Three Mile Island was a PWR (Pressurized Water Reactor) accident that happened 28 March 1979 with the reactor operating a nearly full power. A pump to the steam generators stopped working and the backup pump failed also. A series of blunders included an essential signal lamp being hid by a maintenance tag, and various valves stuck open leaking coolant from the reactor core. Hot uranium fuel was exposed, over-heated and crumbled, releasing explosive hydrogen gas and radioactive fission products. Luckily the ensuing explosion did not burst the concrete containment. The only radioactive substances to escape into the environment were some 17 curies of radio-iodine and 13 million curies of inert gases such as argon and krypton.

The initial concern of the scientists and technicians was to vent molten pieces of the core from bursting the reactor pressure-vessel and destroying the containment. By November 1979, the base of the containment was flooded to a depth of 6 feet, and highly radioactive water was still leaking in at a rate of 1,000 gallons a day, threatening vital equipment.

In June 1980 the United States Nuclear Regulatory Commission decided that krypton gas still trapped in the building should be vented into the atmosphere. The decision led to vehement protests from the population in the surrounding area, but the NRC justified it on the grounds that even greater risks would arise unless their staff were able to gain access.

A month later they began to release the krypton through a tall chimney - some 2,000 people opted to leave the area while the operation was in progress - and on 23 July two engineers clad in protective clothing and using special respirators briefly entered the containment building.

In the following months, experts charged with the task of cleaning up the site gradually began to realize the full magnitude of the challenge that faced them. In August 1979, they believed they would have the reactor back in business by the middle of 1983. The cost of the entire job was estimated to be $400 million.

In December 1980, however, they announced that the clean up could not be completed before mid-1985 and that it would cost at least $1,000 million. The task of removing the highly radioactive debris proved exceedingly difficult and was still only one-fifth completed by the end of 1986, and new problems are still emerging... with 2,500 workers, payroll alone was running $40 million per year... and the estimated number of man hours required to complete the job has since been revised upwards by a factor of six.

By the end of 1987, $700 million had already been spent.

Side note: the Browns Ferry Reactor accident in Alabama was caused by a fire in a cable channel started by workmen searching for air leaks with a candle. It destroyed the control systems of two twin reactors and could have led to a common mode failure. The Three Mile Island accident also involved common mode failure.