RISK INFORMED APPROACHES FOR PLANT LIFE MANAGEMENT: REGULATORY AND INDUSTRY PERSPECTIVES

1. RISK INFORMED APPROACHES FOR PLANT LIFE MANAGEMENT: REGULATORY AND INDUSTRY PERSPECTIVES Björn Wahlström

2. Paper outline • Introduction • Building safety into nuclear power plants • A basis for safety • The deterministic approach • The probabilistic safety analysis • Applications of the risk-informed approach • Regulatory oversight • The basis • Systems of requirements • Prescriptive versus performance based regulation • Challenges for the future • Changes in the regulatory systems • Plant life management • Harmonisation of safety requirements • A new safety philosophy • Conclusions

3. The construction of safety • Threats together with their likelihood and consequences • eliminate some of the threats • decrease the likelihood of the remaining threats • mitigate their consequences • Two roles • the operator has the undivided responsibility for safety • the regulator issues safety requirements and inspects that they are fulfilled • The utilisation of operational experience • Two generic questions • how complete is the analysis? • what is safe enough?

4. Deterministic requirements defence in depth principle design basis accidents single failure criterion New needs a broader set of potential threats logical means for prioritising threats identify and quantify sources of uncertainty adapt to the true complexity of a NPP Probabilistic criteria reliability of RPS core damage probability large radioactive releases Applications of PSAs plant modifications targeting of maintenance and inspection actions prioritizing outstanding corrective actions changes in testing, inspection and monitoring requirements identification of safety importance of components Deterministic and probabilistic approaches

5. The risk-informed approach enhances the traditional deterministic approach Because it is • explicitly considering a broader range of safety challenges • prioritizing the challenges on the basis of risk significance, operating experience, and engineering judgment • considering a broader range of counter measures to mitigate the challenges • explicitly identifying and quantifying uncertainties in analyses • testing the sensitivity of the results to key assumptions

6. Impediments to the increased use of risk-informed regulation • Inadequacies and incompleteness in PSAs. • Costs involved in establishing adequate PSA models. • The need to revisit risk-acceptance criteria. • Lack of guidance on how to implement defence in depth and on how to impose sufficiency limits. • Lack of guidance on the significance and appropriate use of importance measures. • Variation of PSA quality and scope and the need for standards.

7. The comparison of two PSA-studies • nearly identical NPPs (Forsmark 3, Oskarshamn 3) • two different teams responsible for the PSAs • two different projects (purpose, scope, resources, time schedule) • different assumptions and boundary conditions • differences in initiating events (identification, categorisation, frequencies) • differences in event tree analysis (end states, success criteria, models) • different approaches for the qualitative systems analysis • differences in fault tree modelling and analysis • different interpretations and judgements used for the failure data • differences in CCF modelling principles • different performance shaping factors in HRA models Comparison of PSAs from different NPPs is not meaningful!

8. Shortcomings in present PSA models • assumptions are often implicit • simplistic failure models • large variations in models of event sequences • availability of plant specific data • programmable I&C • human reliability models • organisational factors • interpretation of probabilistic safety goals • a PSA-study is never complete PSAs have a good internal validity, but the external validity is seldom satisfactory!

9. Regulatory oversight The basis • national practices and legislation • construction and operation is based on a license • regulatory requirements reflect a collected operational experience Systems of requirements • design targets and conditions for acceptability • the process of verification and validation Prescriptive versus performance based regulation • prescriptions on features, actions and/or programmatic elements • regulation anchored in defined performance goals • a fear that new regulatory burdens will be introduced • finding measurable or calculable performance parameters

10. Characteristics of good safety requirements • aspects important for safety are addressed • a definition of a safety envelope is given • they provide guidance for designers and operators • they provide guidance for regulatory inspectors • they are written with an appropriate level of detail • guides are balanced, consistent and non-contradictory • it is easy to find specific requirements • interpretations are stable over time and independent of inspectors • requirements are harmonised with other countries • regulatory decisions are documented and communicated • requirements on the same level are equally binding • requirements are updated regularly to reflect new experience • changes are introduced in a consistent way • changes in guides are not an end in itself

11. Challenges for the future Changes in the regulatory systems • new regulation (severe accidents, human factors, digital I&C) • an obstacle is the bulk of regulation that has to be changed Plant life management • a deregulated electricity market, additional cost pressures • need to navigate between new requirements and modernisations Harmonisation of safety requirements • a large diversity in national regulatory approaches • harmonisation of safety management in safety critical industries A new safety philosophy • a combination of deterministic and the probabilistic approaches • new components (models, operational experience, safety culture)

12. Conclusions • deterministic safety principles will not be abandoned • better models and tools for the PSAs are needed • a frame for reasoning about safety in many different settings • taking a cost-benefit view not to squander with resources • performance based regulation will not supersede prescriptive regulation • an open dialogue between nuclear utilities and regulators is needed • the challenges is to organise for efficiency, without compromising safety • maintain the nuclear option for electricity generation in Europe