Development of a Cost-Effective Simulation Tool for Loss of Flow Accident Transients in High-Temperature Gas-cooled Reactors

TL;DR

This work addresses the need for fast, 3-D loss-of-flow accident analysis in HTGRs by extending SubChCFD with flow unsteadiness, variable-property, and buoyancy corrections. Using a 1/12th core model, the approach captures buoyancy-driven natural circulation during LOFA and demonstrates good agreement with RANS benchmarks while reducing computational cost by more than an order of magnitude. Case studies with two decay-heat histories show distinct natural- circulation patterns and temperature evolutions, highlighting the impact of decay heat on passive cooling performance. The results indicate SubChCFD as a viable tool for rapid safety analyses and parametric studies in HTGR LOFA scenarios, with identified areas for further refinement in peripheral flow and laminar-transitional regimes.

Abstract

The aim of this work is to further expand the capability of the coarse-grid Computational Fluid Dynamics (CFD) approach, SubChCFD, to effectively simulate transient and buoyancy-influenced flows, which are critical in accident analyses of High-Temperature Gas-cooled Reactors (HTGRs). It has been demonstrated in our previous work that SubChCFD is highly adaptable to HTGR fuel designs and performs exceptionally well in modelling steady-state processes. In this study, the approach is extended to simulate a Loss of Flow Accident (LOFA) transient, where coolant circulation is disrupted, causing the transition from forced convection to buoyancy-driven natural circulation within the reactor core. To enable SubChCFD to capture the complex physics involved, corrections were introduced to the empirical correlations to account for the effects of flow unsteadiness, property variation and buoyancy. A 1/12th sector of the reactor core, representing the smallest symmetric unit, was modelled using a coarse mesh of approximately 60 million cells. This mesh size is about 6% of that required for a Reynolds Averaged Navier Stokes (RANS) model, where mesh sizes can typically reach the order of 1 billion cells for such configurations. Simulation results show that SubChCFD effectively captures the thermal hydraulic behaviours of the reactor during a LOFA transient, producing predictions in good agreement with RANS simulations while significantly reducing computational cost.

Figures (19)

• Figure 1: Benefits gain by using the Brunone model in capturing the inertia effect in accelerating flows. The data used here is based on a typical case in Mathur et al. Mathur2018, with flow accelerating from Reynolds number of 2,760 to 15,200 in a plane channel.
• Figure 2: 5-degree wedge for the RANS simulations.
• Figure 3: Predictions of $C_f$ in transient ramp-up and ramp-down flows at various reference temperatures and transient intensities.
• Figure 4: Comparison of SubChCFD-predicted rod surface temperatures in a 2×2 rod bundle with supercritical water. The corrected model (blue) shows closer agreement with reference RANS data (black) than the uncorrected model (red).
• Figure 5: Workflow for implementation of the Petukhov correlations into SubChCFD.