The Zaporizhzhia Nuclear Power Plant on the Dnipro River.Credit…Daniel Berehulak/The New York Times

The director general of the United Nations’ nuclear watchdog said on Monday he was on his way to Ukraine to assess the situation at the Zaporizhzhia Nuclear Power Plant after a dam breach depleted water levels in the reservoir it uses to cool reactors and spent nuclear fuel.

The director, Rafael Mariano Grossi, said he would meet with President Volodymyr Zelensky of Ukraine and present a plan for assistance in the aftermath of floods unleashed by the destruction of the Kakhovka dam last week. The breach of the dam sent water from the Dnipro River coursing downstream and drastically reduced the volume of water in the Kakhovka reservoir.

Mr. Grossi said over the weekend that while there was no immediate threat to the water supplies at the Zaporizhzhia plant, the U.N. nuclear watchdog was urgently seeking new data about depleting water levels in the reservoir.

He said there were discrepancies between the water level readings taken by Ukrainian officials upstream of the Kakhovka dam, and the readings that were taken at the Zaporizhzhia plant, which is next to the reservoir and is under Russian control. Ukraine controls the western banks of the reservoir, while Russia holds parts of the eastern bank.

At least 14 people have died as a result of the dam disaster, which has also caused widespread environmental damage and left hundreds of thousands of people without access to clean drinking water. The drop in the reservoir’s water level also presents the latest risk to the nuclear plant, the largest in Europe, which was seized by Russian troops near the start of Moscow’s full-scale invasion of Ukraine in February last year.

Even though the nuclear plant has not been producing electricity for several months now, “it still needs access to water and power for cooling and other essential safety and security functions and to avoid the risk of a potential fuel meltdown and release of radioactive material,” the watchdog group, the International Atomic Energy Agency, said in a statement on Sunday.

Water to cool the plant’s six reactors and perform other critical safety functions is supplied via a pond on the facility’s grounds that is fed by the reservoir. The pond, which is more than two miles long, more than a mile wide and around 50 feet deep, contains enough water to meet the plant’s needs for “several months, ” Mr. Grossi said. But it also needs to be supplemented with reservoir water, which means accurate monitoring of the reservoir’s water level is crucial.

Mr. Grossi said that there was a discrepancy of around six feet between water level readings taken at the thermal power plant on the nuclear facility’s grounds and readings taken elsewhere on the reservoir.

Inspectors from the I.A.E.A who have been stationed at the plant since last year need access to the thermal plant to understand the reason for the difference, Mr. Grossi said. Ukrainian workers continue to operate the plant, but security and access is controlled by Russian troops.

Five of the plant’s six reactors are in cold shutdown mode, the safest state of operation, while the sixth remains in hot shutdown to produce steam to support processes that contribute to safety on the site, the I.A.E.A. said in its statement on Sunday. That appeared to contradict an earlier statement from Ukraine’s state nuclear company, which said that the last reactor still producing energy at the plant had been put into a “cold shutdown” — a state in which it no longer generates electricity — as a safety precaution after the destruction of the dam threatened its water supply.

The cooling pond has become even more important for maintaining the stability of the plant since the dam was breached, and Mr. Grossi said last week that nothing should be done to damage it.

Over the past year, shelling has cut external power supplies to the plant and also hit an area where spent fuel is stored. Mr. Grossi has repeatedly warned of the potential for nuclear catastrophe at the plant.

In addition, Kyiv’s forces have recently launched a counteroffensive in southern Ukraine that raises the possibility of military confrontation in the plant’s vicinity.

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