Cold shutdown reduces risk of disaster at Zaporizhzhia nuclear plant – but combat around spent fuel still poses a threat

Energoatom, operator of the Zaporizhzhia Nuclear Power Plant in the Ukrainian city of Enerhodar, announced on Sept. 11, 2022, that it was shutting down the last operating reactor of the plant’s six reactors, reactor No. 6. The operators have put the reactor in cold shutdown to minimize the risk of a radiation leak from combat in the area around the nuclear power plant.

The Conversation asked Najmedin Meshkati, a professor and nuclear safety expert at the University of Southern California, to explain cold shutdown, what it means for the safety of the nuclear power plant, and the ongoing risks to the plant’s spent fuel, which is uranium that has been largely but not completely depleted by the fission reaction that drives nuclear power plants.

What does it mean to have a nuclear reactor in cold shutdown?

The fission reaction that generates heat in a nuclear power plant is produced by positioning a number of uranium fuel rods in close proximity. Shutting down a nuclear reactor involves inserting control rods between the fuel rods to stop the fission reaction.

The reactor is then in cooldown mode as the temperature decreases. According to the U.S. Nuclear Regulatory Commission, once the temperature is below 200 degrees Fahrenheit (93 Celsius) and the reactor coolant system is at atmospheric pressure, the reactor is in cold shutdown.

When the reactor is operating, it requires cooling to absorb the heat and keep the fuel rods from melting together, which would set off a catastrophic chain reaction. When a reactor is in cold shutdown, it no longer needs the same level of circulation.