Among the wide variety of nuclear reactor technologies, molten salt reactors stand out as their nuclear fuel is dissolved in a liquid molten salt. When studying a high and fast reactivity insertion into such a reactor, the velocity of mechanical waves propagating in the liquid fuel must be assumed finite and the salt compressible. Indeed, the time scale of the transient is very low, about 10 ms, since the reactor becomes prompt supercritical. Gas bubbles can also be present and must be considered as they significantly impact the propagation of mechanical waves. A finite wave velocity means a delay in the expansion of the liquid fuel following an increase of its temperature, i.e. a delay in the negative reactivity feedback, as well as the propagation of strong pressure peaks which might damage the reactor vessel. The compressible study of such fast transients is therefore of primary importance for the safety of molten salt reactors. Although various codes were implemented to simulate such excursions and did provide valuable insight into the phenomenon, they rely on limiting assumptions due to its complex multiphysical nature. For instance, the gas phase was assumed homogeneous, the overflow tank and neutron transport effects were neglected and the deformation of the vessel due to fluid–structure interaction has not been simulated yet. Thus, a new neutronics/multiphase-compressible-thermal-hydraulics/structural-mechanics coupling tool based on APOLLO3 and EUROPLEXUS is under development at CEA. In this presentation, the strategy for coupling neutronics and thermal hydraulics will be described. An analytical solution to the coupled problem was derived and simulations will be shown to be in excellent agreement with it, verifying the relevance of the selected numerical methods. Finally, comparisons with experimental data from the SILENE reactor will be considered as a validation step of the tool.