Criticality Preservation Without Using a Classical Control System in a Small-Scale Demonstrator for a Molten Salt Fast Reactor

The Molten Salt Fast Reactor (MSFR) concept, integrated into the iMAGINE framework, offers a transformative approach to nuclear waste management and fuel cycle closure. However, industrial deployment requires a small-scale demonstrator, DEMO, to validate reactor physics and safety under realistic conditions. This study investigates the central challenge of sustaining criticality in a 50 MWth demonstrator over a 20-year lifespan, with the innovative challenge of operating without employing a traditional mechanical control system. Using the HELIOS code package, simulations reveal that a reference burner core with 19.9% enrichment experiences a significant criticality loss of over 2600 pcm during reactor lifetime. Several non-mechanical compensation strategies were evaluated against operational goals. Over-feeding fissile material can stabilize criticality but risks deviating from the salt’s ideal eutectic composition. Relying on negative thermal feedback requires temperature adjustments of approximately 130 K, which may be “too challenging” for a first-of-a-kind system due to increased corrosion risks and narrow safety margins. Furthermore, while online salt clean-up of noble metals is vital for chemical testing, validation, and optimisation, its direct contribution to reactivity is marginal. A central novel insight is the identification of a “design dilemma”: although increasing core size (reducing enrichment to 14%) sustains criticality through enhanced breeding, it increases fuel costs by 2.5 times and extends the time to target burnup from 20 to about 60 years. Since the primary mission of a demonstrator is to produce high-burnup fuel for analysis quickly, larger cores are counterproductive. The results of this study show that no single method is ideal; instead, either a hybrid approach combining several of the investigated approaches or a future disruptive innovation, such as moderator control for HTGR, is essential to balance reactor physics with practical experimental objectives.