Switzerland’s five nuclear power plants must prove the containment buildings surrounding their reactors can withstand a major earthquake despite passing a European Union stress test.

The Federal Nuclear Safety Inspectorate said on Tuesday that it had made the demand following the results of the EU test on atomic facilities in which Switzerland took part.

Officials said that while all of the country’s plants fulfilled the test’s criteria, there were a number of points that demanded further clarification. It pointed out that while containment buildings could withstand an earthquake, the margin of security was not large.

Depressurisation systems in the confinement buildings of the Gösgen and Leibstadt reactors will also be checked for earthquake resistance after it appeared they could be more vulnerable than the buildings themselves.

The operators of the Mühleberg and Gösgen facilties have been asked too to analyse where the flow of the River Aare, which helps cool their reactors, could be blocked by debris.

Mühleberg near the capital Bern was singled out for further requirements. It must provide by the end of the month data on the seismic resistance of the Wohlensee dam, one kilometre upstream from the plant, and its emergency shutdown system.

The EU stress tests were carried out to analyse the security situation of all European nuclear power plants in the wake of last year’s Fukushima disaster in Japan.

In compliance with the JTI standards

More: SWI swissinfo.ch certified by the Journalism Trust Initiative