Development of a SKB burn-up credit methodology for BWR

Burn-up credit (BUC) is used for PWR in many different applications. However, not many companies have developed and licensed a burn-up credit methodology for BWR. The main difficulty for BWR is the large variation of fuel assembly operational history. In addition to the power history, different BWR-assemblies have different void history and different control rod history. These parameters not only effect the axial BU-profile but also have a great impact on the isotopic composition. Furthermore, BWR-assemblies displays a large variety in fuel designs. All this put together makes it very costly to develop a BU-loading curve which is conservative for all possible conditions and for all different fuel types. SKB Burn-Up credit methodology for PWR was approved by the Swedish Radiation Safety Authority (SSM) in 2017. During 2023 SKB and Vattenfall have further developed this methodology to also cover BWR. The main features are: - Differences in axial BU-profile (End effect) is taken care of by analyzing the core follow calculations. A bounding profile is chosen to ensure conservative treatment. - Control rod history affects are based on control rod presence from real operational history. - Conservative values are also used for void history in a bounding manner. - Uncertainties in calculated isotopic composition and BU is based on conservative assumptions. Scale 6.2 is used for criticality calculations and the validation is using a ck-based methodology. This paper presents how SKB has developed a BWR BUC methodology taking into consideration the above described difficulties and the main results of the uncertainty evaluations. (author)