Mixing of helium with air in reactor cavities following a pipe break in HTGRs - High fidelity and engineering CFD model development and validation

The High Temperature Gas-cooled Reactor (HTGR) design is one of the six advanced reactor types selected by the Generation IV International Forum (GIF) for development to be employed in the near future. It is the only design which allows the generation of hydrogen alongside the generation of electricity. HTGR is one of the key reactor designs that are currently supported in the US and a number of other countries. In the UK, the Department for Business, Energy and Industrial Strategy (BEIS) has recently supported feasibility studies for eight advanced modular reactors (AMRs), three of which are HTGRs, to facilitate the UK's engagement for the development of such advanced technologies. HTGRs are designed to avoid fission product release under any conditions, even beyond design basis accidents, by utilizing passive safety systems. However, the air ingress following the depressurization of an HTGR has been identified as an important risk to the core safety. Significant work has been carried out recently to investigate this phenomenon, but most have focused on the later stages of the process including the air-refill of the reactor building and air-ingress into the reactor pressure vessel. In contrast, the first stage, blowdown, has rarely been investigated in detail to date. This is a highly complex transient process, with the flow transitioning from a highly under-expanded supersonic jet, to a weakly under-expanded supersonic jet, and finally to complex natural circulation. This poses a huge challenge to modelling.

This proposal and the associated US proposal are aimed at investigating the spatial distribution of air/oxygen and helium in each reactor building cavity during and after the blowdown phase. The objective of the proposed research by our partners in the US is to obtain experimental validation data on mixing of helium and air in reactor building cavities during and after blowdown in HTGRs such as a General Atomics 350 MWt MHTGR. The purpose of the proposed research in the UK is to carry out numerical simulations to complement the experimental endeavours carried out by the US partners. The UK work is organised into two work packages. Work Package 1 is aimed at developing and validating engineering Reynolds-Averaged Navier-Stokes (RANS)-based CFD models for the simulation of the full transient process during and after blowdown, from the initial pipe break to the time when equilibrium is reached and continuing to the following air-refill phase. This model will be one of the first to look at the complete transient process, and we aim to bring in innovative numerical methods to deal with the transition from compressible to incompressible flows. Work Package 2 is aimed at developing high fidelity CFD models based on well-resolved Large Eddy Simulation (LES) for the study of fundamental flow physics underpinning the air-ingress phenomena in a HTGR. This is to advance the understanding of such phenomena and provide detailed information and data, complementary to experiments, to support the development of engineering CFD models and correlations. In addition, effort will be made to compare computer codes used in the UK and the US to evaluate the consistency and discrepancies between them.

The ultimate goal of this joint project is to advance the high temperature gas cooled reactor (HTGR) technology so it will become available in the near future to produce electricity at high levels of safety and efficiency in an environmentally friendly manner. This will consequently lead to improving quality of life.

The immediate beneficiaries are the engineers in R&D of HTGRs, model developers who are interested in simulating a complex transient process involving flow transitioning from strongly compressible flow to an incompressible flow and researchers who are interested in strongly under-expanded jets flow physics and high fidelity modelling of such flows.

The second (air-refill) and the third (air-ingress) phases of the HTGR Depressurized Loss of Forced Cooling (D-LOFC) processes have been investigated relatively thoroughly recently despite there being still many unknowns. In contrast, the first stage, blowdown, has rarely been investigated in detail to date. This is a highly complex transient process, with the flow transitioning from a highly under-expanded supersonic jet, to a moderately/weakly under-expanded supersonic jet, and to a natural circulation scenario with species/temperature driven double diffusion. The flow in the early phases is high compressible with strong shock waves whereas it is incompressible towards the later stages. This transient flow poses a huge challenge to modelling. Our RANS model developed will be one of the first to look at this complete transient process, and we aim to bring in innovative numerical methods to deal with the transition from compressible to incompressible flows.

The high fidelity LES of the stationary under-expanded jet in a confined space and the double diffusive circulation will extend the current knowledge on such flow systems, and will be the first to reveal the detailed flow behaviour in the blowdown phase. The LES will also provide benchmark data for the development and validation of turbulence models. We will aim to publish quality papers in archived journals for long-term and broad impact and will present results at international conferences and specialist meetings/workshops for immediate impact and influences.

International collaboration is an important feature of this proposal and we will use this opportunity to showcase UK's expertise in the field of nuclear thermal hydraulics and HPC. We will make effort to develop a long term collaboration relationship so as to establish a regular route for our research to have an international impact as well as for us to benefit from the resources and expertise of our international partners

Finally, a youth researcher/engineer will be trained to gain extensive expertise in this important field of nuclear thermal hydraulics by the end of the project, and will be ready to be an independent researcher/engineer to pursue a career in academia or a senior position in an R&D/industrial establishment.