Beznau 2 can continue operating until the start of the annual overhaul in mid-August

Block 2 of the Beznau nuclear power plant does not have to be shut down early. The operator Axpo was able to plausibly demonstrate to the Swiss Federal Nuclear Safety Inspectorate ENSI that even if one were to assume that the reactor pressure vessel of Beznau 2 had similar material defects to those in Beznau 1, the safety operation of the reactor pressure vessel  is still sufficiently guaranteed.

Kernkraftwerk_Beznau_ENSI (1)When demonstrating the safety of the reactor pressure vessel, the operator worked from the assumption that the reactor pressure vessel of Beznau 2 has the same defects as those in Block 1.

Because Beznau 2 has relatively high margins for safety against brittle fracture  of the reactor pressure vessel, the likelihood of an accident during the remaining three weeks until the planned shutdown for the overhaul, even when one assumes the defects are present, is extremely small.

The probability of failure of the pressure vessel is less than once in a billion years, which makes it more than a thousand times lower than the statutory limit for design accidents.

ENSI has arranged for its own estimates independent of this report. “The evidence that Axpo has submitted to us plausibly demonstrates that a short-term preliminary shutdown of Beznau 2 is not necessary”, says Georg Schwarz, Deputy Director of ENSI and Director of the Division for Nuclear Power Plants, as he sums up the assessment.

The present assessment is not a blank cheque

Georg Schwarz emphasises that this is not a kind of “blank cheque”. “This assessment applies to the brief operating time that remains.” For a definitive assessment it will be necessary to carry out a thorough inspection of the reactor pressure vessel of Beznau 2 as well. Axpo plans to shut down Beznau 2 on 12 August 2015 for the annual overhaul. During this overhaul the special inspections of the base material of the reactor pressure vessel that were ordered by ENSI in 2013 are also to be carried out.

Brittle fracture

The irradiation by high-energy neutrons causes changes in the properties of the materials of the reactor pressure vessel. In particular the irradiation leads to a reduction in the toughness of the steels. The reactor pressure vessel becomes brittle and this makes it less resilient to mechanical loads.

This is also why an increased brittleness of the reactor pressure vessel is given central importance in the shutdown directive of the Federal Department of the Environment (UVEK). This directive specifies precisely when a nuclear power plant must be shut down due to this phenomenon of ageing. If the criterion is met the power plant must be “immediately shut down for the time being”, the directive says.

ENSI had asked Axpo to state the grounds why a continued operation of Beznau 2 is possible from the technical safety aspect. Axpo submitted the report to ENSI last week, together with the report about the ultrasonic indications on the reactor pressure vessel of the Beznau 1 reactor.

ENSI brings in international experts

Work on the assessment of the reportable indications in Block 1 of the Beznau nuclear power plant has started. Axpo is obliged to submit to ENSI proof of the safety of the reactor pressure vessel. It then has to examine if and how the reactor pressure vessel may have been weakened because of the findings, and whether it still meets the regulatory requirements. “We have already made plans to bring in the assistance of international experts”, says Georg Schwarz. They will help ENSI in making the assessment. Furthermore, ENSI has been in regular contact with the Belgian authorities since 2012.

Georg Schwarz again stresses the fact that there is no time pressure for Beznau 1 with regard to the safety case of the reactor pressure vessel. “We will only give the green light to Beznau 1 after examining the evidence for safety and only when we are certain that the safety is guaranteed.”