Study and design of a molten salt reactor for power supply in space: first glimpse of the core design

As new space missions are planned, longer and more ambitious, the need for a controllable powerful energy becomes a priority. Though electric propulsion still suffers from its very low thrust, advanced designs like NASA's X3 or Ad Astra Rocket Company's VASIMR are filling the gap at the price of a great power supply need. Given this context, we began the study of a 1 MWe reactor for space application. Because of its inherent safety, as well as its high controllability allowing quick power changes, we choose the molten salt reactor technology. The presented work relates to the building of the core design based on the technology of molten salt reactors, already taking into account technological and safety constraints. Initial calculations motivated the choice of a thermal graphite-moderated reactor using a U²³⁵ highly enriched fluoride salt. A study of different configurations and the sensibility of the criticality to some variations of the design, as well as some identified constraints on the system, then oriented the work toward the proposed design. Once a design chosen, the reactivity reserve and the Doppler effect compensation have been studied. This last point is essential to ensure an autonomous start-up of the core, an easier design and the possibility to shut-down and restart the reactor several times as a solid salt means no density negative feedback. Otherwise, the core could not reach on its own its operating temperature without an additional and independent core heater system to melt the salt. Eventually lifespan issues through Monte-Carlo burn-up calculations on the proposed design have been investigated. (authors)