What was the Three Mile Island Accident of 1979?

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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.

What happened at Three Mile Island Accident?

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.

How did US Environmental Protection Agency respond to the Three Mile Island Accident?

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.

What was the impact of Three Mile Island Accident?

• Impact on the health conditions of local residents: Noble gases got released into the atmosphere. Local population received some radiation dose. Experts from the Three Mile Island Litigation Team calculated and found radiation range within 25 to 50 mrem. Researchers did not find any radioactive iodine in food and milk samples. The Three Mile Island Accident increased the risk of cancer, but at most, maybe one inhabitant may get a fatal cancer.
• Impact on the regulatory norms: After the accident, the Nuclear Regulatory Commission (NRC) did not approve any application for building new nuclear power station in U.S.A. NRC ensured regular testing of existing plants' equipments (pressure relief valves, piping systems, fire protection, auxiliary feedwater systems, and electrical circuit breakers) several times in a year. Federal Emergency Management Agency and NRC participated in emergency preparedness drills along with state and local agencies. NRC managers analysed plant performance for any additional regulatory attention. In the year 1977, NRC’s resident inspector program underwent 360⁰ degree change. According to the new norms, two inspectors always have to live nearby a nuclear plant and work exclusively. Risk assessment protocols were exclusively designed and implemented to identify vulnerabilities of any nuclear plant The Institute of Nuclear Power Operations (INPO) and the Nuclear Energy Institute was exclusively established to provide a unified industry approach to generic nuclear regulatory issues. These institute carried out their functions by interacting with NRC and other government agencies. NRC expanded its international liason to share its nuclear safety and technical knowledge with other nations.

What will prevent another Three Mile Island Accident?

The Nuclear Regulatory Commission along with federal agencies implemented certain changes or steps to prevent another Three Mile Island Accident in future.

• Equipment requirements and plant design were specified.
• Proper training was given to operators to control and monitor the activities inside the plant.
• Fitness-for-duty or emergency drills was made mandatory for plant workers to prevent alcohol or drug abuse.
• Information on plant performance is being released regularly for public awareness.
• Regulatory inspection and enforcement has been made mandatory.

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