Abstract
This report presents a preliminary probabilistic risk assessment (PRA) model for the OECD/NEA WGRISK DIGMORE reference case representing digital instrumentation and control (I&C) systems in a simplified boiling water reactor plant. The reference case covers an I&C architecture with several systems, such as the primary and diverse reactor protection system, operational I&C system, hard-wired backup system, and prioritization and actuation control (PAC) systems. The modelling approach selected in this study is to develop a simplified PRA model including only common cause failures (CCFs) and high-level failure events and to perform complex calculations in the background. The approach was selected due to challenges related to CCF calculations, e.g., some CCF groups include more than 8 components.
In the overall results of the PRA model, the I&C systems do not play very important role. This is however partly because of simplifications made in the reference case. Spurious signals causing the main feed-water system to stop (initiating event) are the most important I&C failure events in the results. Concerning failures of safety functions, PAC systems are the most important I&C systems, because they have less redundancy and diversity than the other systems.
In the main PRA analysis, the aim was to follow the reference case description as closely as possible meaning e.g. that the alpha-factor model was applied to hardware CCFs. However, some of the CCF calculations were very complex with the alpha-factor model. Therefore, use of the modified beta-factor model was studied in complementary analyses. It makes the modelling of CCFs much simpler. The beta-factor parameters were estimated using the partial beta-factor method. The partial beta-factor method produced mostly a bit smaller CCF probabilities than the alpha-factor model, but it depended on the assumptions used in the analysis. A complementary analysis case was also developed for spurious stop signals of safety functions.
In the overall results of the PRA model, the I&C systems do not play very important role. This is however partly because of simplifications made in the reference case. Spurious signals causing the main feed-water system to stop (initiating event) are the most important I&C failure events in the results. Concerning failures of safety functions, PAC systems are the most important I&C systems, because they have less redundancy and diversity than the other systems.
In the main PRA analysis, the aim was to follow the reference case description as closely as possible meaning e.g. that the alpha-factor model was applied to hardware CCFs. However, some of the CCF calculations were very complex with the alpha-factor model. Therefore, use of the modified beta-factor model was studied in complementary analyses. It makes the modelling of CCFs much simpler. The beta-factor parameters were estimated using the partial beta-factor method. The partial beta-factor method produced mostly a bit smaller CCF probabilities than the alpha-factor model, but it depended on the assumptions used in the analysis. A complementary analysis case was also developed for spurious stop signals of safety functions.
Original language | English |
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Publisher | VTT Technical Research Centre of Finland |
Number of pages | 52 |
Publication status | Published - 18 Dec 2024 |
MoE publication type | D4 Published development or research report or study |
Publication series
Series | VTT Research Report |
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Volume | VTT-R-00646-24 |
Funding
The research was funded by the National Nuclear Waste Management Fund (VYR) through SAFER2028 programme.
Keywords
- probabilistic risk assessment
- instrumentation and control
- common cause failure