The UK government considers nuclear energy to play an important role in the country's energy mix to establish a secure, environmentally friendly and diverse energy portfolio and meet the future energy demand. There are currently two reactors being built in the southwest of the UK and a number of others currently under development. In order to ensure nuclear energy safer and economically competitive, the Generation IV International Forum (GIF) has been formed, which has selected six advanced nuclear reactor designs to be the focus of the development by its member countries. Two of them use liquid metal as primary-circuit coolant, i.e., the sodium-cooled fast reactor (SFR) and the lead-cooled fast reactor (LFR). In the UK, BEIS has recently supported feasibility studies for eight advanced modular reactors (AMRs), three of which are Liquid Metal-cooled Fast Reactor (LMFRs).

The knowledge gaps and modelling challenges in LMFR are broadly speaking related to two facts. Firstly, the heat transfer characteristic of liquid metal is markedly different from that of the conventional fluids (air and water) due to the extremely low Prandtl number, which makes the conventional turbulence models invalid under most conditions. Secondly, the special pool-type design gives rise to thermal hydraulic phenomena including natural circulation and stratification, which are unique for such reactors. Additionally, there is a lack of benchmarking data due to the difficulties associated with the measure of the flow and thermal fields in liquid metal.

This proposal, developed in response to the EPSRC's collaborative research call on 'UK/US NEUP 2019', is aimed at addressing the above challenges. This joint research project will focus on developing instrumentation technology and associated modelling for liquid metal cooled fast reactor. The US partners will carry out experimental investigations, while the UK partners will develop computational tools for high fidelity modelling and conjugate heat transfer analysis.

High fidelity large eddy simulation (LES) of stagnation and stratification flow of liquid metal will be carried out to complement physical experiment to provide valuable detailed data for turbulence and engineering model development, as well as to help in advancing the knowledge in such complex flow phenomena. This will be followed by refinement and validation of the 'conventional' CFD models using experimental and numerical data to develop new understanding of turbulent models and numerical methods for the simulation of liquid metal flows. Finally, a highly innovative conjugate heat transfer model of the sodium-to-supercritical-CO2 compact printed circuit heat exchanger (PCHE) will be developed based on the novel coarse-grid CFD recently developed and the immerse boundary method. If successful, they represent a major advancement in modelling of heat exchangers with highly complex geometry and physics and can be used to assist the design and optimisation of such systems effectively.

Potential Impact:
The beneficiaries include engineers in R&D of liquid metal fast reactors (LMFRs), modellers for liquid metal heat transfer in any industry including nuclear energy and engineers who are interested in modelling complex heat exchangers in an efficient detail-efficiency balanced manner.

The knowledge gaps and modelling challenges in LMFR are broadly speaking related to two facts. Firstly, the heat transfer characteristic of liquid metal is markedly different from that of the conventional fluids (air and water) due to the extremely low Prandtl number, which makes the conventional turbulence models invalid under most conditions. Secondly, the special pool-type design gives rise to thermal hydraulic phenomena including natural circulation and stratification, which are unique for such reactors. Additionally, there is a lack of benchmarking data due to the difficulties associated with the measure of the flow and thermal fields in liquid metal. The proposed research will contribute to addressing the above challenges.

The high-fidelity LES of natural circulation of liquid metal will complement physical experiment to provide valuable detailed data for turbulence and engineering model development, as well as help in advancing the knowledge in such complex flow phenomena. The RANS model will result in new understanding of turbulent models and numerical methods for the simulation of liquid metal flows following refinement and validation using experimental and numerical data. The new knowledge and improved turbulence modelling will consequently help the R&D engineers to optimise the advanced nuclear reactors for both safety and economics.

The conjugate heat transfer model of the PCHE is highly innovative and is exploratory in nature using the novel coarse-grid CFD recently developed and the immerse boundary method. If successful, they represent a major advancement in modelling of heat exchangers with highly complex geometry and physics and can be used to assist the design and optimisation of such systems effectively. This will ultimately reduce cost of heat exchangers and improve thermal efficiency and consequently save energy.