Nuclear Accidents

Three Mile Island - March 28, 1979

What happened?

On March 28, 1979, following equipment malfunction, design problems, and worker errors, Three Mile Island’s second reactor suffered a partial core meltdown, releasing a small amount of radioactive gas into the atmosphere.

At 4 a.m. the main coolant water pumps suffered a mechanical/electrical failure, preventing water from being sent to the steam generators that cool the reactor. In the absence of coolant water, the plant’s turbine generator and reactor are automatically triggered to shut down. 

As a result, the pressure began to rise in the reactor vessel. To control that pressure, a relief valve opened. While the valve should have closed when pressure reached proper levels, it malfunctioned and stayed open. 

Instruments in the control room falsely indicated that the valve was closed. As such, plant personnel were unaware of the additional loss of coolant water in the form of steam, still being released out of the open valve. Even when the alarms rang, operators failed to realize they were experiencing a loss of coolant accident. Throughout the chain of events, operators were fed false information because of faulty instruments and poor control room design. Due to this, they were unaware of the widespread systematic failures. 

The plant relied on pressure readings to gauge coolant water level in the reactor. Operators were trained that the pressurizer water level is the only sure indication of the amount of coolant water in the system. So without a water level measuring instrument in the vessel and normal pressurizer water level readings, staff assumed there was sufficient coolant water - this was not the case.

By design, emergency water was pumped into the system. But coolant water was building up in the pressurizer and escaping through the relief valve as steam. To relieve the pressure, operators took actions like removing the emergency coolant water that later resulted in uncovering the core, leaving the reactor without enough cooling water, leading it to overheat. 

In other words, they starved the reactor core of water because they were afraid of the pressurizer exploding.

At 6:22 in the morning, operators closed a valve between the relief valve and the pressurizer. This stopped the loss of coolant water, but superheated gasses and steam blocked the flow of water through the core cooling system. 

Operators tried to force water into the reactor system. But during this time, radioactive gasses were building so plant personnel attempted to move the gasses through pipes and compressors, sending them through high-efficiency particulate air (HEPA) and charcoal filters, which removed the majority of radionuclides. However, the compressors leaked a small amount of radioactive gas into the atmosphere. 

For the next month, operators slowly reduced the pressure of gasses like hydrogen, through gradual release. On April 27th, operators were able to establish coolant water flow without mechanical pumps and the plant entered a cold shutdown. Five years later, with the removal of the reactor pressure vessel head, the investigation revealed that at least 45% of the core had melted. The other reactor at the nuclear plant, TMI-1, continued producing electricity for another 40 years and was decommissioned in 2019. 

Overall, effective response to the incident was prevented due to conflicting implications between instrument information, ambiguous labeling of controls,  inadequate training, and plant procedures that failed to help operators diagnose problems. 

Lasting Effects

Three Mile Island, the most infamous accident in U.S. history, caused no deaths, no environmental damage, and no health effects. 

Media coverage and perceived danger led to increased anti-nuclear attitudes that challenged nuclear construction for decades to come. For many, Three Mile Island was a catalyst for anti-nuclear activism. 

In direct response, a group of famous musicians, Musicians United for Safe Energy (MUSE) formed, and held the “No Nukes: The Muse Concerts For a Non-Nuclear Future.” Joni Mitchell, Bruce Springsteen, Jackson Browne, and Bonnie Raitt performed in front of nearly 200,000 at Madison Square Garden in September 1979. Their goal was to influence public opinion and seek to ban both nuclear weapons and nuclear power.

Chernobyl - April 26th, 1986

What happened?

“Despite the fearmongering, the health impacts of nuclear accidents have been far lower than initially predicted,” Isabelle Boemeke said. “Studies of Chernobyl's aftermath found that besides the immediate casualties and an increase in childhood thyroid cancer (which has a 99% survival rate), the long-term health effects were surprisingly small.”

Fukushima- add date

What happened?

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