The Krško NPP, which was shut down in early October after the discovery of a leak in the primary circuit connection system, is back in operation following repairs to the pipeline, according to operator Nuklearna Elektrarna Krško (NEK). The plant began running at 28% and will gradually increase its power by operating procedures, NEK said. “Organisations authorised by the Slovenian Nuclear Safety Administration (SNSA) gave a positive opinion for the start-up after an independent control of the activities.” After Krško NPP submitted the SNSA Analysis Report After Deviation and supporting studies, SNSA confirmed that the plant” had implemented all necessary and expected measures and fulfilled the prerequisites for safe operation”.

Part of the NPP's safety injection system pipeline, where a leak on the weld location had been detected, was replaced. The part of the pipeline on the second branch of the safety injection system was also replaced as a precautionary measure, although no trace of leakage was found there. “After the successful inspection of the installed materials and welds on the pipelines, the standard procedures for entering the criticality state and further restart of the plant were initiated,” NEK noted.

The short-term measures included non-destructive ultrasonic testing of the removed pipelines, functional testing of snubbers on the safety injection pipelines and installation of a temperature monitoring system on the safety injection pipelines. As part of the long-term measures, the NPP will carry out a detailed analysis of the cause of the leak. The two dismantled parts of the pipeline were sent abroad for destructive and metallographic examinations of the material and welds in a suitable hot cell laboratory. Based on the results of these analyses, the Krško NPP will take additional technical measures if necessary.

NEK said replacement of both pipelines was a complicated procedure due to the radiation in the vicinity of the reactor vessel and the contamination of the pipelines. “No worker received a radiation dose above the legal limits. The measured collective dose of all workers was lower than the dose estimated before the start of the repair work. The NPP personnel were well prepared for the repair work on the pipeline, all protective measures for the workers were taken, and effective decontamination of the work area and the pipelines was carried out.”

The impact of the unplanned outage on the environment and the population was very low and comparable to the impact during normal operation of the NPP. Gaseous discharges to the environment were slightly increased during the unplanned outage, but the levels were still comparable to those during regular outages. All radioactive discharges were well below the legal limits set for the NPP.

Image: Krško nuclear power plant (courtesy of NEK)