An Innovative Sleeve Approach to Influence the Burnup Swing in a Low-Power Molten Salt Fast Reactor Without a Control System
Year: 2026
Type: preprint
Abstract: Small-scale molten salt fast reactor (MSFR) demonstrators are essential for validating reactor physics, fuel cycle integration, and operational characteristics of liquid-fuel fast reactors. However, unlike large commercial MSFR concepts designed for near iso-breeding operation, demonstrators typically exhibit burner behaviour, resulting in a continuous loss of reactivity during operation. This creates a major challenge for concepts aiming to operate without a conventional mechanical control system. To address this issue, we propose and evaluate an innovative stainless-steel sleeve using the HELIOS-based neutronic modelling framework developed for molten salt reactor studies. The sleeve is a stainless steel reflector inserted inside the core vessel, where it simultaneously improves neutron reflection and provides a reserve of fuel volume that can be released gradually by reducing sleeve thickness during operation and increasing the fuel inventory without additional fuel salt. Parametric studies show that an optimised sleeve thickness of approximately 24 cm reduces the critical fuel inventory by about 30%, a massive upfront cost benefit, while maintaining the same initial criticality. Stepwise reductions of sleeve thickness provide up to 3500 pcm of reactivity compensation, exceeding the approximately 1600 pcm reactivity loss expected during the planned 20-year operating period. Beyond reactivity management, the sleeve significantly decreases fast neutron exposure of the reactor vessel. Lifetime neutron fluence is reduced to roughly 25% of the reference case for neutrons above 100 keV and to about 7% for neutrons above 1 MeV. The concept therefore combines passive reactivity compensation, reduced fuel requirements, and enhanced vessel protection. Results indicate that the sleeve offers a promising pathway toward long-term operation of a small-scale molten salt fast reactor demonstrator without reliance on a mechanical control system.
Source: Preprints.org
Language: English
Subfield: Nuclear Energy and Engineering
Field: Energy
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W7214178820