Use of best-estimate methods for licensing purposes in Belgium

No new Nuclear Power Plant (NPP) has been built in Belgium since 1985 but about 55 % of the electric energy is still being provided through the nuclear road. To face the growing electricity demand of the country, the Utilities upgraded already 4 out of the 7 NPPs by approximately 10 % capacity. New steam generators (SG) were put in place at those 4 above-mentioned NPPs, as well as at 2 other NPPs (after having increased those SG plugging rates in order to limit primary to secondary leakage) for which no power uprate was considered. Core design evolutions were introduced, some circuits had to be adapted and some of the safety margins were reassessed, mainly with the help of best-estimate (BE) methods. This paper gives an overview of the licensing process followed for the acceptance of the methodologies and the approval of the results. In the context of the so-called Periodic Safety Reassessments of the Belgian NPPs, re-evaluations of specific transients or accidents were performed as, for instance, the re-verification of the structural integrity of some containment sub-compartments in case of pressure buildup after a loss-of-coolant accident (LOCA) or a steam line break (SLB). Other problems were or are being treated on a generic basis, in order to prepare the plant specific calculation to be performed in the next step: for instance, the steam generator tube rupture (SGTR) accident is actually being re-considered in order to analyze the impact of 'new' phenomena such as atomization at the break or to judge other scenarios (e.g. SG overfilling). After a short description of the licensing approach and of the safety assessment process followed in Belgium, the assessment for licensing purposes of BE codes and of code couplings is described in more details. For each case, the licensing requirements set forth are discussed together with the rationale supporting them. Then, a specific part is devoted to the assessment of computerized systems defaults and their impact on functional diversities to be included in their safety assessment. Finally, the practical incidence of BE methods application on important plant modifications is briefly presented. Methods based on BE codes have been progressively introduced in the licensing process for core design calculations and analyses of large break LOCA, small break LOCA, heatup and cooldown transients, overpressure transients. They allow the safety justification of power uprates with a minimum of equipment modifications. The use of coupled codes represents a further advance towards BE evaluations as it has recently been demonstrated in the frame of a SLB analysis