By Edward Lyman
Nearly a year after Russia’s March 4, 2022 seizure of the Zaporizhzhia nuclear plant in Ukraine, the facility remains in a precarious state. The site has endured fire, structural damage, and five temporary losses of all offsite power as the result of shelling, and the grid connection remains fragile. Unprecedented attempts by Rafael Grossi, the director general of the International Atomic Energy Agency, to create a “safety and security protection zone” around the plant have so far been unsuccessful. And now events many miles away from Zaporizhzhia are posing an additional threat to critical aspects of its operations, reinforcing the need for urgent actions to ensure its safety as fighting intensifies.
Like most nuclear plants, the six-reactor Zaporizhzhia facility is situated near a body of water that serves as its ultimate heat sink (UHS), an assured supply of water to its “essential service water system” that enables removal of the radioactive decay heat from shutdown reactors and spent fuel pools. That water system is also used to cool equipment such as the emergency diesel generators needed to provide electrical power when offsite power is lost. (It’s important to note that the essential service water system is distinct from the residual heat removal system that provides cooling directly to the fuel in the reactors in cold shutdown. The residual heat removal system, a closed loop, transfers heat from the reactor cores through heat exchangers to the essential service water system, which then carries the heat away to the UHS.)
At Zaporizhzhia, the water supply for the UHS is provided to cooling ponds from the Kakhovka Reservoir, 80 miles downstream of the plant on the Dnipro River. But in recent weeks, reports indicated that the reservoir’s water level had decreased and stood at 13.98 meters on February 15, according to the International Atomic Energy Agency. Petro Kotin, the president of Ukraine’s state-owned nuclear utility Energoatom, said that if the reservoir level drops below 12.8 meters, then Zaporizhzhia will face an emergency; below 12 meters the situation would become “critical.” If the water level gets too low, then the cooling ponds themselves will not be replenished, and the essential service water system will fail. (Ukraine has accused Russian forces controlling the reservoir dam of draining its water, although, as is typical in this conflict, Russian authorities have denied responsibility and blamed Ukrainian forces for the drop in water levels.)
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Fortunately, there is a reduced risk today that the current situation at Zaporizhzhia would lead to an outcome as dire as Fukushima. First, all six of the reactors have been shut down for at least several months (four in “cold” shutdown and two in “hot” shutdown). Since the decay heat rate decreases significantly over time in a shutdown reactor—dropping by nearly a factor of 100 a few months after shutdown—operators would have a grace period on the order of days, rather than hours, to mitigate a loss of UHS before temperatures rose high enough to cause reactor fuel damage.
Second, the site is better prepared to deal with such an event today than it would have been before Fukushima. In the accident’s aftermath, Ukraine, like many countries, carried out stress tests and developed plans for keeping its reactors safe indefinitely under Fukushima-like conditions—not only the long-term unavailability of electrical power, but also loss of the UHS.
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