Tightly coupled multiphysics simulations for prismatic reactors

A tightly-coupled numerical approach is desired for safety analysis of Very High Temperature Reactors (VHTRs) since its high-level of passive safety design introduces longer transient scenarios, as compared to current nuclear power plants. The commonly employed operator-splitting approach for multiphysics time integration is susceptible to additional truncation errors, and accumulation of the truncation errors can alter the numerical solution of the physics equation system. Multiphysics core simulations for a prismatic-type VHTR are performed in this study. Our solution scheme is based on the Jacobian Free Newton Krylov (JFNK) method, which enables second-order convergence by solving all the constitutive relations consistently at a new time level without forming a complex Jacobian matrix. Under a Multiphysics Object-Oriented Simulation Environment (MOOSE) framework, the code is written in modular fashion, which allows one to restrict or extend the governing physics at the input stage. As a preliminary example, a thermal-fluid calculation is performed with an idealized two-dimensional symmetric representation of the GT-MHR and compared with the RELAP5-3D simulation results. Also, a neutronics calculation is conducted using the same geometry as the thermal-fluid calculation, and using cross section data obtained from an HTGR benchmark problem. In addition, a coupled steady-state thermal-fluids neutronics calculation is performed. The calculation results showed that the developed prismatic VHTR core simulator can perform tightly-coupled multiphysics simulations efficiently, taking full advantage of the MOOSE framework. It is expected that due to the flexibilities of MOOSE the accuracy enhancement of nuclear reactor simulations via higher-fidelity physics models such as Navier-Stokes and transport corrected neutron diffusion equations can be added and utilized while retaining the speed of simulations. (author)