Application of risk informed safety margin characterization to extended power uprate analysis

In this paper we present some initial results of the application of a risk-informed safety margin characterization (RISMC) approach to the analysis of the impact of an extended power uprate (EPU) on plant safety for selected transient and accident sequences. These initial applications were conducted to demonstrate the feasibility and practicality of using the RISMC approach to analyze the safety impact of EPUs at both a pressurized water reactor (PWR) and a boiling water reactor (BWR). For the PWR application, the analysis focused on the loss of main feedwater (LOMFW) event with failure of auxiliary feedwater (AFW) where feed and bleed (F&B) cooling is required to prevent core damage. For the BWR case study, station blackout (SBO) sequences leading to core damage were analyzed. A consistent and repeatable process was developed and applied to identify those key parameters that would be analyzed. Distributions were constructed to represent the uncertainties associated with each of the key parameters. These distributions were sampled using a Latin Hypercube Sampling (LHS) technique to generate sets of sample cases that were used in the physics simulation runs using the MAAP4 code. Simulation results were evaluated to determine the changes to safety margins which would occur due to the uprated power conditions; the results obtained were then compared to those for the current nominal full power. The results obtained indicate, as expected, that safety margins may be reduced with increases in plant power level. However, for most power uprate levels, these safety margin reductions were found to be small. A limited study of margin recovery strategies was performed for the PWR case that indicated that minor to moderate changes in plant operation or design could be used to recover the safety margin reduction that would occur from the power uprate.