Three Mile Island Accident

In March 1979 an accident at the Three Mile Island Nuclear Generating Station forever changed public opinion of nuclear energy. Though there were a multitude of problems which led to a partial meltdown, it really came down to a stuck valve and a broken indicator light.

Construction of the Three Mile Island Nuclear Generating Station started in 1968 and was completed in 1978, with the commissioning of Unit 2. A scant three months after being commissioned Unit 2 suffered a partial meltdown, an accident which remains the most significant accident in commercial U.S. nuclear energy. The accident started early in the morning on March 28, 1979. A failure in a main pump prevented water from reaching the steam generators which helps cool the reactor. The pump’s failure caused an automatic shutdown of the reactor. Without the water to help cool the reactor, heat and pressure rose causing a relief valve to open and vent steam. Once normal pressure was reached, this valve should have closed, but did not. To complicate issues, the valve indicator light in the control room suggested that it was closed. As more steam escaped from the reactor, the core was no longer properly covered with water and began to overheat. Eventually temperatures reached a point where the nuclear fuel rods began to melt and Unit 2 suffered a partial core meltdown. Eventually operators realized what was going on, and stabilized the reactor.

Despite the partial meltdown, the general public was more concerned with the release of radioactive gases during the accident. Since it was still uncertain what had happened in the plant and how much radiation was released, people most vulnerable to radiation were advised to evacuate from the area.  Air, ground and water around Three Mile Island was extensively tested by numerous federal and independent groups immediately following the accident and for several years. It was concluded that about 2 million people had received a dose of radiation equal to 1 millirem.  For comparison, getting an x-ray is about 6 millirem.

Despite how relatively harmless the accident was, U.S. interest in nuclear energy soured. Soaring costs and stricter regulations caused planned reactor construction across the county to be delayed and/or cancelled altogether. Eight years later the Chernobyl accident brought that souring effect to the rest of the world.

Three Mile Island

Population Dose and Health Impact of the Accident at the Three Mile Island Nuclear Station

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