SMRs | THERMAL POWER
fuel and coolant salts for the FLEX reactor, material properties which have hitherto been little explored. Additionally, MoltexFLEX is working together with external
partners in its research. Thanks to a grant from the Henry Royce Institute as part of the Industrial Collaboration Programme, the company is now collaborating with scientists at the University of Manchester’s Nuclear Graphite Research Group to characterise how molten salt interacts with standard industrial-grade graphite. Using industrial- grade synthetic graphite with high thermal and chemical resistance will deliver significant cost savings for the FLEX reactor. Finally, MoltexFLEX is demonstrating corrosion control
of the alloys proposed for use within the reactor vessel and the fuel tubes. Corrosion of metals by molten salts is a long-standing problem in the industry. This is prevented in the coolant salt by dissolving small amounts of aluminium within it, which scavenges any oxidising agents before they can attack the metal. The FLEX reactor uses a patented eutectic mixture of
Above: This cutaway view emphasizes the design philosophy behind the FLEX reactor – namely, to keep the engineering as simple as possible
exchangers to the GridReserve®
The heat output is passed on using molten salt heat storage tanks, located
outside the nuclear island. This stored heat can then be used to generate superheated steam to drive conventional turbines as used in conventional fossil fuel plants. This configuration is possible because, in contrast to conventional reactor designs, the safety of the FLEX reactor is completely independent of the heat sink provided by the heat storage and power generation plant.
Inside the reactor
Both the fuel – a low-enriched uranium fluoride – and primary coolant consist of molten salt, with corrosion controlled through the salts’ chemistry, preventing oxidation and leaching. The fuel salt is contained in steel tubes, each placed in a separate channel in a graphite matrix moderator, forming the reactor core. The core sits within the reactor tank vessel, which is filled with primary coolant. The vessel is placed in a concrete pit underground and covered with a concrete shield. Convection circulates the molten fuel salt within the fuel tube, with heat transferred through the tube wall to the surrounding coolant. During normal operation, heat from the fuel is removed
from the core by natural convection of the primary coolant salt within the reactor tank. Residual decay heat is continually removed by natural circulation of air around the tank. These passive mechanisms ensure that no active systems or pumps are required for heat removal or shutdown.
Latest development work MoltexFLEX’s in-house development team has made considerable strides over the past year. For example, in concert with scientific instruments manufacturer Anton Paar, the company has recently installed a rheometer inside a climate-controlled inert gas glovebox at the MoltexFLEX laboratory in Warrington, Cheshire – an installation unique in the UK. Capable of measuring the viscosity and density of molten salts at temperatures up to 1,000°C, the rheometer is now generating data on the behaviour of the proposed
38 | May 2023 |
www.neimagazine.com
aluminium fluoride and sodium fluoride as primary coolant. Similar salt mixtures have seen decades of use in the aluminium smelting industry, giving high confidence in their lack of interaction with the graphite moderator. The fuel salt chemistry is redox stabilised by a combination of uranium oxidation states, acting as a redox buffer in a eutectic mixture with sodium fluoride diluent. The buffer enables maintenance of redox potential, and neutralisation of potentially corrosive fission products generated throughout the reactor’s life. Alloy sample tubes containing fluoride salts have been
heated to the FLEX reactor operating temperature of up to 900°C and maintained at that level for many months in the MoltexFLEX laboratory, at the end of which they exhibit little evidence of corrosion – even at the micron level when examined under a scanning electron microscope.
Looking ahead MoltexFLEX is scaling up to build on the progress the company has made over the past year, which is expected to see multi-fold increases in the size of its team to several hundred internal and external staff over the next two years, and is currently having fruitful discussions with private investors to fund the next stage of development. While the company believes it can bring the first-of-a-kind FLEX reactor to fruition via private finance alone over the next 6-7 years, it is eagerly awaiting the establishment of Great British Nuclear and the opportunity to work with them to establish advanced nuclear technologies such as FLEX in the energy mix. In the meantime, the company is increasingly gaining recognition for its ground-breaking technology – for example, in April it was named as a Champion as part of the Green Builders Of Tomorrow initiative, run by the Department for Business and Trade. As part of this initiative, Moltex are joining five other companies in travelling to the UAE to meet potential investors. To conclude, MoltexFLEX believes that its SSR technology, and the FLEX reactor in particular, have a huge contribution to make – not just towards achieving Net Zero by 2050, but also in bringing abundant, clean and low-cost energy to the developing world and restoring Britain’s lead as a nuclear innovator. It’s this vision that drives the MoltexFLEX team every day towards making the FLEX reactor a reality. ■
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