Thermodynamic analysis on xenon stripping to shorten restart time in molten salt microreactors

Xenon-135 is the strongest known neutron poison; its precursor chain is prominent in the set of uranium-235 fission products. Compound dynamics in the decay chain result in a large 135Xe concentration spike following shut-down, which can make restarting the reactor difficult or impossible for a period of days. This may be the single greatest hurdle inhibiting nuclear power from breaking into power peaking and remote grid applications. In conventional solid fueled reactors, operators must wait for the super-equilibrium xenon (or 135Xe in concentration above the full-power equilibrium level) to decay before restarting. Molten Salt Reactors (MSRs), which have their fissile nuclides dissolved in a liquid salt, present the opportunity for advective removal of xenon. A stream of helium gas may strip dissolved and entrained xenon such that the core returns to its power dependent equilibrium 135Xe concentration much more quickly than by beta-decay alone. This would allow the reactor to restart sooner. This study investigates the feasibility of restarting a MSR more quickly than traditionally viable. We derived a numerical solver and a single stage equilibrium model of a 2-phase mass transport unit operation to this end. Results show that the thermodynamics of xenon stripping from molten salt fuel are highly favorable, with a very small amount of helium being theoretically required for practical control of the 135Xe decay chain after shut-down. Additionally, fuel lifetime could be extended by exchanging poison reactivity at end-of-life for deeper burn-up; this would require a relatively small operational cost of helium. Further investigation into the kinetics and unit operation design is warranted by the favorable thermodynamic results.