USER EFFECTS ON THE THERMAL-HYDRAULIC TRANSIENT SYSTEM CODE CALCULATIONS

Large thermalhydraulic system codes are widely used to perform safety and licensing analyses of nuclear power plants to optimize operational procedures and the plant design itself. Evaluation of the capabilitie s of these codes are accomplished by comparing the code predictions wi th the measured experimental data obtained from various types of separ ate effects and integral test facilities. During these comparisons of the code results, there has been a continuous debate on the way how th e code user influences the predicted system behaviour. This rather sub jective element might become a crucial point with respect to the quant itative evaluation of the code uncertainties which is essential if the ''best estimate codes are used for licensing procedures''. The Intern ational Standard Problem Exercises (ISPs) proposed by the OECD/NEA-Com mittee for the Safety of Nuclear Installations (CSNI) and by IAEA (Int ernational Atomic Energy Agency) and thermalhydraulic code assessment activity undertaken by USNRC (US Nuclear Regulatory Commission) under International Code Assessment and Application Program (ICAP) demonstra te the large effort put in this framework by organizations all over th e world. In recent years, some attempts have been made to establish me thodologies to evaluate the accuracy and the uncertainty of the code p redictions and consequently judgement on the acceptability of the code s. In none of the methodologies has the influence of the code user on the calculated results been directly addressed. In this paper, the res ults of the investigations on the user effects for the thermalhydrauli c transient system codes will be presented and discussed on the basis of some case studies. The general findings of the investigations show that in addition to user effects, there are other reasons that affect the results of the calculations and which are hidden under user effect s. Both the hidden factors and the direct user effects will be discuss ed in detail and general recommendations and conclusions will be prese nted to control and limit them.