Development of TERRENUS, a multiphysics code for spent nuclear fuel cask criticality analysis

A new multiphysics code, TERRENUS, is in development at Oak Ridge National Laboratory (ORNL). The purpose of TERRENUS is to simulate the consequences of a spent nuclear fuel (SNF) dual purpose (storage and transportation) canister (DPC) entering a critical configuration while in a geological repository, including calculating heat generation, pressure buildup, and increase in radionuclide inventories. Subcritical configuration in loaded SNF DPCs is usually maintained by using neutron absorber panels placed between fuel assemblies. These absorbers are typically composed of boron carbide and aluminum. Since repository periods of interest are at least 10,000 years long, it is unlikely that aluminum-based absorbers will maintain criticality control for that amount of time in an aqueous environment. Therefore, criticality consequence analysis is needed to support direct disposal of DPCs. TERRENUS currently contains two components: a radiation transport code and a thermal hydraulics solver. The radiation transport code is SHIFT, a high-performance Monte Carlo code developed at ORNL, which will calculate heat generation in the fuel rods. The thermohydraulic solver is COBRA-SFS Cycle 4a. COBRA-SFS solves subchannel equations to compute the pressure, density, and temperature in the system. Initial development is focused on coupling SHIFT and COBRA-SFS for steady-state coupled transport-thermohydraulic simulations. A series of challenge problems is being developed, beginning with steady-state coupled transport-thermal hydraulics simulations of 3*3 arrays of pins, eventually proceeding up to time dependent coupled simulations of fully loaded DPCs. This paper demonstrates the results of the initial efforts in steady-state coupled transport and thermal hydraulics applied to a 3*3 array of pins in a flooded spent-fuel canister