The U.S. Nuclear Regulatory Commission (NRC) recently released a staff paper-SECY-18-0096-seeking Commission approval of a new proposed methodology for establishing “functional containment” performance criteria for non-light water reactors (non-LWRs). This methodology would step away from deterministic containment design parameters and embrace a concept of “‘functional containment’ as a barrier, or a set of barriers taken together, that effectively limits the physical transport of radioactive material to the environment.” If approved, the proposed methodology would be a critical step in developing a framework for licensing non-LWR designs.
Many current NRC regulations and guidance documents are prescriptive in nature and focus on problems unique to LWR technology—that is, the regulations tell you squarely what you need to do, providing limited opportunity to meet the end objective of the regulation in a different way. For example, NRC guidance in NUREG-800 Chapter 3 provides detailed requirements for the strength and thickness of barrier concrete. As the NRC has started to grapple with non-LWR designs, it has shifted its focus to more risk-based and performance-based methodologies—that is, the NRC would define the objective, and an applicant would have more flexibility in showing the NRC how its design meets the objective. Under the performance-based methodology proposed in the SECY paper, the requirements imposed on physical barriers would be determined based on the risk of migration of radioactive materials and the other safety measures being used for containment.
The new methodology proposed by the NRC staff, therefore, does not prescribe the traditional structures, systems, and components (SSCs) required for functional containment, but rather focuses on performance requirements. This methodology will give non-LWR designers more flexibility and provide a more integrated approach for developing a regulatory framework for non-LWRs.
A key component of the methodology is the “identification and categorization of licensing-basis events.” The NRC staff recommends using the set of event categories initially developed under the Next Generation Nuclear Plant Project and used in the Licensing Modernization Project as the baseline for developing performance criteria. Based on these event categories, the performance criteria will be developed to meet fundamental safety requirements. Once the performance criteria have been determined, it appears the developers of non-LWRs would consider the potential consequences associated with the identified events and assess the cost and benefits of potential SSC options to prevent or mitigate the migration of radioactive material. The NRC staff is calling this approach the “Barrier Assessment” or “Bow Tie” method, as depicted in the figure below:
If approved, the prosed methodology for functional containment performance criteria will be incorporated into the draft guidance the NRC staff is currently compiling for non-LWR licensing.
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