Uncertainty Analysis Becoming Common Practice in Safety Assessment

During the recent years an increasing interest in computational reactor safety analysis is to replace the conservative evaluation model calculations by best estimate calculations supplemented by uncertainty analysis of the code results. The evaluation of the margin to acceptance criteria, e.g. the maximum fuel rod clad temperature, should be based on the upper limit of the calculated uncertainty range. For example, due to power increase, licensing limits are approached. Therefore, regulators are looking closer on the way, how calculations are performed to meet these acceptance criteria. Methods have been developed and presented to quantify the uncertainty of computer code results. They are briefly presented in this paper. The present overview considers the international situation of development of uncertainty evaluation of computer code results and their application in licensing. Best estimate analysis plus uncertainty evaluation is used in licensing up to now in approximately seven countries. Demonstrations of applying uncertainty methods have been performed in nine additional countries at least. Most organizations use statistical methods. One statistical method is the GRS method proposing ordered statistics. Several demonstrations to apply the GRS method have been performed by GRS for courses of events in the nuclear steam supply system, calculating experiments as well as nuclear power plants. The method has also been applied for post test calculations of containment behavior, as well as severe accidents. One of the most important conclusions is that care must be exercised in determining ranges and probability distributions of the uncertain input parameters.