Three Mile Island is a nuclear power generating station located in south Harrisburg, Pennsylvania. There were two separate plants, TMI-1 and TMI-2. The plant attracted attention after a major accident on March 28th, 1979. Series of electrical, mechanical and human failures lead to the meltdown of the reactor core of TMI-2. The reactor core had been removed but the site has not been decommissioned yet.
On March 28th 1979, the accident began at 4 AM and the calamity lasted for three days. In the turbine loop, the feed pump stopped working for some unknown cause. This pump was used to remove heat from the steam generator. As a result of this, the control rods dropped automatically to stop the nuclear fission process in the reactor of TMI-2. However, it continued to produce heat which raised the temperature and pressure. The Pilot Operated Relief Valve (PORV) got activated to reduce the pressure. However, it did not close when the pressure dropped to a prescribed value. The heated water flowed to a drain tank. The emergency feed pump was supposed to operate after the failure of the feed pump. This pump was tested and found to be functional. The workers were expected to close a valve and open the valve again after the test. But they forgot to open the valve after the test. As a result, the reactor lost water and the pressure dropped. Water in the reactor turned into steam as pressure dropped. This forced water entered into the Pressuriser and steam surrounded the reactor fuel. The fuel pellets got heated as the steam failed to conduct away heat like water. Due to high temperature, the fuel pellets melted. The zirconium alloy that held the fuel pellets, reacted with the water to produce hydrogen gas. There were also indications of hydrogen burn on the same day. From 1st April 1979, operators were successful in removing the hydrogen gas by opening the valve periodically. There was no chance of a hydrogen explosion due to lack of oxygen. Later, it was found that 60% of the reactor core had melted.
On 28th March 1979, EPA stationed skilled operators with equipments to monitor radiation to assess the risk of radiation exposure among people living around the plant. They continued their work for 8 years and operated radiation monitoring network continuously to ensure safety of people and the environment. In 1989, they transferred their responsibility to Commonwealth of Pennsylvania.
The Nuclear Regulatory Commission along with federal agencies implemented certain changes or steps to prevent another Three Mile Island Accident in future.
For more information on related topics, click on the links below: