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Zaporizhzhia NPP moves reactor unit 4 from hot to cold shutdown

Ukraine’s Zaporizhzhia Nuclear Power Plant (NPP) began transferring reactor unit 4 from hot shutdown to cold shutdown last week, following the detection of a water leak at one of its four steam generators in the containment of unit 4, Director General of the International Atomic Energy Agency (IAEA) Rafael Mariano Grossi said on Thursday (10 August).

The power plant will place unit 4 into cold shutdown to determine the precise cause of the water leak that has been detected and conduct maintenance to repair the affected steam generator. There was no radiological release to the environment, IAEA said in a press release.

Over the past weekend, the power plant’s authorities moved unit 6 to hot shutdown to continue steam production on the site.

Unit 6 has been in cold shutdown since 21 April to enable inspection and maintenance of the safety systems. Last week, IAEA experts were informed that the power plant’s authorities have been conducting specific maintenance on parts of unit 6 safety systems. Additionally, the plant’s authorities said that this work and all tests of the safety systems were successfully completed before commencing the transition of unit 6 to hot shutdown.

The IAEA 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 nuclear facility remain in cold shutdown.

In July, the State Nuclear Regulatory Inspectorate of Ukraine issued regulatory orders to limit the operation of all six units to a cold shutdown state.

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 continues to “strongly encourage” the installation of an external source of process steam, which, from a nuclear safety perspective, would provide the safest longer-term solution for the steam needs at the site.

On Thursday, the site’s 750 kilowatt (kV) Dniprovska power line disconnected on two occasions. First for approximately twelve hours until 13:37 and then again at 16:13 local time. These disruptions mean that the site has had to rely on its only remaining off-site power line, the 330 kV backup line, to supply the electricity that is required, for example, to perform safety functions such as pumping cooling water for the plant. There was no total loss of off-site power and there was no need to use the emergency diesel generators, IAEA said.

“The repeated power line cuts underline the continuing precarious nuclear safety and security situation at the plant,” Director General Grossi said.

The availability of cooling water remains relatively stable at the nuclear power plant. While the height of the site’s cooling pond continues to drop by about 1 centimetre (cm) per day, the height of the discharge channel from the neighbouring Zaporizhzhia Thermal Power Plant (TPP) is regularly being topped up by pumping water from the nuclear power plant’s inlet channel.

During the past week, IAEA experts conducted several inspections at the site. On 4 August, they assessed the dry spent fuel storage facility, verifying fuel cask integrity. On 8 August, inspections included the site’s unit 2 control rooms and safety-related spaces where no unusual items were found, except for military trucks in the turbine hall meant for maintenance.

On Thursday, the IAEA experts visited one of the fresh fuel storage facilities. They confirmed that the fresh fuel was “safely and securely” stored.

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