Importance of uncertainty modelling for nuclear safety analysis

Abstract

The U.S. Nuclear Regulatory Commission (NRC) reviewed the 10CFR50.46c regulations regard the loss-of-coolant-accident (LOCA), and emergency core cooling system (ECCS). In this planned rulemaking named as 10CFR50.46c. New LOCA criteria included the integration of models used to the hydrogen uptake changes equivalent cladding react (ECR), coupled with peak cladding temperature (PCT). This rule inserts the embrittlement mechanism considering the hydrogen buildup as a pre-transient condition, reducing a loss of operational margin. 10CFR50.46c criteria should combine the effects produced from different fields, such as neutronic analysis, thermal-hydraulic, with fuel performance codes. Besides, it should contemplate Best-Estimate Plus Uncertainty (BEPU) practices. Consequently, increases the challenges to safety analysis because of nuclear power plants run for extended periods than planned initially. In these circumstances, nuclear units need to operate on extended life cycles based on safety margins. With a lifespan of 60 years or more, we reviewed the behavior of the structural material on accident scenarios. This work showed the importance of uncertainties created by physical models such as the fission gas release, thermal conductivity, and loss of ductility caused by hydrides.