Scientists at UK-based MoltexFLEX have published new research on how graphite interacts with the molten salt to be used in the company’s FLEX reactor design. Together with scientists at the University of Manchester’s Nuclear Graphite Research Group (NGRG), the researchers used x-ray micro CT scanners to investigate how tiny amounts of molten salt infiltrated pores within standard industrial grades of graphite. This was the first time such scanning has been used for this purpose.
MoltexFLEX has been working with the NGRG on graphite-related research for more than three years. The NGRG team, led by Professor Abbie Jones, is internationally recognised for its particular expertise in graphite research.
MoltexFLEX said in September 2023 that it had “reached a watershed in the development of its small modular FLEX reactor” and would move from the pre-concept science phase into accelerated product and project delivery. Moltex Energy launched its MoltexFLEX subsidiary in 2022 specifically to work on the FLEX reactor – the latest application of the company’s stable salt reactor (SSR) design. This is the thermal spectrum version of Moltex Energy’s SSR technology, which uses graphite as the moderator. That technology is shared with MoltexFLEX’s sister company, Moltex Energy Canada, which is developing a fast spectrum version (the SSR-W).
Graphite is a vital component of the FLEX reactor and many other nuclear reactor designs as a moderator. MoltexFLEX is aiming to use commercially available grades of graphite as part of the company’s ethos of using ‘off-the-shelf’ materials. This will make it easier to mass produce FLEX reactors while keeping costs low, the company noted.
The research, conducted at MoltexFLEX’s Warrington laboratory and the University of Manchester, involved exposing samples of graphite to the molten salt coolant used in the FLEX design. The samples were immersed in the salt within sealed stainless-steel containers for 30 days at temperatures above 750°C – the operating temperature of the FLEX reactor.
“The results were very much as we predicted,” said MoltexFLEX Senior Metallurgist Dr Ciara Fox. “This research is a very promising first stage on the path to predicting and controlling the behaviour of molten salt infiltration within the graphite. It’s an important stepping stone in developing a technique that will allow us to do that.”
The research was published in the journal Materialia. The research paper notd that thermal spectrum Molten Salt Reactors (MSRs), graphite is typically utilised as moderator, reflector, and part of the core support structures. “The inherent porosity within the graphite means that it is prone to infiltration by the molten salt, resulting in changes in its properties. Such property changes must be taken into consideration during reactor design and a good understanding of interaction between molten salt and the graphite microstructure is therefore important in selecting graphite grades for applications in MSRs.”
It points out that graphite/molten salt interaction “has traditionally been investigated using the weight gain after salt infiltration, followed by mercury intrusion porosimetry and post-mortem microstructure characterisations using, for example, Scanning Electron Microscopy”. While this provided an overall description to the infiltration of molten salt into the graphite porosity, a better. understanding of the graphite/molten salt interaction is needed. “How the pore size, shape, connectivity, and location with respect to the molten salt affect the detailed salt infiltration process remains to be understood,” it said.
The new research using a non-destructive 3D microstructure characterisation technique, "can potentially deliver more detailed information for understanding the graphite/molten salt interaction" with the aim of observing the microstructure change due to the salt infiltration. The research was able to provide "a direct comparison of the graphite's microstructure before and after molten salt infiltration”. This made it possible to quantify the relative volume of pores that have been filled by the salt and provided “detailed information about the spatial distribution of the infiltrated salt and the 3D structures of the infiltrated salt”.
The work was supported by a grant from the Henry Royce Institute of Advanced Materials (Royce) under Royce’s Collaboration Programme (ICP) which seeks to boost research, development and innovation activities across the UK. The ICP is part of a £5.6m ($7m) initiative for collaborative, business-led research, development and innovation projects aimed at accelerating progress towards a sustainable future.
Images (a) and (c) show 2D virtual slices of a sample of graphite before (a) and after (c) exposure to molten salt at temperatures of over 750°C. Images (b) and (d) show 3D sections of the region highlighted by the yellow rectangle in (a) and (c). Image (b) shows the structure of pores within the graphite sample, and image (d) shows pores containing salt after exposure highlighted in red. The purple area is the large void visible within the highlighted area in (a) and (c). (courtesy of MoltexFLEX)