# Development of a transient multiphysics solver for nuclear fuel assemblies within a CFD framework

[Examensarbete på avancerad nivå]

The aim of this thesis is to develop and implement a code for solving the coupled multiphysics of PWR fuel assemblies at transient conditions using a computational fluid dynamics (CFD) methodology. The coupled neutronic and thermal-hydraulic problem of nuclear reactors is typically not resolved at all scales in current reactor codes. Due to the complexity of the problem only a coarse coupling between the separate fields of physics is typically resolved. With advances in affordable computer power it has become increasingly feasible to solve the coupled problem with full resolution on the fuel pin cell scale. While not yet practical for full core simulations, such calculations can be applied to fuel assemblies in high performance computing (HPC) environments. This work extends the capabilities of an existing code framework, developed within the project FIRE at Chalmers University of Technology. The FIRE code is based on the open source C++ library OpenFOAM, a continuum mechanics code built on finite volume discretization. While the focus of FIRE has previously been on steady-state solvers, this thesis is focused on solution of transients. The main application couples a neutronic module with a thermal-hydraulic module, solving the coupled problem by iterating between the modules and advancing the time at joint convergence. The length of time steps is controlled throughout the simulation by an adaptive algorithm. Two neutronic solvers have been implemented; one based on the diffusion approximation and the other solving the neutron transport equation using the discrete ordinates (SN) methodology. The thermal-hydraulic module is based on a Reynolds averaged Navier-Stokes (RANS) methodology solving the single-phase conservation equations for mass, momentum and enthalpy. Turbulence is modeled using the standard k-epsilon model. The pressure-velocity coupling is based on the PISO algorithm. In addition to the equations for fluid transport the conjugate heat transfer from the fuel to the moderator is solved. The coupled solver has been applied to a set of inlet temperature transients to demonstrate its multiphysics capabilities. The code is shown to capture both fields of physics as well as their interdependence.

**Nyckelord: **Multiphysics, neutronics, thermal-hydraulics, CFD, deterministic reactor modeling, conjugate heat transfer, diffusion, discrete ordinates method, OpenFOAM

Publikationen registrerades 2015-06-26. Den ändrades senast 2015-06-26

CPL ID: 218901

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