Zaporizhzhia unit 4 was switched to hot shutdown in late July, with unit 5 switched from hot to cold shutdown to allow maintenance work to be carried out. That decision was taken by the operators of the nuclear power plant, which has been under Russian military control since early March last year.
"As a result of moving power unit 4 of the temporarily occupied Zaporizhzhia NPP into a hot shutdown state, in breach of the licence terms and Energoatom requirements, an incident occurred with a primary-to-secondary water leak," Ukrainian nuclear operator Energoatom said. "This happened due to a violation of the integrity of the third steam generator located in the containment of the unit."
The unit is now being placed back into cold shutdown to determine the precise cause of the water leak that has been detected and to conduct maintenance to repair the affected steam generator. The International Atomic Energy Agency (IAEA), which has experts on-site to monitor the plant, confirmed there was no radiological release to the environment.
Unit 6 - which has been in cold shutdown since 21 April to enable inspection and maintenance of the safety systems - will be transferred to hot shutdown to continue steam production. The site uses the steam generated from one reactor unit in hot shutdown for various nuclear safety purposes including the processing of liquid radioactive waste collected in storage tanks.
The IAEA said its team on the site will closely monitor the operations for the transition between the shutdown states of units 4 and 6. The other units at the plant remain in cold shutdown, it noted.
Energoatom noted that bringing unit 6 into hot shutdown "is another breach of the licence terms and Energoatom requirements".
The State Nuclear Regulatory Inspectorate of Ukraine issued regulatory orders in June for all six units to be placed in cold shutdown, given its situation on the frontline of the war, and the breaching of the dam that had helped ensure plentiful cooling water supplies. The Russian operators of the plant say such a move is not required from a "legal or technological point of view". The IAEA has urged investigation into whether an external boiler could be installed at the site to generate the steam required, so all units could be moved to cold shutdown.
Meanwhile, the 750kV Dniprovska power line providing external power to the Zaporizhzhia plant disconnected twice on 10 August, first for about 12 hours and then again for almost three hours. These disconnections meant that the plant had to rely on its only remaining off-site power line, the 330 kV backup line, to supply the electricity that it required.
"The repeated power line cuts underline the continuing precarious nuclear safety and security situation at the plant," said IAEA Director General Rafael Mariano Grossi.
