# Burnup Credit Evaluation for BWR Spent Fuel from Full Core Calculations
## 2 Codes Description  
In the current study, a BWR full core analysis was performed using the Polaris/DYN3D sequence. Polaris is a newly introduced module of SCALE 6.2 code system for the 2D lattice physics computations, adjusted for LWR design [18]. The SCALE package was developed by Oak Ridge Nuclear Laboratory (ORNL, Oak Ridge, TN, USA). It is designed to solve various problems of nuclear safety analysis and design, such as criticality safety analysis or reactor physics computations. Before incorporating Polaris into the SCALE package, lattice physics analysis was performed using the TRITON module within the SCALE package. Polaris has significant advantages in comparison with TRITON, such as an easy input file structure and improved running time without affecting the quality of the results [19]. For these reasons, the Polaris module was chosen for the lattice calculations and the cross-section preparations for the BWR fuel assembly design. Polaris utilises an Embedded Self-Shielding Method (ESSM) based on the Bondarenko interpolation approach for calculating multi-group self-shielded cross-sections [18]. The transport calculations are being performed using the Method of Characteristics (MoC). Polaris is supplied either with 252 or 56 energy groups' nuclear data libraries generated from the ENDF/B-VII.1 library. The study [20] showed that TRITON module of SCALE 6.2 with 252 energy groups' data library has a discrepancy with well-validated commercial code CASMO5 in the range of less than \(\pm 100\) pcm for the BWR pin-cell. Thus, lattice physics computational tools of SCALE 6.2 code used in the current study can be considered as a robust means for cross-section preparation for industrial standard light water reactor problems.  
The simulation of BWR reactor operation was performed in the DYN3D nodal core simulator. DYN3D is a multi-physics, three-dimensional nodal code for steady-state and transient analysis of LWRs developed by Helmholtz-Zentrum Dresden-Rossendorf (HZDR, Dresden, Germany) [21]. The neutron physics model uses the nodal expansion method (NEM) for solving the three-dimensional two-group or multi-group neutron diffusion equations. The code can simulate square and hexagonal fuel assemblies' designs, such as those of BWR and water-water energetic reactor (VVER) fuel assemblies, respectively. This study used the multi-group version of the DYN3D code. The calculated reactor thermal-hydraulics parameters, such as fuel temperature, coolant density and temperature, are fed back to the neutronics solver within DYN3D to estimate the thermal-hydraulics feedback. DYN3D can also simulate the reactor fuel cycle by incorporating burnup and fuel shuffling options into the analysis. DYN3D was initially developed for VVER reactor type, where it is the NURESIM (European Reference Simulation Platform for Nuclear Reactors) reference code [22]. However, DYN3D is currently extensively verified and validated for different types of LWRs [21]. For BWRs, the code was compared against the Nuclear Energy Agency of the Organisation for Economic Co-operation and Development (OECD/NEA) BWR Turbine Trip Benchmark [23].