Cernavodă begins Unit 1 restart procedures

Nuclearelectrica has initiated restart procedures for Unit 1 at Romania’s Cernavodă nuclear power station, following an improvement in Danube water levels that has allowed the operator to begin preparations for the reactor’s return to electricity generation.

The company confirmed on 4 November that the minimum conditions required to initiate the restart had been met. Resynchronisation of Unit 1 with Romania’s National Energy System is scheduled for Friday 6 November, subject to completion of the necessary technical operations and nuclear safety requirements.

Unit 2 remains shut down because the available cooling-water supply is not yet considered sufficient for both reactors to resume operation. Nuclearelectrica expects the restart of Unit 1 to increase water consumption and lower the level in the station’s intake basin, leaving the second reactor dependent on a further sustained rise in the Danube.

The decision marks a change from the operator’s position in late October, when both units were being maintained in safe shutdown conditions. Although water levels had begun rising, the improvement was not sufficiently widespread or sustained to meet the requirements for recommencing electricity generation.

Unit 1 was disconnected from the electricity network on 28 July following drought conditions and reduced Danube flows. Unit 2 was subsequently taken offline in August, leaving the station without its normal nuclear generating output while the operator monitored hydrological conditions.

The two reactors use Canadian CANDU 6 technology and have installed electrical generating capacities of approximately 700MW each. Unit 1 entered commercial service in 1996, followed by Unit 2 in 2007, establishing the station as a substantial component of Romania’s electricity generation infrastructure.

The CANDU design uses natural uranium fuel and heavy water as both a moderator and a primary heat transfer medium. Heat released through nuclear fission is transferred through the reactor’s primary heat transport system to steam generators, where a separate water circuit produces the steam required by the turbine-generator.

After passing through the turbine, the steam must be condensed and returned to the generation cycle. This process requires an external heat sink capable of removing waste heat from the plant, with cooling-water availability forming an important part of the conditions needed for sustained electricity production.

Cernavodă’s cooling arrangements depend on water associated with the Danube and the Danube–Black Sea Canal system. When river levels are unusually low, the amount of water available at the plant’s intake can restrict the operation of its generating equipment.

The external cooling-water system is distinct from the reactor’s primary heavy-water circuit. Restrictions affecting sustained electricity generation do not mean that the station has lost the ability to maintain its reactors safely shut down, as the relevant safety functions are supported by separate engineered systems and operating procedures.

Throughout the interruption, Nuclearelectrica reported that both units remained in a controlled, safe condition. The operator maintained the required nuclear safety functions through redundant equipment, established procedures and personnel assigned to the station’s ongoing operation and maintenance.

Its 29 October assessment recorded a local increase of approximately 10cm in the water level measured in the pump intake basin. However, the improvement was not supported by a corresponding sustained increase across the upstream Danube basin, and forecasts remained too uncertain to justify restarting either reactor.

The company determined that the water level required for safe reconnection needed to be maintained over several days and show an upward trend. A temporary increase in the intake basin was insufficient because the operating conditions must support continued cooling-water availability after generation resumes.

The earlier hydrological assessment left both generating units offline, with the operator continuing to monitor conditions. The subsequent decision to initiate Unit 1 restart procedures represents a distinct operational milestone rather than a repetition of the previous shutdown status.

Restarting a nuclear generating unit requires a controlled sequence of plant checks, equipment operations and authorisations. Relevant systems must meet the operating requirements established for the installation before reactor power can be increased and the turbine-generator returned to electricity production.

Nuclearelectrica has emphasised that the reconnection process must follow the station’s nuclear safety procedures. The decision to begin restart operations does not establish that the reactor has already been synchronised with the electricity network or that commercial generation has resumed.

Grid synchronisation requires the generator’s electrical operating conditions to be coordinated with those of the connected network. Voltage, frequency and phase relationships must satisfy the relevant requirements before the generator can be connected, after which its output can be increased in accordance with approved operating procedures.

Cernavodă exports electricity through Romania’s high-voltage transmission infrastructure, which is operated by Transelectrica. Returning Unit 1 to service would restore a major source of generation, although its initial output following reconnection will depend on the operating sequence and any technical restrictions applied during the restart.

The position of Unit 2 is more complicated because the two reactors rely on the site’s available cooling-water resources. Nuclearelectrica expects the operation of Unit 1 to reduce water levels in the shared intake basin, making it necessary to establish that the Danube can sustain the additional demand before the second generating unit is restarted.

Consequently, the conditions adequate for restarting one reactor are not necessarily sufficient to support both units simultaneously. Unit 2 will require a separate assessment and restart process once the operator is satisfied that the necessary hydrological and operating conditions have been achieved.

The extended interruption has also prompted Nuclearelectrica to examine measures that could reduce the impact of similar conditions in future. The company is working with relevant authorities and technical partners on short-, medium- and long-term options intended to improve the availability of its generating installations during periods of restricted water supply.

Those options have not yet been presented as completed engineering modifications. Any future changes will need to account for the station’s cooling-water requirements, nuclear safety obligations, environmental conditions and the practical operating limitations of the existing infrastructure.

The operator is separately progressing preparations for the long-term refurbishment of Unit 1, a programme intended to extend the reactor’s operating life. That investment project is distinct from the current restart operation, which concerns the restoration of generating capacity following the drought-related shutdown.

As of the latest formal operational update, the immediate milestone remains Unit 1’s planned resynchronisation on 6 November. Confirmation of successful connection will establish the next stage of recovery, while Unit 2’s return to generation remains conditional on a sustained improvement in Danube water levels and completion of its own restart requirements.