from Metropolitan Edison and NRC data, a maximum of PBq 13 MCi of radioactive noble gases primarily xenon were released by the event.

Anti-nuclear political groups disputed the Kemeny Commission's findings, claiming that other independent measurements provided evidence of radiation levels up to seven times higher than normal in locations hundreds of miles downwind from TMI. Gundersen offers evidence, based on pressure monitoring data, for a hydrogen explosion shortly before p. 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 claimed that the control room shook and doors were blown off hinges. However, official NRC reports refer merely to a "hydrogen burn". Twenty-eight hours after the accident began, William Scranton III , the lieutenant governor , appeared at a news briefing to say that Metropolitan Edison, the plant's owner, had assured the state that "everything is under control".

Farmers were told to keep their animals under cover and on stored feed. Governor Dick Thornburgh , on the advice of NRC chairman Joseph Hendrie, advised the evacuation "of pregnant women and pre-school age children The evacuation zone was extended to a mile radius on Friday, March Several state and federal government agencies mounted investigations into the crisis, the most prominent of which was the President's Commission on the Accident at Three Mile Island , created by Jimmy Carter in April Kemeny , president of Dartmouth College.

It was instructed to produce a final report within six months, and after public hearings, depositions, and document collection, released a completed study on October 31, Although Babcock engineers recognized the problem, the company failed to clearly notify its customers of the valve issue.

The Pennsylvania House of Representatives conducted its own investigation, which focused on the need to improve evacuation procedures. In , a television camera was used to see the interior of the damaged reactor.

In , core samples and samples of debris were obtained from the corium layers on the bottom of the reactor vessel and analyzed. However, following the event, the number of reactors under construction in the U.