Open-source CFD for a helium-cooled pebble-bed reactor core

Problem

In a pebble-bed high-temperature gas-cooled reactor (HTGR), helium flows through a packed bed of spherical fuel pebbles and carries heat out of the core. Predicting that flow and its temperatures underpins safety analysis, because it decides whether the fuel stays within its temperature limits.

Resolving every pebble is far too expensive at core scale, so whole-core models treat the bed as a porous medium, with empirical closures for flow resistance and heat transfer. The engineering objective was to find out whether an open-source CFD code could carry this kind of model, and what it would take to trust its results.

My contribution

This was an individual internship project in Allseas' HTGR team. I was responsible for:

• reviewing the open-source CFD landscape and shortlisting candidate codes;
• building an OpenFOAM porous-medium model of helium flow and solid–gas heat transfer in the core;
• designing and running a staged verification programme;
• comparing results with reference codes and assessing the model's limitations;
• presenting findings and recommendations to the engineering team.

Approach

The core model combines compressible helium flow, porous-medium flow resistance, temperature-dependent properties, separate energy equations for gas and solid (local thermal non-equilibrium, LTNE) and a spatially mapped power source.

Rather than trust the full model at once, I verified each ingredient separately before combining them. Each stage isolates one part of the physics:

1. Laminar pipe flow. Checks viscous momentum transport, pressure–velocity coupling and mesh convergence against an analytical solution.
2. Porous-core pressure loss. Checks the porous flow-resistance implementation against analytical values, including the effect of porosity varying towards the wall.
3. LTNE heat transfer. Checks the heat source and the gas–solid heat exchange against an analytical solution.
4. Heated reactor core. Combines everything with a mapped power distribution and checks global mass and energy balances, then compares with reference codes.

Results

The project delivered a working OpenFOAM core model, a staged verification workflow that can be reused as the model grows, and recommendations on the role of open-source CFD in the programme. Numerical results are confidential.

Limitations

• Verification shows that the equations are implemented correctly under the tested conditions. It does not validate the empirical closures, or predicted local temperatures, against experiments.
• Porous-medium models represent the bed through correlations, so their accuracy depends on how well those correlations fit the real core.