Requirements for statistical safety demonstration for loss of coolant analyses

Abstract

During the recent years, an increasing interest in computational safety analysis of Nuclear Power Plants (NPPs) is to replace the conservative evaluation model calculations by best estimate calculations supplemented by uncertainty analysis of the code results. Conservative means unfavorable in the direction of regulatory acceptance limits. The evaluation of the margin to acceptance criteria, e.g. the maximum fuel rod cladding temperature, should be based on the upper limit of the calculated uncertainty range. Uncertainty analysis is needed if useful conclusions are to be obtained from “best estimate” thermal-hydraulic code calculations, otherwise single values of unknown accuracy would be presented for comparison with regulatory acceptance limits. Methods have been developed and presented to quantify the uncertainty of computer code results. The Gesellschaft für Anlagen- und Reaktorsicherheit mbH (GRS) in Germany has proposed a statistical method, propagating input uncertainties by computer code calculations to end up with output uncertainties of the calculated results. The basic techniques of that GRS-method are presented for calculating the peak cladding temperatures of the fuel rods in a reactor core and to compare with an acceptance limit to be met under any design basis accident. A comparison of necessary number of code calculations is provided using a penalized “hot rod”, or a significantly higher number of calculations when all high powered fuel rods are considered in the analysis when conditions of many fuel rods are very close to the most unfavorable fuel rod.