Post-Fukushima response | Upgrading

Results of the stress tests

1 July 2012

One of the most intense multinational safety reviews of nuclear power, the post-Fukushima European complementary safety tests, or stress tests, are now winding up. The unprecedented effort established safety benchmarks across Europe. By Will Dalrymple

Complementary safety assessments of 140 nuclear power plants in 17 countries (15 in the European Union plus Switzerland and Ukraine) were produced, analysed by national regulators, and were in turn analysed in a peer review process organised by the European Nuclear Safety Regulators Group (ENSREG).

In fact the stress tests, which were mandated (and paid for) by the European Commission, also considered security; an ad hoc group on nuclear security evaluated the units’ resistance to attack. Its conclusions were excluded from the public report.

All of the countries included have taken ‘significant steps to improve the safety of their plants,’ said ENSREG’s final report, released on 25 April. However, it noted that there was lots of variation between the countries in their approach and in the degree of action taken so far. Action plans have already been or are soon to be defined for each country covered.

ENSREG said that European regulators should consider four main areas of safety improvement:

1. Standardise extension of safety margins beyond the design basis. Although the peer reviews found that the national reports were generally compliant with ENSREG guidance for earthquakes and flooding, national regulators were found to be inconsistent when it came to events that exceed the safety design-basis, and to assessments of cliff-edge effects, in which a relatively small change has a major effect. It has recommended that the Western European Nuclear Regulators Association (WENRA) develop guidance.

2. Plan to carry out another periodic safety review in 2021. The report said the peer review process demonstrated the utility of carrying out a periodic safety review, particularly for natural hazards. In many countries in Europe, nuclear reactors’ licences are reassessed every 10 years.

3. Start carrying out containment integrity protections now. The ENSREG report calls this a ‘crucial’ issue, and recommends that operators make improving the resilience of containment integrity an urgent priority.

4. Improve defences against natural hazards.

ENSREG’s review-of-the-reviews was as much about evaluating regulators as it was about evaluating nuclear power plants. The stated goals of the peer review were to check compliance of the stress tests to the guidance, to ensure that nothing significant had been overlooked, to identify strengths and weaknesses of existing plant, and to consider post-Fukushima proposals to increase plant safety.

The reviews broke the issue of safety down into three topics: natural hazards, loss of safety systems, and severe accident management. In addition to the overall conclusions above, it produced lots of suggestions of potential improvements, both in general and by country (see table).

In terms of the first topic, natural hazards, it suggested that an appropriate hazard level for external events is a 10,000-year return period, that is, a 10-4/yr annual frequency. It also suggests using this level for back-fitting. It suggests that countries consider a plant flooding analysis using incrementally increased flooding levels. It suggests that operators should consider developing standards to address qualified plant walkdowns with regards to earthquake, flooding and extreme weather to make sure that operational requirements are maintained for event safety cases.

Examples of good practices included:

1. Consider a hardened core philosophy: independent set of structures, systems and components that can withstand beyond design basis earthquake/flooding (see also box, right)

2. Provide rugged mobile equipment to perform safety functions, and make sure that the equipment is stored safely

3. Assess hazard cases and improve plants using periodic safety review

4. Implement permanently-installed seismic monitoring and alarm systems

In the loss of safety systems topic, the report summarised the amount of time European reactors gave before the onset of a loss of power. The amount of time it took for the temperature of the fuel to exceed a safe level following a station blackout (SBO) with no action varied by reactor type and size, from 30-40 minutes for some BWRs, to 1-4 hours for a large operating PWR, to 10 hours for a small PWR. The time it would take for a spent fuel pond to boil off in the worst case, where it contained an entire fuel load with no make-up water, could be as short as 7-9 hours. Based on on-site supplies of demineralised water, diesel generators could last for between 72 hours and eight days. Typical battery life was reported between 1-3 hours. Reactor coolant pump (RCP) seal integrity would typically last a few hours before failing, further draining the primary circuit. Plant ventilation can also be a limiting factor, because after a few hours of stoppage plant equipment could overheat and cut out.

To mitigate loss of safety systems, it made five key recommendations.

1. Availability of a variety of mobile devices, with prepared quick connections, procedures on how to connect and use and staff training for deployment.

2. Using alternative means of cooling, including alternate heat sinks, such as steam generator?(SG) gravity feeding, supply from stored condenser cooling water, alternate tanks or wells on the site or in the vicinity.

3. Ensuring the supply of fuel and lubrication oil, and battery load-shedding.

4. Additional equipment and trained staff made available to deal with issues on multi-unit sites.

5. Consider using the atmosphere as an alternative ultimate heat sink (with cooling towers or spray ponds).

Good practices included the provision of an independent supply of cooling water for pumps and diesel generators, for example using fire-water supply, using borated steel in the spent fuel pond so that injection of fresh (unborated) water would not lead to recriticality concerns, and installing replacement air accumulators on important valves to increase the robustness of valve operation.

In terms of severe accident management, the report acknowledged that countries throughout the study region are aware of the issue, and are making improvements. The basics are being dealt with adequately, it said. However, it also criticised the plans. It said that in general, the prevention aspects of severe accident management are better developed than mitigation aspects. It also said that a post-Fukushima lesson is that the SAM scope needs to be extended to take account of plant shutdown states, multi-unit events, long-duration events and accidents initiated in spent fuel pools (SFPs).

In particular, the means for maintaining containment integrity should be improved, including depressurization of the reactor coolant system, prevention of damaging hydrogen explosions, and means of addressing long-term containment over-pressurization, such as filtered venting.

The SAM section ruled little out in what it expects operators to consider. It says, “Training and exercises aimed at checking the adequacy of SAM procedures and organisational measures should include testing of extended aspects such as the need for corporate and national level coordinated arrangements and long-duration events.”

Even events with low numerical risk should be considered if their consequences are very high, it said.

The peer review process was guided by a seven-member panel of senior nuclear regulatory figures. Each member state plus Switzerland and Ukraine was allowed to nominate one expert for each topical area review team. There were in total 70 reviewers from 24 countries, plus observers from the USA, Canada, Croatia, the UAE and Japan.

The first step of the process was a desktop review of country reports, which generated questions worked through in a two-week seminar in Luxembourg. Then four-day long country visits began, in which teams of eight were assembled based on the principle of at least two reviewers for each topical area, and visited a plant. ENSREG also held a public meeting in January, and held another in May.

The report pointed out that the effort was hugely labour-intensive, and said that it should be considered an ‘exceptional exercise.’

Author Info:

This article was first published in the June 2012 issue of Nuclear Engineering International.

Will Dalrymple is editor of NEI.

Related Articles
First European stress test results published
European stations pass 'stress tests'

Hardened core

French radiological protection agency IRSN proposed a new concept of a "hardened core" that would over-protect the most important safety functions in a plant in its review of the French operator's report in November 2011. The French regulator ASN picked up this strand and has requested that utility EDF define and deploy such a system by end June 2012.

The hardened safety core would form an ultimate hazard protection system, protecting the reactor from beyond-design basis natural threats and long-term loss of electrical source or heat sink systems.

ASN said that the goal of the system would be to prevent a severe accident or limit its progression, limit uncontrolled large-scale releases in an accident scenario, and enable the licensee to perform emergency management.

To do this, it has asked EDF to adopt "significant" fixed safety margins above the current baseline requirements.

IRSN said that only a limited number of hardened safety core structures and equipment designed to withstand natural risks must be over-protected.

The equipment, which needs to be maintained to function, should be protected from accidents and their effects, including falling loads, impacts from other components and structures, fires and explosions, ASN said.

FilesTable 1: Highlights of peer review country reports

Privacy Policy
We have updated our privacy policy. In the latest update it explains what cookies are and how we use them on our site. To learn more about cookies and their benefits, please view our privacy policy. Please be aware that parts of this site will not function correctly if you disable cookies. By continuing to use this site, you consent to our use of cookies in accordance with our privacy policy unless you have disabled them.