RISK INFORMED APPROACHES FOR PLANT LIFE MANAGEMENT:
REGULATORY AND INDUSTRY PERSPECTIVES
Björn Wahlström
VTT INDUSTRIAL SYSTEMS
Paper outline
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Introduction
Building safety into nuclear power plants
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Regulatory oversight
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The basis
Systems of requirements
Prescriptive versus performance based regulation
Challenges for the future
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A basis for safety
The deterministic approach
The probabilistic safety analysis
Applications of the risk-informed approach
Changes in the regulatory systems
Plant life management
Harmonisation of safety requirements
A new safety philosophy
Conclusions
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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?
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Deterministic and probabilistic approaches
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
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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
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Impediments to the increased use of riskinformed regulation
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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.
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The comparison of two PSA-studies
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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!
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Shortcomings in present PSA models
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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!
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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
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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
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Characteristics of good safety requirements
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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
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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)
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Conclusions
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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