Feasibility of the use of frozen walls in molten salt fast reactors (MSFR-FW)

Lead Research Organisation: STFC - Laboratories
Department Name: Scientific Computing Department

Abstract

The overall challenge for the future energy supply is given in UN Goal 7: Ensure access to affordable, reliable, sustainable and modern energy for all is one of the major challenges for the future of human being. The Research Councils UK Energy Research Program describes the problem as the energy 'trilemma' consisting of the challenges to reduce emissions, enhance security of supply, and reducing the cost. Disruptive innovation seems to be the only way to tackle the energy 'trilemma'.
Nuclear reactor technologies have the potential to deliver a promising solution for this challenge. However, the current nuclear energy system is dominated by Light Water Reactors which are not ideal for the required growth in energy production, since their long term operation is not sustainable due to the insufficient use of the natural uranium. The spent nuclear fuel from these reactors still contains ~95% of its original energy content when it is unloaded and considered as waste further on. Making use of the energy content remaining in the spent fuel by closing the nuclear fuel cycle can provide an almost unlimited energy resource. In addition, it can provide a promising use of the Pu stockpile, an unused energetic asset, which is a leftover from the first attempt to achieve a closed fuel cycle in the nuclear system in UK. From an economic point of view, the Pu stockpile is seen as burden due to the requirement of safeguarded storage. Pu utilization could transform this still existing energetic asset into an economic asset.
The objective of this feasibility study is to assess the applicability of frozen wall technology to molten salt fast reactors which is a key technology for this kind of highly innovative reactors. The core of molten salt reactors is designed such that there are no internal vessel structures and that the fuel dissolved in the salt to achieve a critical mass therein. The fuel is pumped through a number of external heat exchangers to extract the heat to be converted to electrical power. Small quantities of fuel salt are continuously fed and withdrawn from the reactor to allow the separation of unwanted fission products from the salt before being recycled into the core. Consequently, as the fuel salt is liquid, it requires less processing before and after use with smaller on site inventories of fuels and waste. A 3 GWth (1-1.5 GWe) reactor is proposed as a direct burner of the spent nuclear fuel accumulated from 60+ years of nuclear power generation in the UK. Thus, the MSFR is intended as an affordable, reliable and sustainable supply of low carbon electrical power that can convert an economic and social burden into an economic asset.
One of the key technical challenges in developing molten salt reactor concepts is that the molten salt corrosively attacks almost all materials at the operating temperatures expected in the reactor. The high neutron flux expected in the MSFR is also known to embrittle structural materials. This study is intended to determine whether the use of frozen wall technology is feasible method of protecting the reactor vessel from corrosive attack, before we can proceed with further studies of design concepts.
To assess the feasibility of frozen wall technology, numerical models of the turbulent thermal hydraulics of the 3 GWth reactor vessel will be coupled to structural models of the vessel wall and the neutron transport. The coupled models will enable us to make observations of regions where the power production contributes to the temperature of the molten salt, which in turn will influence the temperature at the interface between the salt liquidus and solidus interfaces. We will be able to determine regions of thick and thin frozen film depths in order to specify how much cooling is required to maintain the salt film to protect the materials and to propose design modifications that can stabilise the flow and reduce the influence of strong temperature gradients expected over the reactor height.

Planned Impact

The energy trilemma of security (primary supply with reliable, resilient infrastructure), equity (affordability and access) and sustainability (efficiency, renewable and low carbon) is a concern for the future energy mix encapsulated in UK and EU decarbonisation goals, the Seventh UN Millennium goal and the GEN-IV-International Forum goals. The trilemma impacts on society as the supply of energy affects employment, wealth creation, food production, climate change, pollution levels, health, poverty and technological development.
The UK stockpile of spent nuclear fuel (SNF) and the already separated plutonium accumulated over the last 60 years can contribute to tackling the energy trilemma. SNF and TRU require long term safeguarding due to nuclear proliferation risks and their radiological impact on the environment, while the safety and security risk for plutonium alone is such that it will cost the UK £73 million (undiscounted) per year to store for the next 100 years. Nevertheless, SNF can provide a secure supply of fuel, as 95 % of the fuel's energy content is unused after one operational cycle.
The current options for SNF and the included trans uranium isotopes are deep geological disposal (DGD). For the separated Plutonium its conversion to mixed oxide fuels for use in a second operational cycle before long term on site storage prior to disposal. The social impact of DGD was shown by recent host community rejection and subsequent legislation. Therefore, suitable technologies that can reduce the complexity and security issues surrounding DGD could ease such concerns. Particularly as the respective cost to the taxpayer and the consumer for a depository for legacy and new build wastes is considerable for a vague solution to be built by 2040. Thus, a technology that tackles the energy trilemma and reduces the complexity of dealing with SNF and Pu will have strong societal and economic impact.
To meet these objectives we intend to develop a molten salt fast reactr (MSRF) that can burn SNF with plutonium as the fissile material. MSFRs are a highly innovative and disruptive technology that aims to close the nuclear fuel cycle and reduce the volumes of SNF to make a safer more secure society with fewer worries over storage and keeping the lights in the long term. 3000 MWth (1000-1500 MWe) MSFRs, operating on plutonium or SNF over an operating lifetime of 60 years, will make a significant contribution to the economy and the energy mix through the generation of a secure supply of low carbon energy rather than being a burden on the economy, the consumer, the taxpayer and the host community. However, material and technical breakthroughs are necessary for MSFR development due to the corrosive nature of molten salts. One such breakthrough for all kinds of MSRs would be the application of frozen wall technology on the reactor vessel. Viable frozen salt films will demonstrate this application of coupled neutronics and thermal hydrodynamics models and it will help determine whether further studies into the long term burn-up fuels and the associated change in the neutron cross section will influence reactor operational parameters and safety.
A supported proposal will improve the existing UK research strength by producing highly capable scientists who can apply the expertise they gain from software development and coupling of fluid dynamic and neutron kinetic codes to the modelling techniques required in the virtual engineering design of any nuclear power plant. Thus, contributing to NIRAB objectives of people, processes and tools to virtual engineering design of the next generation fleet of nuclear reactors. The scientists can then contribute to relevant national and international research programmes via the development of projects that explore the design envelop of new nuclear power plants. The trained scientists can then train or organise the training of other researchers in these techniques.
 
Description This one-year project was about assessing the feasibility of the use of frozen walls in molten salt fast reactors (MSFR). MSFRs are one of the several designs of the Generation IV reactors, which could be up and running by 2040. The nuclear fuel is mixed with the molten salt and is mobile within the reactor circuit. Their main advantage apart from the lack of solid fuel and the potential for a meltdown, is the simplification of the nuclear fuel cycle, e.g. the progression of fuel through a series of differing stages, as the molten salt allows a continuous feed of a small amount of fuel and removal of unwanted fission products.

The first objective of the project was to develop a coupled capability for thermal hydraulics and neutron kinetics, with application to MSFR. This coupling has been achieved using an open-source library called MUI, with the Computational Fluid Dynamics being carried out using the multipurpose software Code_Saturne, coupled with DYN3D to simulate the neutronics of the reactor core behaviour under steady state and transient conditions. This coupling was applied to the 3GW EVOL reactor design.

The second objective was to investigate the impact of power distribution, fluid mixing, local fuel properties on the temperatures observed in the core and the impact that this can have on frozen salt film formation on reactor vessel walls.

A third objective was to demonstrate the ability of an MSFR to operate on plutonium and natural uranium. The natural uranium was considered representative of spent nuclear fuel, while the plutonium was representative of the separated plutonium stored in an above ground repository in the UK.
Exploitation Route One of the main findings of this research is the lack of available data for solidification (phase change during the freezing stage) for highly turbulent flows, with application to the freezing of salt on molten salt fast reactor walls. A new proposal has just been submitted, by the same team, with addition of the University of Manchester to study the problem and generate several databases. The University of Manchester will carry out the experimental work, STFC perform the DNS and LES simulations which will be used to improve RANS modelling, and the University of Liverpool will provide their expertise in neutronics to study apply the finding of the new proposal to frozen walls of a generic reactor design that can operate on spent nuclear fuel.
Sectors Energy,Environment

 
Description The main non-academic impact relates to public engagement and the following article meant for children has mentioned the research carried out during the project, and also some of its findings: https://www.sciencejournalforkids.org/uploads/5/4/2/8/54289603/nuclear_waste_article.pdf
First Year Of Impact 2018
Sector Energy,Environment
Impact Types Societal