Nuclear Power Safety and Accidents Pt. 2: Three Mile Island Accident

A simplified PWR schematic

The first accident I’ll look at is the Three Mile Island Accident, which was a Pressurized Water Reactor. The accident is the most significant one in the USA. Even though there were no deaths caused by the accident itself or the radiation emitted due to the accident,  the public backslash was so huge that in the following two decades after the accident there hasn’t been any nuclear plant productions. The events led to the accident, is believed to start with a blockage in a condensate polisher, which are filters designed to clean the minerals and impurities in water. After the blockage, the heat and pressure rose in the reactor. At that time, SCRAM was performed, but the decay heat couldn’t be eliminated because water wasn’t going to the steam turbines. They activated the auxiliary water pumps, however, because, they had been closed for routine maintenance a few days ago, the system was unable to pump any water. So in order to decrease the pressure, pressure realizer valve was planned to be open for 8 seconds, but because of a malfunction in the system it got stuck and went unnoticed. The indicator in the control room only showed whether a close signal was sent, not whether it was actually closed or not. Coolant was lost rapidly through the open valve, turning this into a Loss of Coolant Accident (LOCA). Steam voids due to high heat formed in the coolant channel, further preventing the flow of coolant and heat transfer. Since the voids were forming more rapidly than the coolant lost through the open valve, operators were lead to believe water levels were increasing. In fact the situation was the exact opposite. This resulted in the cut of Emergency Coolant Flow, which could have prevented the formation of voids if it hadn’t been cut. At the end, with of the coolant lost, reactor core became exposed, and zirconium oxidation took place.

The fuel rods in a nuclear plant are covered with zirconium, which is an alloy with low neutron absorption, high hardness, ductility and resistance. However, when exposed the steam at high temperature the following reaction takes place and hydrogen forms: Zr + 2H2O → ZrO2 + 2H2. Hydrogen explodes when mixed with air. This reaction also melts the fuel pellets and releases radioactive ions.

All of these happened in Three Mile Island, yet only when a shift change happened and workers with a new mindset came into the plant, did they recognize that what they had in their hands was actually a LOCA. They closed the backup valve to stop coolant loss, but by that time plant was already seriously contaminated and partial meltdown had occurred. So what they did next was to release the radioactive steam into the atmosphere to drop the pressure. Considering all of the mistakes made by the operators, it is actually a miracle that no one bwas harmed by the accident.the staff it is a miracle that no one was actually harmed or got sick due to radiation poisoning.

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