Reviewing the periodic safety review

Since the first generation of commercial nuclear power plants started operating in the 1950s there have been substantial developments in safety standards and practices, and in technology, resulting in new scientific and technical knowledge, better analytical methods and lessons learned from operating experience. At the same time, for assessing the degree of plant conformity to those standards and practices, periodic safety reviews (PSR) are increasingly valuable.

Operational nuclear power plants in many states are subject to both routine and special safety reviews. It is a challenge to organise safety reviews that take into account improvements in safety standards and operating practices, modifications at the plant, cumulative effects of plant ageing, feedback from operating experience, and possible developments in science and technology. In many member states in the European Union (EU), the PSR has been introduced as part of the regulatory system, and frequently runs on a 10-year cycle. While every country through its regulatory body is free to choose its own safety review methodology, the minimum standards in the EU have to comply with the recommendations of the IAEA [1].

In 2003 the IAEA developed the Safety Guide NS-G-2.10 [2] according to which the objective of a PSR is to determine, by means of a comprehensive assessment of an existing nuclear power plant, the extent to which the plant conforms to current international safety standards and practices, the extent to which the licensing basis remains valid, the adequacy of the arrangements that are in place to maintain plant safety until the next PSR or the end of plant lifetime, and the safety improvements to be implemented to resolve the safety issues that have been identified. The purpose of this Safety Guide is to provide recommendations and guidance on the conduct of a PSR for an existing nuclear power plant. It proposes 14 nuclear safety factors to be screened in the scope of the PSR (Table 1). Through these safety factors, almost all of the nuclear safety aspects of a nuclear power plant are analysed. The 14 safety factors (SF) are a means to divide the safety assessment of the plant in homogenous areas.

However according to discussions to which Joint Research Centre - Institute for Energy and Transport (JRC-IET) participated in late 2010, the application of the IAEA safety guide may present some difficulties to the utility, more specifically the way some safety factors are to be screened and analysed. It was decided to call a workshop that would provide the opportunity for the participants to thoroughly discuss the identified issues and to share approaches and solutions. Such a workshop was convened 1-2 December 2010 at the JRC Petten site.

To better understand the way this safety guide is used by different utilities, JRC-IET collected information and experiences on practical application of IAEA NS-G-2.10 in the EU Member States in advance to the workshop. To facilitate information collection and use, a questionnaire was developed that concentrates especially on the practical application of IAEA NS-G-2.10. The dedicated questionnaire was distributed among the NPPs, utilities and technical support organizations in different EU and associated countries: AXPO (Switzerland), Bohunice NPP (Slovakia), Borssele NPP (Netherlands), Dukovany NPP (Czech Republic), EDF-British Energy (UK), Electrabel (Belgium), Fortum (Finland), Iberdrola (Spain), IRSN (France), Kozloduy NPP (Bulgaria), and Paks NPP (Hungary), representing in total 11 countries.

Results

Almost all participants develop a specific methodology for determining the status (strengths and weaknesses) of the safety factors that are defined in the IAEA Safety Guide NS-G-2.10. In general the methodology is based upon a systematic gap analysis against the general reference framework, regulatory guides, against international codes & standards and international practice. Shortfalls are ranked based on three or four categories of safety significance. In addition, works, which have already been identified by normal business processes, that is, existing prior to PSR, are recorded and ranked using a similar broad categorisation.

In particular, in relation to actual condition of systems, structures and components (SF2) the following methodologies were shared:

• The list of systems, structures and components to be considered during PSR is generally limited to the systems important to nuclear safety and one layer below. The lowest detail level is generally not considered. However, it may be if long term operation is a part of the review.
• A 15-step review process is defined starting with the definition of the relevant requirements and finishing with the summary evaluation of the findings.
• A review of in-service inspection and maintenance rule results of the last 10 years is carried out, as is a review of preventive maintenance and surveillance requirement results.
• The following items are reviewed against current national and international standards:
• significant developments over the review period
• relevant issues from other PSRs
• existing documentation (safety case, procedures, etc.)
• plant walk-down results
• plant performance and national and international operating experience
• shortfalls and enhancements
• ageing and obsolescence issues.
• A specific assessment based on the current approach is performed to ensure that there is sufficient maintenance of primary-circuit components, the reactor building, automation and electrical systems.

In relation to hazard analysis (SF7), the following methodologies were discussed:

• A four-step review process is carried out to review hazards including natural phenomena and man-initiated events
• Deterministic/probabilistic safety analyses include specific scenarios related to hazard analysis.

In the light of the Fukushima accident, it appears that the hazard analysis safety factor is becoming of utmost importance (see also box).

In relation to safety performance (SF8), participants reported reviewing licensee event reports, with focus on number and root causes, in search for negative trends (special attention paid to scrams, to unavailability of safety related systems, disturbances, and analysis of disturbances). Extensive review of radiation doses to personnel, to activity situation, activity releases, occupational exposure was also carried out.

In relation to use of experience from other plants, and of research findings (SF9), participants usually carry out a review of operational experience feedback processes, of internal and external operating experiences for the last few years (for example, for the last 10 years) and a review of actions taken or changes implemented based on these analyses.

In relation to human factors (SF12), participants offered different practices:

• A human factors programme defined between industry and regulators some years ago is reviewed, along with the evolution of the programme and future plans
• Only a limited review is carried out at present based on key safety cases, but this area is to be developed for future PSRs
• Human factors are reviewed with other personnel-related factors such as personnel availability at training, criteria for choosing personnel, procedures, man-machine interface, and so on.

Most participants said that they also perform a global assessment of safety. Such assessment is usually based on ranking of weaknesses, counterbalancing some weaknesses by some strengths, engineering judgments (to evaluate the combined effect of strengths and weaknesses), privileging the corrective actions that have a direct impact on nuclear safety, taking into account of previous PSR arrangements and of legislative requirements.

There are many different ways of performing the global assessment: probabilistic safety analysis (PSA) is widely used; both deterministic and PSA approaches are used with respect to defence in depth; and assessment of cost/benefit can be included. Other methods could serve, depending on the nature of the findings. One example given was a systematic method where gaps between guidelines and existing condition at the plant are evaluated according to a risk matrix describing potential effects and frequency of occurrence, after which potential effects are categorized.

Having done the research, all of the safety-relevant measures flowing from the global assessment, including all the technical, organizational, procedural, and administrative measures, are defined by their deadlines and compiled into an integrated implementation plan. This could include, for example, further development of the equipment qualification processes and procedures, minor hardware changes, configuration and organizational changes, documentation modifications and creation, processes improvements or establishment of new processes.

The most challenging issue raised at the meeting was related to the question of how to rank safety improvements. Practical advice suggested at the workshop included:

• Develop safety significance classification of PSR findings based on estimation of their real/potential influence on the defence-in-depth concept
• Findings identified as safety issues are to be assessed with respect to all safety issues and their interrelationship
• Multi-disciplinary team meetings are to be held for safety assessment of all identified safety issues in consideration of safety area interactions.

New standard

To overcome the shortcomings of NS-G-2.10 a new safety guide on PSR has been under development by the IAEA. Some utilities have been already using the draft of the new Safety Guide (DS426) [3] as a reference document for their PSR. The new document provides some more detailed information on PSR input in assessing long-term operation as well as general methodology of assessment of the safety functions. It emphasizes that during assessment, the priority should be given to meeting the requirements of national safety standards rather than international ones. Also, the topic of global assessment becomes its own section for more detailed explanation of the process, including assessment of the risks associated with the findings during review of particular safety factors. Also, responsibilities of the regulatory body are more precisely specified. For instance, it will be clearly stated that the regulatory body should establish requirements for PSR, as well as to approve milestones and timeframes provided by the operating organization.

Conclusion

The workshop demonstrated that well-targeted exchanges among utilities about practical use of IAEA safety guides such as NS-G-2.10 and its successor enhances the diffusion of good practices.

Christiane Bruynooghe, European Commission, DG for Research and Innovation - CDMA 4-160 BE-1049, Brussels, Belgium (formerly at JRC-ITE, where among other tasks she served as organiser and moderator of the 2010 utility conference); Benoît Lance, Electrabel (GDF-SUEZ), 34 Boulevard Simon Bolivar, B-1000 Brussels, Belgium; Alexander Renev, Senior Manager, JSC Rusatom Overseas, 23A, 21st floor Tarasa Shevchenko Emb., Moscow 121151 Russia

References

[1] Council Directive 2009/71/Euratom of June 2009 establishing a Community framework for the nuclear safety of nuclear installations.

[2] IAEA, Periodic Safety Review of Nuclear Power Plants, Safety Guide NS-G-2.10, Vienna, 2003.

[3] IAEA, Periodic Safety Review of Nuclear Power Plants, Draft Safety Guide DS426