Effect of 135Xe and 135Xem Migration on Reactivity in Molten Salt Reactor

Fissile material dissolved in the molten salt is employed in molten salt reactor(MSR), which circulates through the primary loop. Considering the flow characteristic of fuel in MSR, the neutron poison calculation model such as Xe is different from traditional solid reactor. As an important neutron poison, 135Xe is free to migrate with the flow of molten salt in MSR. The helium bubble system of molten salt reactor can blow the fission products of krypton, xenon and other gases out of the reactor core to improve the neutron economy. The recent TENDL2021 nuclear data library estimates the thermal neutron absorption cross section of 135Xem, which is much higher than 135Xe thermal neutron absorption cross section. In order to predict the effect of xenon on core reactivity more accurately, the effect of 135Xem thermal neutron absorption cross section was added into the Xe poison model based on the lumped volume method. The migration model is made up of cover gas calculation model and xenon worth calculation model. In order to calculate the xenon worth, the loop void fraction was calculated firstly, and the loop void fraction was inserted into the xenon worth calculation model. According to the rate balance equations and mass transfer equations, the concentration of xenon in loop could be calculated to compute the xenon worth. Firstly, the model results considering the flow effect of fuel salt were verified with the experimental results of 8 MW molten salt reactor experiment (MSRE). As the results shown, with the increase of loop void fraction, the steadystate xenon worth increases first and decreases later, which is in good agreement with experimental results of MSRE. Then the influence of fuel salt types and 135Xem on steadystate xenon worth was considered in this paper. The steadystate xenon worth of the 233U system with and without 135Xem is made and compared to a 235U system, indicating that the influence of 135Xem on xenon worth cannot be ignored, which is much higher with the increase of power. Finally, the contribution of graphite, fuel salt and helium bubble to Xe worth was evaluated by the interaction between graphite and helium bubble, indicating that the interaction between helium bubble and graphite cannot be ignored.