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Multi-Unit Risk - EPRI 3002032224 [70] The addition of new reactors to existing sites or new nuclear power plant site plans with multiple smaller reactors may challenge current risk assessment methods and tools. Advanced reactors also present a wider range of technologies that can be significantly different from current water-based reactors than existing multi-unit research has focused on. The objective of this report is to expand the processes and insights provided by the existing EPRI multi-unit risk assessment framework to address the unique considerations of advanced reactors.

EPRI 3002032033 [68] As the nuclear industry advances toward modular and scalable reactor technologies, the potential distinction between “multi-unit” and “multi-module” configurations has been raised as a question when considering the development of probabilistic risk assessments (PRAs) or probabilistic safety assessments (PSAs) for such designs. This white paper explores regulatory definitions, design characteristics, and operational considerations that influence risk modeling for tightly coupled reactor modules. It also evaluates current guidance from the U.S. Nuclear Regulatory Commission (NRC), international standards, and the perspectives of several advanced nuclear technology developers to identify potential gaps and challenges in applying existing multi-unit PRA/PSA methodologies to multi-module scenarios.

PSA for High-Temperature Gas Reactors (HTGR) and Molten Salt Reactor (MSR) - EPRI 3002034425 [69] Many of the emerging advanced reactor designs will depend on advanced nuclear fuels and passive safety systems. Some such designs will utilize coolants other than water such as helium, liquid sodium, or molten salt, and can be designated as non-light water (NLW) plants.

In EPRI 3002034425, EPRI partnered with KAERI and other Korean organizations to assess current PSA methods and tools for their readiness to support advanced reactor designs through design, licensing, construction, and operation. This report documents the evaluation of the applicability of existing PSA standards, data, methods, and tools to selected NLW-SMR designs of interest to the Korean team. A high-temperature gas reactor (HTGR) was selected for the primary evaluation, with a molten salt reactor (MSR) included as a secondary design.

• The following features of NLW-SMRs are important to the evaluation of several PSA aspects:
  • Extensive and advanced use of passive features
  • Lack of operating experience
  • Potential industrial applications
  • Flexible siting
  • Multiple units at a site
  • Digital environments

• HTGRs, MSRs, and other NLW-SMRs may also include further distinct characteristics impacting PSA modeling:
  • Online refueling
  • Non-static core and fuel
  • Higher operating temperatures
  • Higher or lower operating pressures
  • Non-applicability of traditional PSA end states (CDF and LERF)

• Current methods for several PSA elements (developed for current water-based reactors) are generally applicable for NLW-SMRs. For example, the current LWR PSA methodology appears to be suitable for initiating event identification and quantification, development and use of reliability data, and off-site probabilistic consequence analysis.

• However, for some PSA elements and some portions of those PSA elements that are generally applicable, potential gaps in the current methods and tools were identified. For example, a lack of a consensus method for identifying initiating events in low-power or shutdown operating states.