A blast on June 6, 2023, destroyed the Kakhovka dam on the Dnieper River in eastern Ukraine. The rupture lowered water levels in a reservoir upriver at the Zaporizhzhia Nuclear Power Plant in the city of Enerhodar. The reservoir supplies water necessary for cooling the plant’s shutdown reactors and spent fuel, which is uranium that has been largely but not completely depleted by the fission reaction that drives nuclear power plants.
The International Atomic Energy Agency, which has inspectors on-site to monitor effects of the war at the plant, issued a statement saying that there was no imminent danger. Nevertheless, the destruction of the dam increases the risk of a disaster at the plant, a risk already heightened by ongoing combat in the area.
The Conversation asked Najmedin Meshkati, a professor and nuclear safety expert at the University of Southern California, to explain what the dropping water level means for the safety of the nuclear power plant and the ongoing risks to the plant’s spent fuel.
The immediate situation is becoming very precarious. The dam is downstream from the plant, meaning that the flooding will not jeopardize the plant. But the plant draws water from a major reservoir on the river for its cooling system. This reservoir is draining because the downstream dam has been damaged.
The plant doesn’t need the massive amount of water it otherwise would because its six reactors are in cold shutdown. But the plant still needs water for three purposes: to reduce the residual heat from the shutdown reactors, to cool the spent fuel, and to cool the emergency diesel generators if the plant loses off-site power.
The plant’s operators pumped water from the reservoir into a cooling pond, which is why the IAEA said the plant has enough water for several months. But that’s the last resort, which is why the agency also said that it’s vital that the cooling pond remains intact. If the plant loses the cooling pond, the only hope would be to try something like they did at the Fukushima nuclear power plant after the earthquake and tsunami in Japan in 2011. They brought in huge water pumps to pump saltwater from the Pacific Ocean into the reactors to cool them down. The plant operators may need to try to pump water from the Dnieper River.
The two lifelines of any nuclear plant, whether operational or closed down, are water and electricity. The newly launched Ukrainian counteroffensive puts these two lifelines in further jeopardy. Since the Russian occupation, the plant has suffered a lot and lost off-site power seven times. My immediate concern is that if the plant loses its last remaining power line, which powers the cooling pumps, then it needs to rely on emergency diesel generators. There are 20 generators with on-site storage of only 10 to 15 days of fuel supply. Getting fuel while the counteroffensive is going on is another major challenge.
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.