Cover: Earthquake of Basel (1356) in the “Chronicon Helvetiae” by Christoph Silberysen, 16th century. Aargau Cantonal Library

(translation by the Blog author)

ENSI communication https://www.ensi.ch/fr/2021/06/16/centrale-nucleaire-de-beznau-non-respect-de-prescriptions-de-montage-sur-des-generateurs-diesel-de-secours/

In an announcement dated 16 June 2021, ENSI provides information on an incident at the Beznau nuclear power plant: “Beznau nuclear power plant: non-compliance with installation regulations for emergency diesel generators: Non-compliance with installation regulations in the engine mountings of emergency diesel generators at the Beznau nuclear power plant, which had existed since the emergency systems were installed in 1992 and 1993, led to a reduction in seismic resistance. Nevertheless, thanks to various upgrades and other measures, the legal requirements for seismic safety were met, with the exception of the months of April and May 2012. This was the conclusion reached by ENSI following the examination of the December 2020 event, when the Beznau nuclear power plant immediately shut down both units after discovering the non-compliance in order to rectify it.”

Missing shock absorbers

“Shock absorbers are used to limit the movements of the diesel engine. If they are missing, the earthquake resistance of the whole system is reduced. Despite the missing shock absorbers, an earthquake that could occur every 10,000 would not have resulted in impermissible relative displacements under the relevant seismic hazard assumptions valid until 2012. This does not apply to the new seismic hazard assumptions, which have been significantly increased following the Fukushima accident. An earthquake that could occur every 10,000 years under the new seismic hazard assumptions would not have been controlled without a shock absorber.”

Safety significance of the incident according to ENSI

ENSI assessed the event and came to the following conclusion: “The event is safety relevant and is therefore classified as INES-1 on the international event scale. This is the lowest classification on the seven-level INES event scale. It describes deviations from normal operation that do not require the intervention of safety systems.

The particularity of this incident is that the deviation in the assembly had not been detected for a long time. ENSI will therefore examine whether the relevant guidelines defining the scope of ENSI inspections and the scope of supervision by the Swiss Association for Technical Controls need to be adapted.”

Comment by the authors of the blog

In the aftermath of the Fukushima disaster, ENSI found that the seismic protection of Swiss nuclear power plants was inadequate. As a result, upgrades were required in 2012 and 2015 to meet the new post-Fukushima requirements.

Two remarks:

– Firstly, it must be assumed that Swiss nuclear power plants were exposed to seismic risks for all the decades after they were commissioned and up to the time of their upgrades (i.e. not only in April and May 2012, as suggested by ENSI).

– We note that once again the control failed, and a defect remained undetected for decades. The present incident is reminiscent of the incident of 24 July 2014, entitled “KKL: damage to primary containment”, when the perforation of the primary containment during the installation of fire extinguishers in 2008 was only discovered after six years.

Our conclusion on the fundamental principles according to which ENSI carries out its supervisory function: After the retirement of Hans Wanner, Director of ENSI and long-time President of WENRA, it is time to revise the famous motto of the former Director: “The question is on what working assumption we base our supervisory function. There are two options: either ‘Swiss nuclear power plants are basically safe’ or ‘Swiss nuclear power plants are basically unsafe’. As I have already explained on several occasions, we proceed on the basis of the first working hypothesis, which we continually substantiate with data and facts in an ongoing internal process”. It would certainly be useful for the safety of the installations if upgrades and other maintenance work (including that of previous years) were systematically checked and re-checked. Who knows how many such or even more serious errors have occurred in Swiss nuclear power plants and continue to exist undetected to this day?