Investigation of MELCOR and STAR-CCM+ modeling capabilities to accurately predict air ingress rates from small breaks in high temperature gas-cooled reactors

In a High Temperature Gas-cooled Reactor (HTGR), the study of postulated accident scenarios is necessary for licensing, deployment, and public perception. One specific scenario of concern is the air ingress following a primary loop break. Recent research has determined that small breaks are more likely than large double-ended guillotine breaks. The primary driving mechanism for the air ingress is the buoyant force due to the density differences of helium and air. This leads to a stratified flow through the break, with the lighter helium flowing above the heavier air. This thesis aims to investigate the modeling capabilities of MELCOR and STAR-CCM+ for small break air ingress events. Initial tests present a generic model to test the effects of forced flow and natural circulation through a break connecting a helium volume with an air volume. These tests range in diameters covering large breaks and sizing down to small breaks. The generic models provide a basis for confidence when modeling experimental results from literature, while testing the impact of break diameter, break length, and break orientation. When modeling the small breaks in MELCOR and STAR-CCM+, good agreement is seen between both codes and experimental values for larger diameters, but both codes show lower accuracy for smaller diameter cases when attempting to replicate the experimental results. To test the reason for disagreement, an investigation into the diffusive effects is carried out. The simulation setup, parameter sensitivity, and current limitations are discussed for both codes. Future work is needed to model the combination of buoyancy and diffusive forces correctly in MELCOR and STAR-CCM+. Further studies should also test the extensibility of MELCOR for cases beyond the current break orientation limit.

Name thesis:degree_name

M.S.

Level thesis:degree_level

Thesis

Discipline thesis:degree_discipline

Nuclear, Plasma, Radiolgc Engr

Grantor

University of Illinois Urbana-Champaign

Year dc:date

2025

Author dc:creator

• Boyd, Anthony Joel

Contributors dc:contributor

• Brooks, Caleb S
• Grunloh, Timothy P
• Kozlowski, Tomasz

• Stratified Flow
• Computational Fluid Dynamics
• System Analysis

Statement dc:rights

• Copyright 2025 Anthony Joel Boyd

Language dc:language

en

Handle dc:identifier

https://hdl.handle.net/2142/132595

OAI identifier oai:identifier

oai:www.ideals.illinois.edu:2142/132595