Impact of photoneutrons on reactivity measurements for TMSR-SF1

Abstract

The solid-fueled thorium molten salt reactor (TMSR-SF1) is a 10 MWth test reactor design to be deployed in 5–10 years by the TMSR group. Its design combines coated particle fuel and molten FLiBe coolant for great intrinsic safety features and economic advantages. Due to a large amount of beryllium in the coolant salt, photoneutrons are produced by (γ, n) reaction, hence the increasing fraction of effective delayed neutrons in the core by the photoneutrons originating from the long-lived fission products. Some of the delayed photoneutron groups are of long lifetime, so a direct effect is resulted in the transient process and reactivity measurement. To study the impact of photoneutrons for TMSR-SF1, the effective photoneutron fraction is estimated using k-ratio method and performed by the Monte Carlo code (MCNP5) with ENDF/B-VII cross sections. Based on the coupled neutron–photon point kinetics equations, influence of the photoneutrons is analyzed. The results show that the impact of photoneutrons is not negligible in reactivity measurement. Without considering photoneutrons in on-line reactivity measurement based on inverse point kinetics can result in overestimation of the positive reactivity and underestimation of the negative reactivity. The photoneutrons also lead to more waiting time for the doubling time measurement. Since the photoneutron precursors take extremely long time to achieve equilibrium, a “steady” power operation may not directly imply a “real” criticality.