The International Atomic Energy Agency (IAEA) warned of the "precarious" situation at the Zaporiyia nuclear power plant, in southern Ukraine and under occupation by Russian troops, which only has an external power line for its safety tasks.
"Nuclear safety at the Zaporiyia nuclear power plant remains precarious," said IAEA Director General Rafael Grossi of Argentina.
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"I call again for the commitment of all parties to ensure nuclear safety at the plant," he added in a statement.
Europe's largest nuclear power plant, with six reactors, has been under Russian occupation for a year. All reactors are now shut down, but a steady flow of electricity is needed for essential cooling work.
There has been shelling around the plant for which Russia and Ukraine have blamed each other and the IAEA has unsuccessfully advocated establishing a safety zone.
For the past three weeks, the nuclear power plant has obtained electricity only through the only external line that is still functioning, the IAEA warns.
Should this be damaged, there would be a total loss of external power supply and the plant would be dependent on its fuel-powered generators, "its last line of defense," according to Grossi.
The plant's other remaining emergency power line has been offline since March 1 and is being repaired.
"This shows once again how threatened nuclear safety is at Europe's largest nuclear power plant," Grossi stressed.
Already on March 9, the plant was without external power for eleven hours when the supply was interrupted, making it completely dependent on generators.
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According to the Ukrainian nuclear operator, the plant was left without regular power supply as a result of a major Russian missile attack.