| Nuclear power


The heat output is passed using molten salt heat exchangers to the GridReserve®


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 fuelled power 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


Reactor building


Heat exchangers


Tank top (shield)


Tank pit (shield) Reactor core


Reactor cutaway. This cutaway view emphasises the design philosophy behind the FLEX reactor – namely, to keep the engineering as simple as possible. © MoltexFLEX


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 1000°C, the rheometer is now generating data on the behaviour of the proposed fuel and coolant salts for the FLEX reactor, material properties which have hitherto been little explored. Additionally, MoltexFLEX is involving 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. MoltexFLEX is also 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.


Passive


‘thermometer’ device regulates nuclear reaction


Natural


convection molten salt heat exchange


Heat exchanger


Natural convection molten salt cooling


Molten salt


thermal storage tanks


Grid electricity


Molten salt fuel contained in vented pins (unpressurised)


Reactor core


Pump


Industrial and district heat


Nuclear licensed site


Conventional non-nuclear plant and equipment


Schematic drawing. This schematic drawing again shows the simple nature of the FLEX reactor design – keeping the radioactive fuel salt in tubes simplifies the corrosion control challenge, and pumped flow is replaced by natural convection in the reactor island. © MoltexFLEX


www.modernpowersystems.com | April 2023 | 25


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