Stress tests take on Fukushima lessons

19 September 2011

European regulators have been publishing progress reports on the program of stress tests being carried out at nuclear power plants in response to the Fukushima accident.

In the weeks following the Fukushima accident, the European Council (EC) requested a review of safety at European nuclear power plants when faced by challenging situations. The criteria for the reviews, now known as stress tests, were produced for the European Commission by the European Nuclear Safety Regulatory Group (ENSREG). Progress reports were due to be submitted to the European Commission by 15 September, and many nuclear regulators and in some cases plant operators have published summaries, including regulators in Belgium, France, Hungary, Romania, Slovakia, Slovenia, Spain, Sweden and the UK.

The reports vary from country to country, but the take-home story emerging from the reports is that Europe's nuclear plants are generally well placed to withstand beyond-design-basis events. Some plants have already put into practice initial measures to improve safety in response to Fukushima, and the tests are bringing to light more measures that need to be taken to improve resilience on a plant-by-plant basis.


Some measures that have already been identified are simple to put into place: for example, housekeeping routines have been changed to reduce the potential for seismic interactions (where non-safety related equipment could impact or fall onto seismically qualified equipment) at UK power plants.

The stress tests focus on three areas highlighted by events in Japan: external threats from earthquake and flooding, specifically tsunami; the consequences of loss of safety functions, that is, a total loss of electricity supply (also referred to as station black-out, or SBO), the loss of ultimate heat sink (UHS), or both; and issues connected with the management of severe accidents. The UHS is a medium to which the residual heat from the reactor is transferred, for example the sea or a river.

While tsunami are not foreseen as a problem in Europe, the plants have been obliged to consider other external and internal initiating events that could trigger a loss of safety functions.

In France, a total of 150 nuclear facilities including operating reactors, reactors under construction, research reactors and other nuclear facilities are affected. In its progress report, French regulator Autorité de Sûreté Nucléaire (ASN) notes that the risk of similar phenomena to those that triggered the Fukushima accident is negligible and says that it prefers to submit a more comprehensive report for all of its affected installations later in the year. However, reports for the 80 facilities identified as priorities have been submitted and those for the country's 58 operating power reactors have already been published on the ASN's web site.

No fundamental weaknesses in the definition of design basis events or the safety systems to withstand them has been revealed for UK nuclear power plants from either the stress tests or from earlier national reviews, according to the progress report from the UK's Office for Nuclear Regulation (ONR). However, lessons are being learnt about improving resilience for beyond-design-basis events and removing or reducing cliff-edges, and will be applied in a timely manner, the regulator says.

Measures under consideration in the UK include the provision of additional local flood protection to key equipment and the provision of further emergency back-up equipment to provide cooling and power, while EDF Energy, operator of the country's AGRs and single PWR plant, is preparing additional studies to reconsider flood modelling for specific sites and to review recent climate change information that arrived subsequent to recent routine ten-yearly safety reviews. The main focus for the country's Magnox reactors will be to improve the reliability of cooling systems in the face of a variety of beyond-design-basis faults to reduce or minimise the potential for cliff-edges.

Evaluations of findings are still ongoing. Operators have up to 31 October to make their full report back to their national regulator, and regulators have until 31 December to make their full reports to the European Commission.

Researched and written

by World Nuclear News