Abstract
This review paper highlights approaches and tools available to the nuclear industry for dynamic probabilistic risk assessment (DPRA) using dynamic event trees. DPRA is an emerging methodology that has advantages as compared to traditional, static PRA predominantly owing to the addition of time dependent modeling. Traditional PRAs predefine events and outcomes into Event Trees (ET) and Fault Trees (FT), that are coupled with various combinations of Initiating Events (IE), Top Events (TE), branches, end states and sequences. A more complete depiction of the system and accident progression behavior can be quantified using DPRA to account for dynamic events such as those involving human actions. This paper discusses the strengths and needs of existing DPRA tools to align with the risk informed methodology currently used in the nuclear industry. DPRA is evolving during an exciting time in the nuclear industry with emerging advanced reactor designs also coming on the scene. Advanced nuclear (Gen IV) designs often incorporate passively safe systems that have less readily available data for traditional PRA due to their limited operating history. DPRA is a promising methodology that can address this challenge and demonstrate to the regulatory bodies and public that advanced designs operate within safety margins. In this light, the paper considers the historical role of PRA in the nuclear industry and motivation for considering dynamic PRA models. An introduction to the differences inherent in DPRA and how it complements and enhances existing PRA approaches is discussed. Additionally, a review of research from U.S national laboratories and universities features recent DPRA tool advancements that could be applied in the nuclear industry. These DPRA approaches and tools are summarized and examined to thoughtfully provide a path forward to best leverage existing research and integrate DPRA into advanced reactor design and analysis.
Highlights
Probabilistic risk assessment (PRA) continues to play a critical role in refining and quantifying risk for the nuclear industry
DPRA maintains a similar methodology to traditional PRA, with fewer constraints that allows for assessment to consider scenarios such as equipment degradation, human performance, adversarial actions, or numerous mitigating actions that may vary over time
As the availability of computational resources advances, it is likely that DPRA may be more attainable across the nuclear sectors with less access to high performance computing
Summary
Probabilistic risk assessment (PRA) continues to play a critical role in refining and quantifying risk for the nuclear industry. The use of PRA techniques in the nuclear industry began back in 1977 when the “Lewis Committee” formally endorsed the risk-informed methodology of the 1975 Reactor Safety Study (WASH 1400) (Rasmussen, 1975). Dynamic PRA in the Nuclear Industry methodology advanced and the assessment of risk refined, conservative engineering margins were allowed to be reduced while ensuring safety It was in 1995 that the NRC issued a formal ‘PRA policy statement’ officially adopting PRA as a commitment to their risk informed approach to regulation. A Level 1 PRA systematically evaluates numerous sequences of events, and depicts the subsystems needed to respond to events, in order to evaluate the probability that the outcome damages the core This process involves analyzing systems such as the steam generator, reactivity control, and plant monitoring, among others. Other trends in PRA models include the consideration of multiple external hazards (i.e. flooding and seismic), human action(s) or inaction(s), and the addition of multiple sites (Miller et al, 2020)
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