As advanced nuclear technology (ANT) progresses with the development of small modular reactors (SMRs), non-light water reactors, and the like, the term “multi-module” has been broadly used by the nuclear industry. The U.S. Nuclear Regulatory Commission’s (NRC’s) “Draft Technical Guidance on Multi-Module Risk” (ML14150A330) provides the following definition of multi-module:

"A nuclear power module is a nuclear power reactor and its associated safety and control features. A multi-module nuclear power station, or nuclear power plant, is one that is composed of multiple nuclear power modules wherein: a) each module can be safely operated independent of other modules, and b) all modules are located within a common building structure such that a module can be physically impacted by events occurring at another module due to proximity and lack of a physical barrier providing separation."

Regarding the analysis of risk, one important question that arises is if multi-module and multi-unit designs are fundamentally different such that additional or novel risk analysis considerations are required. In short, are the current multi-unit risk analysis frameworks sufficient to analyze the risk of multi-module designs? In the context of current research, the distinction between a module and a unit seems to identify “tight coupling” between systems and components within a common structure. Today’s nuclear power plants (NPPs) have various degrees of coupling between systems, structures, and components. For example, coupling can include shared systems and components, shared structures, shared resources, common procedures and emergency response, similar make and model of components, similar approaches to testing and maintenance, exposure to similar external hazards, and the like. Current multi-unit risk analysis frameworks (for example, EPRI report 3002020765, IAEA safety report 110) consider this coupling.

To develop a framework for addressing multi-module risk, the first step is to better understand the variety of multi-module designs. For example, what do these new designs have in common with the current NPP fleet? What are the differences, and are these differences relevant to the analysis of risk? Finally, is there a clear distinction between multi-unit risk, and multi-module risk, such that additional or novel considerations are required? The goal of this investigation is not to identify every design detail, but rather to focus on those details that are important to the analysis of risk. To achieve the stated goal, this investigation focuses on the following questions:

• What does “multi-module” really mean?

• Is multi-module risk analysis fundamentally different from multi-unit risk analysis?

• What are the key elements that differentiate multi-module risk from multi-unit risk?

• How do current multi-unit frameworks and guidance support multi-module risk analysis?

• How do various dependencies impact risk and the analysis of risk for “tightly coupled” modules/units?

EPRI’s multi-unit framework (EPRI report 3002020765) serves as a well-established foundation for the analysis of multi-unit risk and is the basis for this investigation.