Case study

Thermal hydraulic modelling

Frazer-Nash led a multi-phase R&D programme, running until 2021, in the field of nuclear thermal hydraulics under the UK government's Nuclear Innovation Programme.


The UK Government’s 2013 Nuclear Industrial Strategy described significant ambitions to grow the UK’s nuclear capability.

To help fulfil the strategy’s objectives, a ‘Nuclear Innovation Programme’ (NIP) was launched, with the aim of developing the UK as a significant partner in the next generation of nuclear reactor design and deployment.

Frazer-Nash Consultancy led a multi-phase R&D programme running until 2021 in the field of nuclear thermal hydraulics under the NIP. We were supported in this delivery by a team of organisations including both UK and international industrial and academic partners.

Our NIP programme has brought the UK thermal hydraulics community together, provided an impetus for model development and experimentation in this field, and initiated a number of international activities. The technical outputs of this work are captured within technical volumes and case studies, summarised below.


Technical volume summary

  1. Introduction to the technical volumes and case studies
  2. Convection, radiation and conjugate heat transfer
  3. Natural convection and passive cooling
  4. Confidence and uncertainty
  5. Liquid metal thermal hydraulics
  6. Molten salt thermal hydraulics

These technical volumes do not only cover modelling guidance – they also describe how analysis fits into the graded approach of regulatory frameworks, and the use of conservative vs best estimate plus uncertainty (BEPU) models.

Technical volume 2 - convection, radiation and conjugate heat transfer

Technical volume 2 - convection, radiation and conjugate heat transfer

Case studies

The technical volumes are supported by four case studies, providing illustrative reactor-specific ‘worked examples’ of thermal hydraulic analyses, with an emphasis on passive cooling applications and practical guidance showing how the methods outlined in several technical volumes can be combined and applied. The case study applications cover different thermal hydraulic applications and advanced nuclear technologies.


A – Liquid metal CFD modelling of the TALL-3D test facility (TALL-3D):

Using CFD to model forced and natural circulation in liquid metals in the TALL-3D test facility at the Royal Institute of Technology (KTH), Stockholm. A range of fidelities were used, from large eddy simulation (LES), to 2D steady state models, and compared to the experimental data to improve confidence in analysis results.


B – Fuel assembly CFD and UQ for a molten salt reactor (MSR):

Flow through a fuel assembly under forced and natural circulation is analysed to demonstrate CFD of molten salts, the derivation of a porous model representation of the fuel, and the application of uncertainty quantification (UQ) and sensitivity analysis (SA) to the salt thermophysical properties. This study features a molten fuel salt within solid pins, immersed in molten coolant salt.

Case Study A: Liquid Metal CFD Modelling of the TALL-3D Test Facility

Case Study A: Liquid Metal CFD Modelling of the TALL-3D Test Facility

C – Reactor scale CFD for decay heat removal in a lead-cooled fast reactor (LFR):

A number of advanced reactor designs use a passive heat removal system to cool the reactor following a significant thermal excursion as a result of loss of coolant or where the heat removal system has failed. This study investigates natural circulation modelling of the whole primary circuit of a LFR to analyse flow paths and associated heat transfer for decay heat removal.


D – System code and CFD analysis for a light water small modular reactor (SMR):

Emergency cooling injection into the LSTF test facility is modelled and compared to the OECD/NEA ROSA test data. The link between a system code model providing boundary conditions for the CFD case, and how the conjugate heat transfer prediction of the reactor vessel and pipework temperatures from CFD model relates to pressurised thermal shock (PTS) stress prediction is shown.

Case study C: Reactor-scale CFD for decay heat removal in a lead fast reactor

Further Modelling R&D

Three specific research projects were selected for further study, based on interest from industry and international researchers, potential for near-term industrial exploitation and applicability to a range of reactor technologies:


Natural circulation loops:

Exploring the limits of modelling of stable and unstable buoyancy driven loop configurations, using several different modelling tools and turbulence modelling strategies. This work simulates the L2 experimental loop at the University of Genoa to investigate the transient evolution of flow and instability using system code and CFD techniques.


Course-grid CFD:

Further development, demonstration, and validation of SubChCFD as a modelling tool. The two key areas that were expanded are:

  • Coupling the coarse-grid method with resolved CFD and porous media approaches
  • Developing the capability for buoyancy-influenced flows.

Liquid metal heat transfer:

Examining the modelling of low Prandtl number fluids with CFD and generation of high-fidelity benchmark results.

This R&D intends to address gaps in knowledge to support the technical volumes and case studies, and to focus on areas of industrial interest that are applicable to a range of reactor technologies.

Mesh system in sub channel CFD

These outputs are freely available for you to use and benefit from

They can be found on our project website below and we will be holding dissemination events including Q&A sessions from late 2021