The accident at the Fukushima nuclear power plant in Japan triggered the need for an immediate and coordinated response from the EU, aiming to identify potential further improvements, in line with the fundamental principle of the continuous improvement of nuclear safety.
On 25 March 2011, the Heads of States and Governments of the EU Member States, reunited in the European Council, concluded that the safety of all EU nuclear plants should be reviewed, on the basis of comprehensive and transparent risk and safety assessments ('stress tests'):
"The European Nuclear Safety Regulators Group (ENSREG) and the Commission are invited to develop as soon as possible the scope and modalities of these tests in a coordinated framework in the light of lessons learned from the accident in Japan and with the full involvement of Member States, making full use of available expertise (notably from the Western European Nuclear Regulators Association); the assessments will be conducted by independent national authorities and through peer review; their outcome and any necessary subsequent measures that will be taken should be shared with the Commission and within the ENSREG and should be made public; the European Council will assess initial findings by the end of 2011, on the basis of a report from the Commission".
WENRA is a network of Chief Regulators of 16 EU Member States and Switzerland, as well as of other interested European countries which have been granted observer status. During their plenary meeting of 23-23 March 2011, WENRA developed a technical definition of the 'stress tests', as well as an initial approach on how they should be applied to nuclear facilities across Europe in terms of methodology and timeframe.
A dedicated WENRA Task Force met on 5-6 April 2011 to refine and further elaborate the initial WENRA proposal related to the scope, methodology and timeframe of the 'stress tests'. Following its endorsement by WENRA, this technical proposal is currently presented on the WENRA website for stakeholders' information (document dated 21 April 2011). The objective is to present the finalised WENRA paper to the 15th ENSREG meeting of May 2011, as a basis for discussion and possible endorsement.
Objectives of the proposed WENRA 'Stress tests' specifications
The WENRA paper contains a general section presenting the definition of the 'stress tests', their technical scope and the process to perform the 'stress tests' and their review. The next sections of the document set out the general information required from the licensees and the issues to be considered by the licensees for each considered extreme situation. In summary, the 'stress tests' are defined as a targeted reassessment of the safety margins of nuclear power plants in the light of the events which occurred at Fukushima: extreme events challenging the plant safety functions and leading to a severe accident.
This reassessment will consist in a verification of the preventive measures and in an evaluation of the response of a nuclear power plant when facing a set of extreme situations, chosen following a defence-in-depth logic (initiating events, consequential loss of safety functions, severe accident management issues). The preferred approach is deterministic, i.e. sequential loss of defence is assumed in the defined extreme situations. For a given plant, the reassessment will report on the effectiveness of the preventive measures and on the response of the plant, noting any potential weak point and cliff-edge effect, for each of the considered extreme situations. The licensees have the prime responsibility for safety. Hence, it is up to the licensees to perform the reassessments, and to the regulatory bodies to independently review them.
During the regulatory reviews, interactions between European regulators will be necessary and will be managed through WENRA or ENSREG as needed.
WENRA proposal – 'Stress tests' specifications
For reference - WENRA First proposal about European “stress tests” on nuclear power plants