One of six units remains in hot shutdown to produce steam and heating
Unit 5 at the Zaporizhzhia nuclear power station in Ukraine is being moved from hot shutdown to cold shutdown as operators aim to determine the cause of boron detected in a cooling circuit, the International Atomic Energy Agency said.
This still leaves one of the station’s six reactors in hot shutdown to produce steam and heating, IAEA director-general Rafael Grossi said.
Unit 5’s transition to cold shutdown began on 20 November and was expected to be completed on 22 November. Unit 4 will remain in hot shutdown. There are no plans to bring a second unit into hot shutdown to replace Unit 5.
Once in cold shutdown, staff will carry out tests to identify why low levels of boron were found in the secondary cooling circuit of one of the unit’s steam generators.
Zaporizhzhia’s operators told IAEA experts at the site that the boron concentration in the affected cooling circuit remained below the limits permitted by its technical specifications. No radioactivity has been detected in the secondary cooling circuit.
Borated water is used in the primary coolant to help maintain nuclear safety.
Operators decided to move the unit to cold shutdown after one of the three 17.4 MW diesel boilers located offsite started operating on 17 November, providing additional heating to the nearby town of Enerhodar, where many station staff live.
Zaporizhzhia had been keeping Units 4 and 5 in hot shutdown to provide heating and steam for nuclear safety purposes on site, as well as heating for Enerhodar.
Background: The Need For Cold Shutdown
The IAEA wants Zaporizhzhia to find an alternative source of steam generation. Ukraine’s national regulator, the State Nuclear Regulatory Inspectorate of Ukraine (SNRIU), issued regulatory orders in June to limit the operation of all six units at Zaporizhzhia to a cold shutdown state.
When a reactor is in cold shutdown, the fuel is almost cold and operators do not need to constantly run the primary cooling pumps at the same level to circulate cooling water in the primary cooling loop.
If the plant loses offsite power – something that has happened frequently at Zaporizhzhia – the operators will not have to worry about cooling an operating reactor with unreliable backup diesel generators.
With hot shutdown, the nuclear fuel remains reasonably hot because it continues to react and therefore needs constant cooling.
Separately, IAEA experts on the site are continuing to gather information to fully understand the cause of the event that occurred last week which resulted in Unit 6 losing power and relying on a diesel generator for 90 minutes.
Zaporizhzhia is near the frontline of fighting in southern Ukraine. It has been occupied by Russian forces since March 2022.
