The Three Mile Island Unit 2 reactor, near Middletown, Pa., partially melted down on March 28, 1979. This was the most serious accident in U.S. commercial nuclear power plant operating history.

What Happened?

• The plant experienced a failure in the secondary, non-nuclear section of the plant (one of two reactors on the site).
• Either a mechanical or electrical failure prevented the main feedwater pumps from sending water to the steam generators that remove heat from the reactor core.
• This caused the plant’s turbine generator and then the reactor itself to automatically shut down. Immediately, the pressure in the primary system began to increase.
• To control that pressure, the pilot-operated relief valve opened. The valve should have closed when the pressure fell to proper levels, but it became stuck open.
• Other instruments available to plant staff provided inadequate or misleading information. As a result, plant staff assumed that instruments showed water level was high enough and the core was properly covered with water too. That wasn’t the case.
• Unaware of the stuck-open relief valve and unable to tell if the core was covered with cooling water, the staff took a series of actions that uncovered the core. 

Analysing the Root Cause:

1. Failure of automatic control valve
2. Equipment and instrument malfunctions
3. Human errors in operating procedures

Impact:

1. The TMI-2 reactor was destroyed.
2. Radiation-induced health effects in the area surrounding the plant.

What could have been done to avoid this?

1. Adopting preventive maintenance to eliminate any potential machinery defects which could lead to failure.
2. Deploy a condition monitoring system to gain insight into the health of key assets
3. Carry out safety inspections periodically

What should be done to avoid such incidents in the future:

Have a third party professional electrical installation safety testing and inspection service done for your establishment. Include Electrical Safety Audit and EICR Certification to evaluate electrical wiring, connections, and systems for potential electrical hazards, design, and structural flaws. 

Provide comprehensive safety training to all operators and electricians.

Practise workplace’s safety culture and develop strategies to increase their safety levels

Every time an Electrical Contractor completes a new electrical installation, extension, or modification to an existing installation he/she is obliged to test and certify that the installation complies with current standards.

Pictures and References:

https://www.nrc.gov/reading-rm/doc-collections/fact-sheets/3mile-isle.html#summary

https://www.britannica.com/event/Three-Mile-Island-accident

http://content.time.com/time/subscriber/article/0,33009,920196,00.html