External Hazards

Purpose

Taking External Hazards into account is an important part of quantifying the reliability of advanced reactor designs. While extreme external hazards can be very rare, it is important that they are still considered as one extreme external event can quickly lead to total plant failure, as seen in the Fukashima Daiichi accident.

Scope

External events usually encompass a variety of weather and geographical phenomenon, such as seismic events, flooding events, and high winds, but can also be related to non-geographical factors such as proximity to another plant. When considering external hazards, it is important that no stone goes unturned in identifying possible failure scenarios.

External Hazards

One of the main conclusions of TECDOC 1487 [42] is that “the development of an external event PRA in parallel with the early plant design may help to identify the vulnerabilities as well as potentially overly conservative design features at an early stage, leading to a well-balanced and cost-effective improvement in safety”. Therefore a core issue that needs to be clearly addressed moving forward is the balance between the cost and benefit of developing detailed External Events PRA models with insights that can be used to adequately and robustly screen external events that may otherwise be expected to be fully included (e.g., is such an approach feasible, justifiable; can a graded approach be provided that accounts for increased margin to safety goals with sufficient consideration of uncertainty and degree of confidence). In relation to the section describing passive system reliability, a large part of determining the uncertainty of a passive system is being able to determine the risk associated with certain external events and how it relates to the designed passive safety systems. For example, external events have the potential to change assumed system boundary conditions which could result in the failure of the passive system to successfully perform its intended safety function.

The impacts of external hazards will likely provide a substantially greater fraction of the contribution to risk to the public than is the experience from the current fleet of LWRs. Because the current state of knowledge of potentially dominant very low frequency external hazards (phenomenology, event occurrence frequencies) is less developed than for internal events, the larger fractional contribution to overall risk will have a greater level of uncertainty. This situation also may require modifications to processes and criteria that have been used for the existing fleet of plants to assess and screen external hazards from consideration as providing a negligible contribution to PRA results. Some external events may be screened out through a deterministic and probabilistic process.

Research Roadmap (EPRI 3002026495) actions supported for passive system reliability include:

• Develop Enhancements to Licensing Process
• Establish Decoupling Framework for Nuclear Beyond Electricity (NBE) Users
• Demonstrate Risk-Informed and Performance Based Approach

A higher level of detail for these actions can be found in the report.