On unprotected loss-of-flow accident in a generic molten-salt reactor

Unprotected loss-of-flow (ULOF) transients in molten salt reactors (MSRs) are governed by a unique interplay between thermal-hydraulics and neutronics due to the circulation of the fuel salt, which carries the delayed neutron precursors (DNPs). This study analyzes a generic fast-spectrum molten chloride reactor, similar to the LOTUS design, during a ULOF accident using the SIMMER-III multi-physics code. The work investigates the previously observed double-peak in the power evolution and examines the conditions under which this behaviour occurs. The first power peak arises primarily from the retention of DNPs in the core as flow decreases, while the second peak is driven by the return of colder fuel salt from the heat exchanger, inserting positive reactivity. Between these peaks, a power valley occurs when negative temperature feedback temporarily dominates. A comprehensive parametric study examines the influence of key operational parameters on the magnitude, timing, and presence of these characteristic features. Results demonstrate that higher mass flow rates and longer coast-down times generally increase the amplitude of the first power peak, while lower heat exchanger temperatures and lower initial powers enhance both peaks and prolong oscillations. ... mehrFor high-power operation​, and certain flow conditions, the double-peak behavior can be significantly damped or even eliminated. For MSRs operating at low power, the risk of significant power oscillations increases. For example, for the reference conditions, an initial power of 0.8 MW results in a first peak of approximately 104%, a subsequent valley of 55%, and a second peak of 108% of the initial power. Increasing the initial power to 3.2 MW under the same conditions eliminates the first peak, while the power decreases directly to 24% before recovering to 40% of the initial value. The analysis highlights the strong sensitivity of MSR transient behavior to steady-state design parameters and underscores the importance of accurately modelling DNP advection and heat exchanger performance in liquid-fuel reactor safety assessments.