Estimation of the impact on safety margins of a NPP power uprate accounting for probabilistic and thermal-hydraulic uncertainties

The adoption by regulators of the risk-informed decision-making philosophy opened up the debate on the role of the deterministic and probabilistic approaches to support regulatory matters of concern to Nuclear Power Plants safety. RG 1.174 is a key document in this framework, which provides guidance in support of licensee request for changes to a plant's license basis, such as a power uprate. This regulatory guide presents five principles of risk-informed decision making to be used for making decisions regarding plant-specific changes to the licensing basis. The third principle states that the proposed change has to maintain sufficient safety margins, and is the one we are concerned with in this paper. Development of methods for the assessment of safety margins within this framework has attracted attention only in recent years with an aim at using probabilistic safety analysis results into requirements and assumptions in deterministic analysis, and vice versa, to provide a more comprehensive and realistic measure of reactor safety. In previous works the authors have proposed a new method to estimate safety margins based on Monte Carlo runs of best estimate thermal hydraulic models together with data and models from a level 1 PRA and simultaneously considering the uncertainty associated to both types of models, which can be of application for both design and beyond design basis accidents. This paper presents a case of application of such methodology consisting of analyzing the impact of a power uprate on safety margins for an accident scenario in a typical pressurized water reactor. (author)