This report details the progress of INL on the creation of a reference plant multiphysics model for the ABTR. The model was developed within Task 13 of the NRC project “Development and Modeling Support for Advanced Non-Light Water Reactors” and is an extension of the reference plant developed in Task 4b. Several improvements were made to the reference plant model developed in Task 4b. (1) The discrete ordinates method was used in lieu of SPH-corrected diffusion approximation to better capture the anisotropic scattering contribution and the change in neutron leakage due to thermal expansion. (2) The novel neutronic spatial discretization approach named the ring-heterogeneous approximation was introduced to capture the differential expansion of the materials in the core. The new technique was proved to preserve fission rates and the eigenvalue within 2.5\% percent and 266 pcm, respectively. The separation of the different material in the core allows to explicitly account for the differential expansion of materials, therefore eliminating the need for problem-specific cross section functionalization techniques. (3) The SAM model for the core and system thermal hydraulics analysis was updated to include 61 channels instead of 4 representative channels. This allows the user to obtain improved spatial resolution for sodium temperature and density. (4) All the mesh files were created with the MOOSE reactor module, therefore eliminating the reliance on external tools for mesh creation. (5) Finally, the fuel axial expansion now leverages the HT9 and UPuZr material properties that have been validated against experimental data. The reference plant model was used to perform a full-core ULOF transient calculation including neutronics, thermal and mechanical feedback mechanisms. Future work will be devoted to further enhance the model. Potential improvements to the model include the addition of the control-rod driveline expansion feedback and the improvement of the support plate model to explicitly include three-dimensional effects. Additionally, a NEAMS-funded parallel effort will completely automate the creation of the ring-heterogeneous mesh from the fully-heterogeneous geometry, therefore maximizing user friendliness for the Sodium Fast Reactor workflow.
