[23] 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.

When RCS pressure rose to 2,255 psi (155.5 bar), the pilot-operated relief valve (PORV) opened, relieving steam through piping to the reactor coolant drain tank[28] in the containment building basement.

[54] 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 radioactivity releases.

[56] Readings from instruments at the plant and off-site detectors had detected radioactivity 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.

[57] 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.

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.

[62] 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 8:00 a.m., roughly half of the uranium fuel had already melted.

A hydrogen explosion could breach the pressure vessel and, depending on its magnitude, might compromise the integrity of the containment building leading to a large-scale release of radioactive material.

According to the Rogovin report, the vast majority of the radioisotopes released were noble gases xenon and krypton resulting in an average dose of 1.4 mrem (14 μSv) to the two million people near the plant.

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.

[80] The commission consisted of a panel of 12 people, specifically chosen for their lack of strong pro- or anti-nuclear views, and headed by chairman John G. Kemeny, president of Dartmouth College.

[81] According to the official figures, as compiled by the 1979 Kemeny Commission from Met Ed and NRC data, a maximum of 480 PBq (13 MCi) of radioactive noble gases, primarily xenon, were released by the event.

[85] 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.

[88][verification needed][89] Gundersen offers evidence, based on pressure monitoring data, for a hydrogen explosion shortly before 2:00 p.m. on March 28, 1979, which would have provided the means for a high dose of radiation to occur.

"[91] Work performed for the Department of Energy in the 1980s determined that the hydrogen burn (deflagration), which went essentially unnoticed for the first few days, occurred 9 hours and 50 minutes after initiation of the accident, had a duration of 12 to 15 seconds and did not involve a detonation.

[92][93] The investigation strongly criticized Babcock & Wilcox, Met Ed, General Public Utilities, and the NRC for lapses in quality assurance and maintenance, inadequate operator training, lack of communication of important safety information, poor management, and complacency, but avoided drawing conclusions about the future of the nuclear industry.

[98] On Wednesday, March 28, hours after the accident began, Lieutenant Governor Scranton appeared at a news briefing to say that Met Ed had assured the state that "everything is under control".

[99][100] Based on the advice of the Chairman of the NRC and in the interest of taking every precaution, I am advising those who may be particularly susceptible to the effects of any radiation, that is, pregnant women and pre-school aged children, to leave the area within a five-mile radius of the Three Mile Island facility until further notice.

We've also ordered the closings of any schools within this area.Governor Thornburgh, on the advice of NRC chairman Joseph Hendrie, advised the evacuation "of pregnant women and pre-school age children...within a five-mile radius of the Three Mile Island facility".

Additionally, as a result of the earlier 1973 oil crisis and post-crisis analysis with conclusions of potential overcapacity in base load, 40 planned nuclear power plants already had been canceled before the accident.

During the lengthy review process, complicated by the Chernobyl disaster seven years later, federal requirements to correct safety issues and design deficiencies became more stringent, local opposition became more strident, construction times were significantly lengthened and costs skyrocketed.

The first major phase of the cleanup was completed in 1990, when workers finished shipping 150 short tons (140 t) of radioactive wreckage to Idaho for storage at the Department of Energy's National Engineering Laboratory.

"[113] According to health researcher Joseph Mangano, early scientific publications estimated no additional cancer deaths in the 10 mi (16 km) area around TMI, based on these numbers.

[117] The Radiation and Public Health Project, an organization with little credibility among epidemiologists,[118] cited calculations by Mangano that showed a spike in infant mortality in downwind communities two years after the accident.

[86] John Gofman used his own, non-peer reviewed low-level radiation health model to predict 333 excess cancer or leukemia deaths from the 1979 Three Mile Island accident.

[11] The ongoing TMI epidemiological research has been accompanied by a discussion of problems in dose estimates due to a lack of accurate data, as well as illness classifications.

In the previous May, an estimated 65,000 people—including California Governor Jerry Brown—attended a march and rally against nuclear power in Washington, D.C.[125] In 1981, citizens' groups succeeded in a class action suit against TMI, winning $25 million in an out-of-court settlement.

[128] Under a plea-bargaining agreement, Met Ed pleaded guilty to one count of falsifying records and no contest to six other charges, four of which were dropped, and agreed to pay a $45,000 fine and set up a $1 million account to help with emergency planning in the area surrounding the plant.

[129] According to Eric Epstein, chair of Three Mile Island Alert, the TMI plant operator and its insurers paid at least $82 million in publicly documented compensation to residents for "loss of business revenue, evacuation expenses and health claims.

"[130] However, a class action lawsuit alleging that the accident caused detrimental health effects was rejected by Harrisburg United States district court Judge Sylvia Rambo, and the final appeal of that decision failed in 2002.