Use of Best Estimate Codes for Analysis of PWR Large Break LOCA

Abstract

Use of the best estimate codes for PWR large LOCA analysis has increased considerably in recent years. This paper describes a series of calculations 05 a postulated large LOCA in a large four loop PWR carried out with the TRAC-PF1/Mod 1 best estimate code. Results are found similar to results of earlier Licensing analyses using the Evaluation Model approach, provided similar boundary conditions and data assumptions are adopted. Sensitivity of peak fuel clad temperature to break flow, axial power shape modelling and effects of injection of accumulator nitrogen are quantified. A net benefit on peak clad temperatures due to core radial power shape is indicated, although the magnitude is sensitive to modelling assumptions and is therefore difficult to quantify. The new calculations are consistent with USNRC best estimate calculations of a large LOCA in a similar PWR plant. Differences in quoted peak clad temperatures arise because the US results are appropriate for fuel rods with a lower peak Linear rating.