Three Mile Island is an 814 acre island on the Susquehanna River near Harrisburg, Pennsylvania. The name is most commonly associated with an accident at the Three Mile Island Nuclear Generating Station on March 28, 1979 when reactor TMI-2 suffered a partial core meltdown.
The plant was run by Metropolitan Edison Company. The two reactors on site where both pressurized light-water reactors built by Babcock & Wilcox, TMI-1 was a 786 MW unit and TMI-2 was capable of generating 900 MW.
The accident was not serious, no injuries due to radition occured, but it held the potential to have been a disaster and started a serious decline in the public popularity of nuclear power.
The Accident
The accident was started at 4 a.m. when pumps running the secondary loop of the reactor's heat transfer system shutdown due to a malfunction. Backup pumps came into immediate operation but they were not connected to the system. The turbine was tripped and the natural build up of pressure in the system led to the automatic venting of steam through a release valve into a quench tank.
The release valve then remained open despite indicators showing it as closed, and so coolant continued to be passed out of the reactor. The makeup system and then the emergency injection water system soon became active and began to increase coolant flow into the reactor. In the control room a faulty water level meter reported the water level as rising and the emergency water was cut off, in reality the water level was falling. After around eight minutes the control room noticed the backup pumps for the secondary loop were not connected and rectified that problem, but water loss from the primary loop continued.
After almost eighty minutes of slow temperature rise the primary loop pumps begin to judder 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. But steam in the system locked the primary loop and as the water stopped circulating it was converted to steam in increasing amounts, after around 130 minutes since the first malfunction the top of the reactor core was exposed and the heat and steam drove a reation evolving hydrogen and other radioactive gases from reaction with the zirconium rod clading, the quench tank ruptured and radioactive coolant began to leak out into the general containment building. At 6 a.m. there was a shift change in the control room, a new arrival noticed that the temperature in the holding tanks was excessive and used a backup valve to shut off the coolant venting. Around 250,000 gallons of coolant had already been lost 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 which time the radiation levels in the primary coolant water were around 300 times expected levels.
It was still not clear to the control room 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 thermocouiples and obtained a sample of primary loop water. Around seven hours into the emergency new water was pumped into the primary loop, the pressure in the system was so high that the backup release valve was opened to reduce pressure. At around nine hours the hydrogen evolved within the reactor exploded, but this was largely un-noticed. After almost sixteen hours the primary loop pumps were turned back on and the core temperature began to fall. A large part of the core had either melted or vapourized and the system was still dangerously radioactive. Over the next week the steam and hydrogen were removed from the reator using a recombiner and also, more controversially, by venting straight to the atmosphere. It is estimated that 2.5 million curies of radioactive gas was released by the accident.
The reactor cleanup started in August 1979 and officially ended in December 1993 at a cost of around $975 million. From 1985 to 1990 almost 100 tonnes of radioactive fuel were removed from the site. TMI-1 was restarted in 1985.