Ukraine says it has shut down the last working reactor at Europe’s largest nuclear power plant as a safety measure while ongoing fighting in the area continues to escalate the chances of a serious radiation accident.

The step has been urged for weeks by the International Atomic Energy Agency, the UN nuclear watchdog, whose inspectors gained access to the site only last week. While a direct military strike on one of the reactor cores could still trigger an accident, the effects would be greatly reduced if the units are not operating, experts say.

The six-reactor Zaporizhzhia nuclear plant lost its outside source of power a week ago after all its power lines were disconnected as a result of shelling. Since then, it had been  operating in what engineers call “island mode” for several days — generating electricity for crucial cooling systems from its only remaining operational reactor.

The shutoff is meant to put the last of the plant’s reactors into a relatively safe state as fighting continues around the facility in southeastern Ukraine. But if the plant is once  again isolated from external power — as has happened twice in the past three weeks — it would mean that it would be forced to rely on its 20 diesel generators to power cooling and safety equipment.

Energoatom, Ukraine’s national nuclear operator, estimates the backup generators have enough fuel to last 10 days.

“A decision was made to shut down power unit No. 6 and transfer it to the safest state — cold shutdown,” Energoatom, said in a statement released Sunday.

In comments to the Associated Press, plant management acknowledged that the risk remains high for outside power to again be cut off.

The Biden administration had been urging Ukrainian nuclear authorities to move ahead with full shutdown for weeks, according to The New York Times, but Kyiv had been reluctant to take that step. At full operational capacity, the plant supplied 20 percent of Ukraine’s electricity.

Ukrainian officials have worried that, should the plant be shut down entirely, Russian troops might seek to connect the plant to the Russian power grid, and disconnect it from Ukraine’s.

Such a move would place it under the purview of Rosatom, Russia’s state nuclear monopoly, which in the past has denied plans to take over the plant’s operation. Nonetheless, Rosatom technicians are present at the plant and the hostage Ukrainian staff is made to follow their orders. Rosatom public relations staff were likewise responsible for guiding last week’s IAEA safety mission to the facility.

On Friday, it was clear to Ukrainian nuclear officials that they had little choice but to shut the last reactor down. For a time, all external power was cut off from the plant, including the sources of power used for cooling systems. Full power restoration seemed unlikely and Ukrainian officials argued that Russia was seeking to engineer a crisis at the plant and then blame it on Ukrainian plant workers.

Rafael Grossi, the IAEA’s director general, who visited the plant last week and left two monitors in place full time, said Friday that, “the shelling around Zaporizhzhia nuclear plant must stop.”

He said that the monitors were aware of the reactor shutdown and confirmed that the restoration of a backup powerline to the plant means that the facility has the “electricity it needs for reactor cooling and other functions.”

Last week’s shelling around the plant severed transmission lines that provide external  electricity to the plant, thereby disconnecting it from the Ukrainian grid. Ukrainian engineers elected to use the plant’s one remaining reactor to power cooling and safety systems — a more reliable solution than the backup diesel generators, Petro Kotin, Energoatom’s head, told The New York Times.

As of Monday, the plant was reconnected to the national grid, allowing engineers to take the step of shutting down that last reactor, which places it in a safer state than when it is producing energy. The plant’s other five reactors have also been cycled down since the outbreak of war.