Investigating the Accuracy of Porous-Medium Treatments in the Analysis of Nuclear Thermal Propulsion Systems

Porous-medium thermal-hydraulic modeling has become more popular in recent years for reactor analysis due to its flexibility and the ease of its implementation within multi-physics, PDE-oriented libraries like OpenFOAM and MOOSE.A porous-medium approach often provides similar results to that of heterogeneous (multi-region) models while only taking a fraction of the computational resources.However, nuclear thermal propulsion (NTP) systems feature core heterogeneities and temperature gradients that challenge the use of a porous medium approach.This work presents the results of using the OpenFOAM-based porous medium solver GeN-Foam against results obtained using a heterogeneous solver on a fuel assembly in a reference NTP system.We employ two different porous-medium models with different levels of complexity, and we propose and test a GeN-Foam extension that allows modeling the axial and radial heat diffusion through a sub-scale structure.This heat transfer extension provides increased accuracy and the ability to simulate temperature redistribution in the solid structures.for instance after reactor shut-down.Results show that the inaccuracies associated with using a porous-medium approach are relatively small and generally acceptable in most cases, notably when considering the extra computational cost associated with heterogeneous simulations.Additionally, the heat diffusion extension to GeN-Foam shows great agreement with multi-region heat transfer solvers in OpenFOAM, verifying its implementation.