FINIX - Fuel behavior model and interface for multiphysics applications:Code documentation for version 0.15.6

The FIN IX fuel behavior code has been updated to version 0.15.6. The focus of the update has in improving the usability of the code. The interface and data structures have been rewritten and input and output file formats have been defined and added to the code package. The FINIX code has been designed to provide a fuel behavior module for other simulation codes in multiphysics simulations. The intended use is the improvement of fuel behavior description in neutronics, thermal hydraulics and reactor dynamics codes, without having to employ full-scale fuel performance codes. FINIX couples with the host code on a source code level, and provides an interface of functions that can be used to access the fuel behavior model from the host code. At VTT, FINIX has been integrated into the Monte Carlo reactor physics code Serpent 2, and reactor dynamics codes TRAB-1 D, TRAB3D and HEXTRAN. FINIX consists of several interconnected models that describe the thermo-mechanical behavior of the fuel rod. FIN IX solves the transient heat equation, with couplings to the cladding and pellet mechanical behavior through the gap conductance and pressure. Publicly available experimental correlations are used for the material properties, and simple models for the heat transfer from the cladding to the coolant have been included. FINIX has been verified against the FRAPTRAN fuel performance code in RIA scenarios, and compared against experimental Halden reactor data. Results indicate good agreement with the FRAPTRAN code and experimental measurements. Limitations of the present version in the simulated scenarios have also been identified. Detailed account of the validation results are presented in a separate validation report