EXPLAINER: Ukrainian nuclear power plant shutdown reduces risk

Kyiv, Ukraine — The forced shutdown of the endangered and crippled Ukrainian nuclear power plant in Zaporizhzhia – the largest in Europe – significantly reduces the risk of a radioactive disaster that has haunted the world for weeks.

The last of six nuclear reactors at the Russian-occupied Zaporizhzhia power plant was shut down on Sunday because Russia’s military actions in Ukraine had repeatedly cut off reliable external power sources. This power is needed to prevent reactors from overheating to the point of causing a meltdown that could puncture surrounding concrete and steel containment buildings and spew radiation across Ukraine, Russia and other neighboring countries.

Since a fire on September 5 caused by bombings knocked the plant off all external transmission lines, the sixth reactor has had to continue to operate – at reduced power – to supply reactor cooling and other plant equipment. crucial security. This “island mode” is unreliable and not designed to be more than a stopgap measure, according to Ukrainian officials. On Sunday, a plant’s connection to the Ukrainian power grid was restored, so power to the sixth reactor was not needed for safety systems.

Here is an overview of the risks, impact and what could be done if the external power supply were cut again.


Ukrainian operators at the Zaporizhzhia plant apparently decided it was too risky to operate any of the six reactors because fighting had endangered external power supplies for cooling and other safety systems. But when all external power was lost, they couldn’t shut down all the reactors. They needed power from at least one reactor to run the security systems. When external power was restored using a reserve transmission line, they executed a “cold shutdown” of the sixth reactor – inserting control rods into the reactor core to stop the reaction of nuclear fission and the generation of heat and pressure.


With the cessation of all nuclear reactions, the temperatures and pressure inside the reactors gradually decrease, reducing the required intensity of water cooling of the radioactive fuel. This is the safest mode of operation of a nuclear power plant.

“A cold shutdown greatly reduces the risk of collapse,” Steven Arndt, president of the American Nuclear Society and scientist at the US Oak Ridge National Laboratory, said on Sunday.

Still, the Director General of the International Atomic Energy Agency, Rafael Grossi, said on Sunday that he remained “seriously concerned about the situation at the plant, which remains in danger as long as the bombardment continues”.


With all reactors shut down, if fighting or other problems cause another loss of external power, plant operators would have more time to organize a backup power source to run the cooling systems, and the cooling load would be reduced, Arndt said. This is because the heat comes from the decaying fuel, not from an active fission reaction.

“With each passing hour, the possibility of fuel meltdown becomes less and less,” he said.

Cooling systems are also needed for spent nuclear fuel, i.e. fuel that has already been used in reactors but must be stored under water until it has cooled sufficiently to be moved outside the reactor containments to dry storage.

If the Zaporizhzhia plant’s external power is cut off again, engineers could turn to 20 emergency diesel generators, as they have done at least once since the start of the war. The IAEA said only one diesel generator power per reactor is needed to maintain safety. While Ukrainian authorities estimated they had enough diesel fuel in stock to run safety systems for at least 10 days, Petro Kotin, the head of plant operator Energoatom, said the week last: “The diesel generators are in fact the last defense of the plant before we can expect a radiological accident.

Commenting after external power was restored on Sunday, Arndt gave a more optimistic assessment: “We are cautiously optimistic that the plant is in a relatively stable location due to the cooling of offsite power and diesel generators from help.”


Although power outages struck Ukraine several times during the war, they appear to be largely the result of the bombing of other power plants and power generation infrastructure, rather than the loss of production from the center of Zaporizhzhia. Zaporizhzhia regional administration spokesman Volodymyr Marchuk said villages around Enerhodar, where the plant is located, are without electricity due to the shutdown of the plant, but alternative power plants , such as the Dnipro hydroelectric power station, supply the electricity network.


Katell reported from New York.


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