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| NKS Programme Area: | NKS-R | | Research Area: | Risk analysis | | Report Number: | NKS-172 | | Report Title: | Probabilistic Safety Goals. Phase 2 - Status Report | | Activity Acronym: | Safety Goals | | Authors: | Jan-Erik Holmberg, Kim Björkman, Jukka Rossi, Michael Knochenhauer, Xuhong He, Anders Persson, Helena Gustavsson | | Abstract: | The second phase of the project, the outcome of which is described in this project report has mainly dealt with four issues:
• Consistency in the usage of safety goals
• Criteria for assessment of results from PSA level 2
• Overview of international safety goals and experiences from their use
• Safety goals related to other man-made risks in society
Consistency in judgement over time has been perceived to be one of the main problems in the usage of safety goals. Safety goals defined in the 80ies were met in the beginning with PSA:s performed to the standards of that time, i.e., by PSA:s that were quite limited in scope and level of detail compared to today’s state of the art. This issue was investigated by performing a comparative review was performed of three generations of the same PSA, focusing on the impact from changes over time in component failure data, IE frequency, and modelling of the plant, including plant changes and changes in success criteria. It proved to be very time-consuming and in some cases next to impossible to correctly identify the basic causes for changes in PSA results. A multitude of different sub-causes turned out to combined and difficult to differentiate. Thus, rigorous book-keeping is needed in order to keep track of how and why PSA results change. This is especially important in order to differentiate “real” differences due to plant changes and updated component and IE data from differences that are due to general PSA development (scope, level of detail, modelling issues).
Goals related to CDF and LERF are surrogates to societal risk level criteria. To fully validate these goals, calculations of environmental consequences of release sequences would need to be made. The on-going international survey conducted by the OECD/NEA WG Risk shows that acceptance criteria for results from level 2 PSA differ considerably between countries. Both definitions for large release and probability values differ. Further, the status of criteria differs from mandatory requirements to informal targets. Some countries do not use probabilistic criteria at all.
The probability limits used in level 2 PSA vary from 10-7/year to 10-5/year. The highest criteria (10-5/year) have been defined for old reactors only. For new reactors, targets between 10-7/year and 10-6/year have been defined. These numbers can be compared with risks experienced or accepted otherwise in society. From the individual risk point of view, these numbers are acceptable. To validate the target values from the societal risk point of view, level 3 PSA assessments need to be made. Results from such assessments are strongly dependent on population data, weather data, and whether or not countermeasures are accounted.
The aim of the definition for large release of the severe reactor accident is such that, first of all, the release magnitude shall be reduced to such an amount that no acute health effects are caused in the environment. It follows from this requirement that only stochastic late effects can be expected. The criterion “100 TBq Cs-137” used in Finland and the differently worded but almost identical criterion “0,1 % of the core inventory of Cs-137 in an 1800 MWt BWR” used in Sweden are examples of criteria fulfilling the above requirement. Test calculations with environmental data from a Finnish nuclear power plant site shows that this particular release limit would not cause acute heath effects and that late effects would be minor.
The on-going OECD/NEA Working group RISK task group on probabilistic safety criteria has the objectives to review the rationales for definition, the current status, and actual experiences regarding the use of probabilistic safety goals and other PSA related numerical risk criteria in the member states. The NKS project participates actively in the task. At present, responses have been received to a questionnaire and processing and compilation of answers has been initiated. The activity has already provided valuable input to the NKS project, and is expected to provide further valuable input during the third and final project phase.
In order to provide perspective on the project’s detailed treatment of probabilistic safety goals for nuclear power plants, some information from other areas has been collected, with the focus on the use of probabilistic risk criteria in European offshore oil and gas operations and in the European railway industry.
In offshore oil and gas operations both the number of precursor events requiring handling and of accidents requiring mitigation is high compared to the nuclear industry, resulting in a relatively high focus in the criteria on consequence mitigation. Criteria have a large scope, i.e. they apply to a wide range of accident events and consider a wide range of safety functions. Defence in depth aspects are considered in the criteria by stating requirements for different safety functions. Finally, the ALARP principle is often applied, involving a safety goal with a limit and an objective.
For European rail systems, a standardisation of safety goals has been prompted by the expressed aim of making it possible for trains and personnel to cross national borders. The harmonisation has been achieved by letting an industry working group propose safety goals, which have then been accepted by authorities. The goals suggested are consensus requirements based on an amalgamation of national practices, mainly from Germany and France. Basic principles are based on comparison to general health risk (MEM principle) and a requirement for continuous improvement of safety (GAMAB). Systematic procedures are in place for creating subsidiary goals, which is done by defining a tolerable hazard rate (THR) for each subsystem forming part of the overall system. Finally, it is worth noting, that a framework for cross-acceptance is under development, i.e., development of an agreed common approach for demonstrating the safety levels of the railway system (in addition to the common risk criteria already in place). | | Keywords: | Safety Goals; PSA; Safety Targets; ALARP; Decision criteria; Risk informed decision making | | Publication date: | 01 Jul 2008 | | ISBN: | ISBN: 978-87-7893-238-9 | | Number of downloads: | 3269 | | Download: |  NKS-172.pdf |
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