STELLARIA’S MSR | SMRs & ADVANCED REACTORS
Above: After 40 to 50 years of operations the accumulation of fission products affects the neutronics to the point that the vessel must be lifted out and replaced
waste-management capability – “they can reprocess almost indefinitely the spent fuel from existing reactors” – the combination of high core costs and long, complex reprocessing chains “has not been commercially viable so far.”
Breyton’s argument is that the limiting factor is not fast- spectrum physics but the choice of fuel form. A liquid-fuel system, he says, allows homogeneous fuel composition, continuous in-core breeding, and long residence times without the degradation constraints of solid pins.
Designing a fast-spectrum molten salt reactor The core of Stellaria’s design is a core containing a mixture of fertile and fissile chlorides - primarily UCl₃ and PuCl₃ - dissolved in a purified sodium-chloride-based carrier salt. The design requires strict control of isotopic composition.
For example, natural NaCl contains both Cl 35 and Cl 37 and in a high-flux environment, Cl 35 captures neutrons to produce Cl 36, a long-lived radiological hazard. “Our reactor could work with chlorine 35 but not well,” Breyton says. To avoid the generation of Cl 36, the fuel salt would use “only chlorine 37,” necessitating isotopic separation at industrial scale. This requirement is non-trivial, as Cl 37 separation is rare in current industrial supply chains. The purified NaCl must also be free of moisture and
oxygen to prevent unwanted chemical reactions and to improve corrosion performance. The fuel mixture includes further unspecified additives which Breyton describes as “our secret sauce”. Breyton emphasises the parallel development needed between reactor hardware and fuel-salt manufacturing. “We are a disruptive innovation; we need to have both the track and the road on time,” he says, adding: “The track is the reactor, and the road is the fuel.” Coordination with Orano, which Breyton says possesses the only commercial-scale facility capable of producing chloride fuel precursors, is viewed as essential. As with the classic fast-spectrum breeder configuration
as the plutonium decays one neutron induces fission in another plutonium nucleus, maintaining the chain reaction. The second neutron is absorbed by uranium 238 in the salt, converting it to plutonium 239. “Because the fuel is fully mixed”, says Breyton, “plutonium 239 is replenished in situ and in a spatially uniform manner”. That homogeneity is the distinguishing feature and enables the fissile inventory to remain nearly constant
across decades of operation. In this system, operators add small quantities of U 238 over time to maintain the fertile- to-fissile ratio as fission products accumulate. This results in what Brayton describes as ‘isoreactivity’. “We always have the same amount of fissile atoms from the beginning to the end,” he said. Because the fissile population is held nearly constant by on-line breeding, there is no initial excess reactivity and no depletion reactivity swing. In principle, the reactor could maintain a stable power output for decades without the extensive fuel assembly management typical of light-water reactors. The design thus allows for long-term operation without fuel replacement until fission-product poisoning limits reactivity, potentially as long as 40 or 50 years.
Passive safety design features Within the Stellaria design the heat-transport system relies on natural convection within the core vessel. As the salt heats up, its density falls and it rises through the fluid column. It then flows through integral primary heat exchangers, is cooled and then descends along the outer annulus of the fission chamber before re-entering the central region. This establishes a buoyancy-driven circulation loop without pumps, reducing mechanical complexity.
One element of this design with particular significance
for passive safety is the thermal expansion coefficient of the molten salt carrier. Breyton stresses the importance of the salt’s finely balanced physical properties as a mechanism in ensuring both passive operations and safety. “Dilation of the salt is very interesting and important” he says, adding: “If you add 20°C, the density will be slightly lower and it’s enough to stop the fission.” Acting as an intrinsic negative reactivity coefficient, Breyton describes the response time as occurring “at the speed of sound,” as the density changes propagate within the fluid. A gas-filled zone on top of this liquid salt allows it to easily dilate if needed. Breyton contrasts this with the behaviour of solid-fuel
reactors, which can experience positive reactivity transients in certain fast-spectrum regimes. In a homogeneous liquid, no internal power peaking arises from cracked or relocated fuel, nor is there a risk of cladding-driven reactivity changes. “The core is already melted,” Breyton says, adding: “so you cannot have a meltdown”. Nonetheless, the design does incorporate rapid salt- drain capability to distribute the fuel and salt mix into
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