The EU stress test is part of the review process which Switzerland initiated immediately after the reactor accident in Japan.
As a direct consequence to Fukushima, ENSI issued three orders to the Swiss nuclear power plants on 18 March 2011, on 1 April 2011 and on 5 May 2011, in which immediate measures and additional re-assessments were required. The immediate measures include setting up an external storage facility for emergency equipment for the nuclear power plants, including the necessary plant-specific connections and back-fitting of feed lines for the external supply of the spent fuel pools. The re-assessments to be conducted immediately (among others on the basis of the provisional shutdown Ordinance) focused on the design of the Swiss nuclear power plants in respect of earthquakes, external flooding and the combination of both events, as well as on the coolant supply for the safety and auxiliary systems and the spent fuel pool cooling. In parallel with these investigations, inspections related to key issues were carried out, namely inspections of the existing spent fuel pool cooling systems, of the protection against external flooding and of the filtered containment venting systems.
In a fourth order on 1 June 2011, the Swiss licensees were asked to take part in the EU stress test. The content and timeframe for the EU stress test were stipulated by the EU Commission in a Specification on 25 May. This specification was drawn up by WENRA (Western European Nuclear Regulators Associations) and adopted by ENSREG (European Nuclear Safety Regulators Group), which comprises members of the nuclear supervision authorities of the EU member states.