Unit 6 at the Zaporizhia NPP (ZNPP) temporarily lost power recently, forcing it to rely on an emergency diesel generator for the electricity needed for cooling and other vital functions, International Atomic Energy Agency (IAEA) Director General Rafael Mariano Grossi has reported.

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. 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 of the ZNPP to a cold shutdown state. Units 1, 2, 3 and 6 are currently in cold shutdown but units 4&5 are needed to supply heat for vital safety operations.

ZNPP is investigating the cause of the 90-minute power outage that occurred at unit 6. The IAEA experts at the site are also gathering information to make their own independent assessment. The unit is in cold shutdown, but still needs access to power. None of the other five reactors lost power.

“While this was not a total loss of off-site power, as we have seen seven times before during the conflict, it once again highlights the precarious nuclear safety and security situation at the ZNPP,” Grossi said. “The IAEA will continue to collect information so we can inform the international community about the situation at the plant.” The day after the power outage, ZNPP informed the IAEA experts that part of the safety system of the same unit was placed under planned maintenance.

ZNPP continues to be connected to the electricity grid through a single 750 kilovolt (kV) main power line – one of four that operated before the conflict – as well as one back-up 330 kV line, compared with six less than two years ago. IAEA experts present at the ZNPP are continuing to hear explosions on a near-daily basis some distance away from the site, on the frontline of the conflict.

Separately, the IAEA experts have been informed that boron has been detected in the secondary cooling circuit of one of the steam generators at reactor unit 5, which is currently in hot shutdown. Borated water is used in the primary coolant to help maintain nuclear safety. The site has increased the frequency of boron measurements in the secondary cooling circuit of unit 5. The measurements remain relatively stable and are within the limits permitted by the reactor’s technical specifications. No radioactivity has been detected in the secondary cooling circuit.

ZNPP stated that, as the boron concentration remains within the allowable limits, the site intends to keep unit 5 in hot shutdown, which will be reassessed after all the boilers, used for heating in the nearby town of Enerhodar, have started operating. At that time, the site will determine whether to move unit 5 to cold shutdown.

ZNPP has been keeping reactor units 4 and 5 in hot shutdown to provide heating and steam for nuclear safety purposes on site, as well as heating for the nearby town of Energodar, where most plant staff live. The IAEA continues to follow ZNPP’s progress to find an alternative source of steam generation. Ukraine’s SNRIU issued regulatory orders in June to limit the operation of all six units of the ZNPP to a cold shutdown state.

In other activities conducted by the IAEA experts over the past week, the team performed – for the first time – a walkdown of all six main reactor control rooms at the ZNPP, one after the other. It provided the team with an opportunity to gather more information about staffing there and to confirm the status of each reactor unit.

“This has been a positive development regarding access. I strongly encourage the plant to ensure that timely access and information sharing take place regularly. It will enhance our capability to report about the overall situation at the plant,” Grossi said.

The same day, the IAEA experts also conducted a walkdown of the turbine hall of unit 5, but their access was partially restricted, as was the case also during a visit to the turbine hall of unit 1 last week, and of the turbine halls of units 1, 2, 4 and 5 during walkdowns in October. The IAEA experts continue to request access to all six turbine halls together as part of their activities to monitor compliance to the seven indispensable pillars and the five concrete principles for protecting the ZNPP.

Following last month’s closure of the reactor vessel of unit 3, the plant informed the IAEA experts that testing of the reactor’s primary cooling circuit had been completed, and pressure testing of the secondary cooling circuit is expected to be completed in the coming days.

Over the past week, up to seven of the nine mobile diesel boilers installed at the ZNPP to provide additional heating during the winter have been in operation most days. Their usage depends on the requirements for steam at the plant and for heating in Energodar.