Three Mile Island Historical Marker in Middletown, Pennsylvania

Early on the morning of March 28, 1979, a series of failures at the Three Mile Island nuclear power plant led to a partial meltdown that released radioactive gases and radioactive iodine into the atmosphere. The accident began with failures in the non-nuclear secondary system, followed by a pilot-operated relief valve that was stuck in the open position, which resulted in large amounts of water escaping from the pressurized isolated coolant loop. Plant operators failed to recognize the situation as a loss-of-coolant accident, which was later attributed to inadequate operating procedures that left them ill-equipped to respond to the potentially catastrophic situation that unfolded in TMI Unit 2. Design flaws, such as poor control design, the use of multiple similar alarms, and a failure of the equipment to indicate either the coolant-inventory level or the position of the stuck-open PORV, were also discovered during follow-up investigations. While official reports indicate that incident did not cause any serious health concerns, cleanup of the incident took over 14 years and cost roughly $1 billion. TMI Unit 2 was shut down following the accident. TMI Unit 1 was shut down in September 2019, but as of September 2024, a 20-year deal was signed by Constellation Energy with the goal of providing power to support Microsoft’s data centers by 2028.

Jun 5, 2025 - 07:00

Three Mile Island Historical Marker in Middletown, Pennsylvania

Three Mile Island nuclear power plant.

Early on the morning of March 28, 1979, a series of failures at the Three Mile Island nuclear power plant led to a partial meltdown that released radioactive gases and radioactive iodine into the atmosphere.

The accident began with failures in the non-nuclear secondary system, followed by a pilot-operated relief valve that was stuck in the open position, which resulted in large amounts of water escaping from the pressurized isolated coolant loop.

Plant operators failed to recognize the situation as a loss-of-coolant accident, which was later attributed to inadequate operating procedures that left them ill-equipped to respond to the potentially catastrophic situation that unfolded in TMI Unit 2.

Design flaws, such as poor control design, the use of multiple similar alarms, and a failure of the equipment to indicate either the coolant-inventory level or the position of the stuck-open PORV, were also discovered during follow-up investigations. While official reports indicate that incident did not cause any serious health concerns, cleanup of the incident took over 14 years and cost roughly $1 billion.

TMI Unit 2 was shut down following the accident. TMI Unit 1 was shut down in September 2019, but as of September 2024, a 20-year deal was signed by Constellation Energy with the goal of providing power to support Microsoft’s data centers by 2028.