Work

A coupled compressible multiphase thermohydraulic–neutronics solver for simulating rapid reactivity insertions in molten salt reactors

Year: 2026

Type: article

Abstract: Molten salt reactors (MSRs) present specific challenges for safety analysis, arising from the liquid and circulating nature of the fuel, the strong coupling between neutronics and thermohydraulics, and the potential presence of gases. During rapid reactivity insertions, compressibility effects — including pressure and shock wave propagation — can play a significant role that is not captured by conventional incompressible or quasi-incompressible approaches. This work presents Cognacc, a reduced-order coupled neutronics–thermohydraulics code developed for fast transient simulations of MSRs. The thermohydraulic module solves the compressible homogeneous relaxation model (HRM) equations using an HLLC Riemann solver, closed by the Noble–Abel Stiffened Gas equation of state, enabling accurate representation of pressure waves, phase change, and non-condensable gases. A dedicated equation of state for an actinide chloride molten salt is derived from molecular dynamics calculations. The neutronic module is based on improved point kinetics equations accounting for the convective transport of delayed neutron precursors throughout the fuel circuit. The code is verified and preliminary validated through a progressive procedure: classical shock-tube test cases confirm the correct numerical implementation; the CANON depressurization experiment validates the phase-change and variable flow area models; and code-to-code comparisons against LiCore under slow transient conditions verify the multiphysics coupling. The results demonstrate the ability of Cognacc to accurately reproduce coupled neutronics–thermohydraulics transients while maintaining low computational cost, supporting its use as an exploratory tool for MSR design and safety studies.

Source: Annals of Nuclear Energy

DOI: 10.1016/j.anucene.2026.112819

Authors: Anna Maître, Nathalie Seiler, Frédéric Bertrand, Axel Laureau, Elsa Merle