Repair work under way at Krško nuclear power plant

The reactor was shut down on 6 October at 05:30 as a precaution after indications there was a leak in the primary system in the containment building. Subsequent investigation discovered what the company said was a "hole the size of a sewing needle at the weld of the reducing part of the piping connection system of the primary circuit, one metre from the reactor vessel".

Possible causes of the problem were identified as "a hidden defect in the welding process, a defect in the material, thermal fatigue of the material, stress corrosion, and mechanical stress". All 121 fuel elements were removed from the reactor vessel and the pipeline ultrasonically inspected to discover the cause of the leak.

NEK said the investigations suggested an "additional anomaly in the pipeline material was identified in the leaking pipe" which led to the decision to replace the affected pipeline. A "conservative" decision was taken, together with original equipment supplier Westinghouse, to also replace pipeline on the other line of the system.

"The possibility of an increased scope of remediation was already considered when ordering the necessary material ... all the material is already available in Krško, remediation is in full swing. The first pipeline is successfully removed; preparations are under way for the removal of the second and the installation of new pipeline segments. For now, plans to reconnect the power plant to the grid in mid-November remain unchanged," said NEK.

The repair work is being overseen by the Slovenian Nuclear Safety Administration. In its report on the initial discovery of the leak it explained: "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."

While the shutdown takes place some outage activities which the safety administration had approved for the scheduled shutdown next year are being brought forward, which may shorten the duration of the 2024 outage.

Krško is a 696 MWe Westinghouse pressurised water reactor located on the Sava river. It is unusual in being jointly owned by two countries: Slovenia, where it is located, and Croatia, both of which were parts of the former Yugoslavia when Krško came online in 1981 - NEK is jointly owned by Croatia's Hrvatska elektroprivreda (HEP Group) and Slovenia's GEN Energija. It supplies as much as 40% of Slovenia's electricity and, with a lifetime extension, is due to operate until 2043.