SOARCA Process, Step 1: Selecting Accident Scenarios to Model

The purpose of the State-of-the-Art Reactor Consequence Analyses (SOARCA) is to evaluate the realistic consequences of postulated severe reactor accident scenarios that might cause a nuclear power plant to release radioactive material into the environment. Toward that end, the project team uses this step of the SOARCA process to focus on select severe accident scenarios.

The SOARCA team uses core damage frequency (CDF) to select scenarios. This approach makes sense because the fuel rods in the reactor core must be damaged before a significant release of radioactive material could occur. The fuel rods comprise the first barrier in the "defense-in-depth" concept the NRC uses to help protect public health and safety. (For additional detail, see Defense-in-Depth Barriers and Their Importance to SOARCA.)

To help identify scenarios with a relatively high CDF, the project team used the Enhanced Standardized Plant Analysis Risk (SPAR) models. The NRC uses SPAR models to quantitatively measure the likelihood of core damage as part of a probabilistic risk assessment (PRA). These models include specific information about each plant's design, systems, and components, and how they all interact. (For additional detail, see the Fact Sheet on Probabilistic Risk Assessment.) In addition, the project team considered the licensees' PRAs and previous NRC-sponsored studies.

The SOARCA project team selected accident scenarios with a CDF higher than 10^-6 (i.e., "one-in-a-million") per year to focus attention on the more likely accident scenarios. This selection method allowed SOARCA to analyze the most likely, yet very remotely possible, severe accident scenarios, improving understanding of a severe accident's likely consequences.

The team also selected some lower probability accidents for analysis because of their potential to result in very high consequences. Thus, for the less likely severe accidents (such as containment bypass or early containment failure scenarios) that could have significantly greater consequences, the staff used a lower CDF criterion of 10^-7 (i.e., "one-in-ten-million") per year to select scenarios for analysis.

Apart from the SOARCA project, the NRC continues to study aspects of severe accidents to support our risk-informed regulatory framework.