the european stress test for nuclear power plants

The approach of ENSREG and of the Commission

Frame of the “Stress test“

The original goal was redefined by ENSREG with the help of the Western European Regulators Association (WENRA): 

“For now we define a “Stress test“ as a targeted reassessment of the safety margins of nuclear power plants in the light of the events which occurred at Fukushima: extreme natural events challenging the plant safety functions and leading to a severe accident.” 4 

By this agreed definition a “comprehensive risk assessment” as was foreseen by the EU-Council had gone out of focus. In principle only initiating events being reviewed are those that have been highlighted by the Fukushima accident: earthquakes and flooding. 

Independent of any special kind of event the loss of the components that are needed to transfer the remaining heat after reactor-shutdown safely into the environment shall be additionally assumed.

According to the Declaration of ENSREG the risk of airplane crashes on nuclear power plants will not be considered: 

“Risks due to security threats are not part of the mandate of ENSREG and the prevention and response to incidents due to malevolent or terrorists acts (including aircraft crashes) involve different competent authorities, hence it is proposed that the Council establishes a specific working group composed of Member States and associating the European Commission, within their respective competences, to deal with that issues. The mandate and modalities of work of this group would be defined through Council Conclusions.” 6 

Technical Goal of the “Stress test“

Under the limited extreme scenarios of the ENSREG approach the “test” will consist in finding out what means will remain in the case of the narrowly defined Fukushima scenario for each plant, to prevent or to mitigate radioactive emissions. This is the main and superordinate target of the whole test. To achieve this goal some key questions need to be answered, in particular how to maintain the three fundamental safety functions, control of reactivity, fuel cooling and confinement of radioactivity.

Therefore a key question of the test will be how the recriticality of the reactor can be avoided even if control systems are no longer available, such was the case with Fukushima. Recriticality means the restart of fission and the additional production of heat that could quickly destroy all barriers. 

Another key question will be by what additional devices and procedures the core and the fuel storage pool can be cooled, and what could be done if cooling gets insufficient. 

The third key question of the test asks with which devices or procedures the radioactivity can be kept within the containment, or by which means can emissions be mitigated if a plant were faced with the Fukushima scenario.8 

One important new feature which is applied in this test is the evaluation of the so called “cliff edge effects”. A cliff edge effect is a qualitative degradation of the plant’s safety conditions. 

“A cliff-edge effect could be, for instance, exceeding a point where significant flooding of plant area starts after water overtopping a protection dike or exhaustion of the capacity of the batteries in the event of a station blackout.” 

Another important feature is the evaluation of how long it takes until critical situations arise when cooling is insufficient, for example, how long it takes before fuel rods start to melt. 

Such effects and their consequences for the safety of the plant were – untilnow – not under consideration in the frame of evidence for licensing conditions or periodic safety reviews. In this respect the “Stress test“ goes beyond the borders of the ordinary safety analysis’ of the past and may give new insights of the plants response on these extreme situations. The test may therefore result in technical and organizational recommendations enabling plants to be better prepared in the case of such accidents.

Structure of the test

The report shall consist of four main parts:10

  • an up-to-date plant description 
  • a description of the provisions taken in the design basis of the plant and a description whether the plant copes with its design requirements
  • an assessment of the so called “robustness” beyond the design basis in the case of earthquake and flooding and loss of electrical power. The assessment shall give information on how the plant specific defence in depth concept contributes to safety when it faces the scenarios of flooding, earthquake and the loss of electrical power.
  • Potential improvements.

The review process of the reports of the 143 European plants shall be finalized within about four months, beginning in January 2012 and ending at the end of April 2012. To assure an equal level of assessment some experts shall be nominated members in each of the peer review teams.11


4 ENSREG (European Nuclear Safety Regulators Group), Declaration of ENSREG, Annex 1, EU

“Stress test” specifications, Brussels 31.05. 2011,, page 1

5 That means the loss of the ultimate heat sink, loss of electrical power, ENSREG, page 4

ENSREG (European Nuclear Safety Regulators Group), Declaration of ENSREG, Brussels 31.05.2011,

7 See fn. 4, page 4 (downloadable document)

8 See id. (downloadable document)

9 See id., page 2 (downloadable document)

10 See id., page 5 (downloadable document)

11 Fn. 4, page 3 (downloadable document)