# Burnup Credit Evaluation for BWR Spent Fuel from Full Core Calculations
For most spent fuel types, the criticality safety analysis is performed under the so-called \"fresh fuel\" approach. It implies that the SF system contains unirradiated (fresh) fuel without burnable poison. The approach results in a substantial overestimation of the calculated system's criticality, especially for the fuel with increased enrichment and high discharged burnup [1]. The reduction in the system's reactivity associated with the fuel burnup can be taken into account using the burnup credit (BUC) method [1]. During fuel irradiation, the total amount of fissile material present significantly decreases, which leads to a noticeable reduction in reactor core criticality. At the same time, transuranic elements are accumulating and fission products content is rising. If the fuel contains burnable absorber, such as gadolinium in case of BWRs [12], its concentration is declining, leading to an increase in criticality until all burnable poison has been depleted. The IAEA defines four frequently used levels of the burnup credit [13]:  
* \"Credit for the net decrease of the fuel fissile content, taking into account both burnup and build-up of the different fissile nuclides (net fissile content level). * Credit for the net fissile content and for the absorption effect of actinides (actinide only level). * Credit for the actinides and the neutron absorption in fission products (actinide plus fission product level). * Credit for the presence of integral burnable absorbers in the fuel design (integral burnable absorber level). This credit uses the maximum reactivity of the fuel, which is often not the initial reactivity.\"  
According to the data published in [14], the BUC on the actinide-only and actinide plus fission product level is widely applied to pressurised water reactors (PWR) fuel at the various stages of SF management. For BWR spent fuel, the BUC on the integral burnable absorber level is mainly applied to wet storage. However, the method is still under development or consideration in some countries for the dry storage and transport of BWR spent fuel [14]. Oak Ridge National Laboratory has recently finished their five-year program regarding the burnup credit development for the transport and storage of BWR spent fuel [15]. The project aimed to analyse the specific points of BWR operation and estimate their impact on BUC. The research was focused on the influence of various operating parameters, such as axial moderator density distribution, control blades movements and operating conditions, on the validation of the isotopic predictions and effective neutron multiplication factor (k\({}_{\text{eff}}\)) calculations. The series of newly published studies [16; 17] was focused on the development and benchmarking of an advanced BWR lattice model with further application to the criticality analysis of the BWR storage task, along with burnup credit, sensitivity and uncertainty studies. The research was conducted for BWR spent fuel discharged at the peak reactivity burnup where the burnable absorber gadolinium does not affect fuel assembly criticality anymore [17]. The current study aimed to create the best estimate for BUC in application to BWR spent fuel discharged at realistic final burnups. For this purpose, the FA behaviour in the reactor core was evaluated by performing full core nodal simulations with different fuel reloading patterns, which provided the comprehensive data for each assembly in the core. The project is divided into two parts. The first part, which is presented in this paper, is focused on the simulation of the BWR reactor operation and optimisation of the burnup distribution of the discharged FAs. In addition, the paper evaluates the benefit of burnup credit application for the realistic BWR assembly model at the estimated discharged burnups.