Evaluating code uncertainty - I: Using the CSAU method for uncertainty analysis of a two-loop PWR SBLOCA

When best-estimate calculations are performed, the uncertainties need to be quantified. Worldwide, various methods have been proposed for this quantif ication. Rather than proposing a new uncertainty methodology, a contributio n is made to the existing code scaling, applicability, and uncertainty (CSA U) method. A small-break loss-of-coolant accident with the break in the col d leg of a Westinghouse-type two-loop pressurized water reactor was selecte d for the analysis, and the CSAU methodology was used for uncertainty quant ification. The uncertainty was quantified for the RELAP5/MOD3.2 thermal-hyd raulic computer code. Some tools suggested by the uncertainty methodology b ased on accuracy extrapolation (UMAE) method were successfully applied to i mprove the CSAU methodology, particularly for nodalization qualification. A critical scenario with core uncovery was selected for the analysis, which showed that when uncertainty is added to the peak cladding temperature, the safety margin is sufficient. The tools developed by the UMAE method showed that the structure of the CSAU method is universal because it does not pre scribe tools for the analysis.