Table of Contents
Three Mile Island
The Three Mile Island (TMI) Nuclear Power Plant is located in Pennsylvania, USA. The site was formerly made up of two units. Unit 2 has been shut down since 1979, following an accident. However, Unit 1 continued to generate electricity until 2019.
Both units were Pressurised Water Reactors (PWRs), a type of nuclear reactor that relies on water to moderate the rate of fission and keep the reactor core cool.
Three Mile Island Unit 1 (TMI-1) was commissioned on 2 September 1974. The reactor remained one of the most productive across the USA until it finally entered decommissioning in 2019, following 45 years of electricity generation.
Three Mile Island Unit 2 (TMI-2) was commissioned on 30 December 1978. It was permanently shut down following an accident in 1979.
The Accident
On the night of 28th March 1979, a malfunction in the cooling circuit caused the temperature in the primary reactor vessel to rise. Whilst the engineered controls automatically responded and shut down the reactor, the relief valve failed to close. This resulted in the leak and drainage of coolant from the primary reactor vessel and the unrestricted rise of temperature due to decay heat from the reactor core. The incident was classified at Level 5 on the International Nuclear Event Scale (INES).
A combination of
unpreparedness for
emergencies, and a lack of
adequate training meant that
nobody could understand the
cause for the irregularities in
the temperature and pressure
of the reactor.
The reactor core reached such
high temperatures that the
fuel rods were irreversibly
damaged and the reactor
components, including the
fuel and the protective
cladding suffered a partial
meltdown.
Radioactive material was also
released into the cooling
water; however, the leak of
contaminated water was only
detected at 6:45 am by which
point radiation levels in the
coolant water were 300 times
greater than normal levels.
- Simplified interactive representation of the TMI-2 reactor at Three Mile Island. Courtesy of World Nuclear Association (WNA).
The accident led to concerns, paranoia and psychological stress surrounding the nuclear energy industry. This arose from a lack of communications combined with mixed media reports regarding what happened. Locally, this caused confusion in Pennsylvania.
This was at a time when new nuclear technologies were being portrayed with immense scepticism in the media (in the form of films, amongst other means) in the shadow of the Cold War nuclear arms race. Consequently, the events at TMI-2 sparked a rise in anti-nuclear demonstrations across the United States, with attention and support from public figures, musicians and politicians.
‘The China Syndrome’ was released in 1979, just prior to the event at TMI-2, about an accident at a nuclear plant. The film broke blockbuster records and fed into the negative public perception of the nuclear industry at the time.
The World Nuclear Association concluded that there were no direct casualties due to this incident in the local area or across the population.
It was estimated that 2 million people may have been exposed to radiation from the accident. However, the Pennsylvania Department of Health monitored the health of the 30,000 residents within 5 miles of the accident for approximately 20 years, and no major health abnormalities were detected. Nonetheless, $25 million was paid in insurance to the population. The New York Times concluded in 1993, after the clean-up had been completed, that 2.3 million gallons of wastewater had been removed from the site.
The TMI-2 unit was shut down and has not been operational since 1979. The reactor core has since been removed and the unit is being decommissioned.
The Institute of Nuclear Power Operators
The US Nuclear Power Industry set up the Institute of Nuclear Power Operations (INPO) in 1979, following investigations into the incident at TMI-2. The aim was to incorporate learnings from the event and promote excellence across the fleet of US Nuclear Power Plants. Formation of the institute was complemented by the establishment of its National Academy for Nuclear Training in 1985.
With a mission to set industry-wide performance objectives and promote standards of best practice in the industry, INPO continues to be funded by the US nuclear industry to this day. Its work includes encouragement and facilitation of sharing of operational experience, to the benefit of performance and safety.
Learning from Experience
Since 1979, learnings and takeaways from the events at TMI-2 have helped to ensure that such an incident will not be repeated:
- Increased Regulation: INPO has significantly contributed to the nuclear industry’s self-regulation within the United States.
- Emphasis on Training and Awareness: INPO provide guidance, facilitate lessons being learned and set standards for the skills necessary in the industry; this helps employees and their attitudes to be safer.
- Safety Culture: Accountability, escalation of questions and a continuous commitment to safer attitudes are embedded in the industry.
- Emergency Controls: Now continually reviewed and monitored, with contingency plans rehearsed across all nuclear industries globally.
To find out more about the key events that shaped the development of the nuclear industry in the UK and across the globe, why not check out our dedicated page?
Arushi Sharma
