Ukraine’s Zaporizhia NPP (ZNPP) recently lost all off-site power overnight and had to rely on emergency diesel generators for the electricity needed to cool its reactors and for other essential nuclear safety and security functions, International Atomic Energy Agency (IAEA) Director General Rafael Mariano Grossi reported in his latest update.

ZNPP lost the connection to both of its external power lines – the eighth time the plant suffered a complete off-site power outage during the military conflict, heightening concerns about nuclear safety and security. The IAEA team of experts at the site reported that the ZNPP’s connection to its sole back-up 330 kilovolt (kV) power line was cut around due to an external grid fault. It was followed around five hours later by the loss of the plant’s sole 750 kV line, its main supplier of external electricity. The cause appeared to be in the outside grid far away from the ZNPP.

As a result, the site’s 20 diesel generators automatically started operating. ZNPP staff then reduced the number in operation to eight – enough to ensure that the plant’s six reactors (all in shutdown) had enough power for essential cooling.

The affected 750 kV power line – the only remaining main power line at the ZNPP compared with four before the conflict – was re-connected the following day. After the re-connection the eight diesel generators were gradually shut down. The power supply is currently being provided by the 750 kV line with no external back-up, IAEA said.

“The most recent external power outage is yet another reminder about the precarious nuclear safety and security situation at the plant, which can be affected by events far away from the site itself. The IAEA continues to do everything it can to help prevent a nuclear accident. I also call on all parties not to take any action that could further endanger the plant,” Director General Grossi said.

The operation of the four main coolant pumps of one of the ZNPP’s reactors – unit 4 – was interrupted during the time of the off-site power loss. The unit is now being brought from semi-hot shutdown back to hot shutdown to produce heating and steam for the site and the nearby town of Energodar, where most plant staff live. The other five reactors remain in cold shutdown. It was the first complete external power outage since May.