SMRs & ADVANCED REACTORS | STELLARIA’S MSR
Right: A graphic showing the design of Alvin, Stellaria’s fission experiment for the development of the Stellarium commercial reactor
subcritical storage tanks. “In case of an accident, it’s
very easy to remove the fuel from the fission chamber and put it in fifteen reservoirs that are, of course, subcritical,” Breyton says. The drain down tanks provide a subcritical configuration
with large surface area for passive decay-heat rejection and in high-threat scenarios, Breyton claims operators could drain the core within fifteen minutes. In addition, mechanically actuated shutdown rods are
retained in the design, although not for fine reactivity control. “You don’t need control rods,” he says, describing them instead as “start and stop rods”. Their purpose is to secure a safe criticality approach during initial startup or to hold the system in shutdown while maintaining fuel in the vessel. If operators require a more conservative safety posture, for example, in the case of extreme external events, the drain-down function provides a final passive shutdown state.
The reactor is designed around multiple negative
feedback processes, including the natural convection within the primary circuit, the vessel radiation, and the cooling of lead reflectors by a network of water inner pipes from passive pools. To support the sustained breeding of Pu 239 “we have 50-70 cm of lead around the reactor reflecting neutrons,” Breyton says. The reactor building is also half-buried with 15 metres
sited below grade behind three concrete barriers and 15 metres above grade. The above-ground portion houses a crane capable of lifting the vessel for periodic replacement
while the half-buried section contributes to mechanical stability under seismic loading and aircraft-impact. According to Breyton, the various safety features allow
for a plant design requiring no evacuation beyond the fence line even in the event of severe accidents including earthquakes, tsunamis and aircraft impact. Extensive simulations of seismic and impact loads, he says, support the safety case. As he says: “When these events happen, you can still do agriculture beside the reactor”.
Reactor life and re-processing Within a Stellarium – the name of the commercial reactor design as envisaged - the effective operating life of a core load is limited not by fissile depletion but by accumulation of fission products. During operations a wide variety of isotopes accumulate within the salt, some of which have high neutron-absorption cross-sections. “After 40 to 50 years, 1% or 2% of these atoms start to pollute the neutronics,” says Breyton. When neutron economy becomes insufficient to sustain criticality at the rated power, additional U 238 can be added to the salt directly as fertile feedstock. “It’s not radioactive,” Breyton says, “at least in terms of decay heat and handling risk. As a result, small periodic additions of U 238 chloride are operationally straightforward, avoiding the enrichment and fabrication infrastructure used for today’s reactors”. The design also includes two mechanisms that allow for
Right:
Industrial operators and data centre operators are increasingly seeking long-term power price predictability for capital-intensive facilities
removal of a significant fraction of the fission products that accumulate. Firstly, a substantial proportion of the non- soluble fission products adhere to the vessel walls. “One- third of the fission products… will stick on the walls of the vessel,” Breyton says, noting that this mechanism effectively reduces the specific concentration of the problematic decay products. Secondly, volatile species also accumulate in the cover gas. “The second third [of the decay products]… will be in the gas above the liquid, so you remove two-thirds of the fission products without changing the salt,” he says. Even so, every 10 years, the inner vessel and primary heat exchangers would be replaced. This schedule, Breyton argues, aligns with current regulatory inspection practices and also goes some way to mitigating the corrosion risks inherent to chloride salts. To enable replacement, the reactor incorporates a double-walled vessel arrangement on the primary side. Both walls are replaceable.
36 | December 2025 |
www.neimagazine.com
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