Krško NPP performs the analyses of the cause of the primary coolant leak that led to the plant shut down on 5 October 2023. The primary coolant leak was detected at a weld location on the safety injection system (Figure 1), at a distance of one meter from the reactor (Figure 2).

The primary coolant leak detected during full power operation did not exceed the operational limit that would require shut down of the plant. The integrity of the primary system is continuously controlled during plant operation by various leak testing systems, mainly: radioactivity of particles and atmosphere conditions in the containment, measurement of liquid level at the bottom of the containment sump and water inflow from the volume control system. The indication of the leak was identified from the trend of the water inflow from the volume control system. The leak rate was approximately a factor of 4 below the operating limit, however, the plant personnel decided to shut down the plant as a precautionary measure. All other parameters (containment pressure and temperature, primary coolant radioactivity, etc.) were within normal conditions during this situation and were not affected by the leakage. Similarly, all parameters were within normal conditions during shut down of the plant.

The weld location of the leak (Figure 1) will be a subject of a thorough analysis. A non-destructive ultrasonic examination of the location of the leak is expected to be carried out tomorrow. Also the other line of the safety injection system connected to the opposite side of the reactor will be examined, as nuclear safety is ensured also by the redundancy of several equivalent safety systems.

The fuel assemblies are foreseen to be transferred from the reactor to the spent fuel pool early next week. Once the fuel has been transferred and the cause of the leak will be determined, corrective actions will begin. The weld repair will be performed in a challenging area, particularly due to the proximity to the reactor pressure vessel and the resulting high radiation level. Ensuring the radiation protection of the workers carrying out the inspection and repair work is one of the priorities of the Krško NPP and of the supervision of the SNSA.

The unplanned shutdown of the Krško NPP will also be an opportunity to carry out some outage activities. Some modifications and upgrades will be carried out by the Krško NPP for which the SNSA has already issued an approval and for which all conditions for implementation are currently met (materials and parts are supplied, subcontractor support ensured, etc.). Some other minor activities will also be performed, which may shorten the duration of the regular outage and fuel replacement planned in 2024.