Ukraine’s Zaporizhia NPP (ZNPP) is transitioning its reactor unit 5 to cold from hot shutdown in order to determine the cause of boron detected in a cooling circuit. This still leaves one of the plant’s six reactors in hot shutdown to produce necessary steam and heating, International Atomic Energy Agency (IAEA) Director General Rafael Mariano Grossi reported. Unit 4 will remain in hot shutdown. There are currently no plans to bring a second unit into hot shutdown to replace unit 5, the plant said.

Once in cold shutdown, ZNPP 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. ZNPP informed the IAEA experts at the site that the boron concentration in the affected cooling circuit remained below the limits permitted by its technical specifications. In addition, no radioactivity has been detected in the secondary cooling circuit. Borated water is used in the primary coolant to help maintain nuclear safety.

The ZNPP decided to move the unit to cold shutdown after one of the three 17.4 MW diesel boilers located off-site started operating on 17 November, providing additional heating to the nearby town of Energodar, where many plant staff live.

The ZNPP had been keeping reactors – 4 and 5 in hot shutdown to provide heating and steam for nuclear safety purposes on site, as well as heating for Energodar. The IAEA continues to follow the ZNPP’s progress 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 demanding that all six units of the ZNPP be placed in cold shutdown.

Russia took control of ZNPP in March 2022 as part of its special military operation in Ukraine. In October 2022, Russian President Vladimir Putin signed a decree formally transferring ZNPP to Russian jurisdiction under nuclear utility Rosenergoatom (part of Rosatom). A Russian Federal State Unitary Enterprise. Zaporizhia NPP was established by Rosenergoatom to operate the plant. However, Ukrainian nuclear utility Energoatom still claims ownership of the plant.

Separately, the 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.

Later this week, the IAEA team has been invited to observe the ZNPP’s planned emergency exercise. “We look forward to observing the emergency response exercise at ZNPP from both the temporary emergency control centre and in the field,” Grossi said. “Emergency exercises are very important for nuclear safety, especially in these times of heightened risk caused by the conflict.”