Unstructured Mesh–Based Neutronics and Thermomechanics Coupled Steady-State Analysis on Advanced Three-Dimensional Fuel Elements with Monte Carlo Code iMC

A thermomechanical fuel performance analysis module is implemented in the Korea Advanced Institute of Science and Technology Monte Carlo (MC) neutron transport code iMC. The module is designed particularly for advanced three-dimensional (3-D) fuel concepts, so an unstructured tetrahedral mesh grid is adopted for geometry flexibility. The cellwise detailed power density distribution is tallied from the MC transport and transferred to the thermomechanics module for the heat transfer, thermal expansion, and stress analysis. In this paper, a recently proposed 3-D fuel concept called the centrally shielded burnable absorber (CSBA) model was considered for numerical studies. Several fuel models were solved by the iMC code: a single CSBA pellet, a three-ball–loaded CSBA pellet, and a CSBA fuel-loaded 17 × 17 fuel assembly. From the analysis results, it was discovered that the uncertainty of the detailed power density distribution hardly affects the uncertainty of the thermomechanical analysis due to dissipation via conduction. Also, the importance of using detailed intrafuel power distribution data in such a thermal neutron spectrum has been demonstrated, showing about 30 K overestimation of peak temperature compared to the conventional uniform power assumption.