The “incredible” difficulty of proving “incredibility” - Example of fire-induced multiple spurious operations

“Risk-informed” regulation is often an alternative to “deterministically-based” regulation that offers relaxation in criteria for acceptability while possibly requiring greater analytical effort. “Risk-informed determinism” is an attempt to meld the best of both worlds by using risk information to set deterministic acceptance criteria a priori. A recent joint effort by the US Nuclear Regulatory Commission's Office of Nuclear Regulatory Research (RES) and Electric Power Research Institute (EPRI) originally endeavored to do this for several examples involving fire-induced multiple spurious operations (MSOs) in electrical circuits at nuclear power plants. While a noble effort, this did not consider the actual distributions involved in the events, originally limiting the analysis to mean values and, in some cases, qualitative considerations. A much more comprehensive and defensible approach is performed here where the probabilistic distributions for all the factors are considered via simulation to meet quantitative acceptance criteria related to the concept of “incredibility” that is often the figure of merit that must be met in a deterministic world. The effort demonstrates that it can be “incredibly” difficult to prove “incredibility” in this context.