Transient behavior of a molten salt fast reactor under two-phase flow conditions with helium bubbling

Abstract

In the present study, transient behaviors of a chloride molten salt fast reactor without control rods under two-phase flow conditions are analyzed assuming a bypass of the safety protection system. The two-phase flow in the core is caused by helium bubbles injected to remove the gases of the fission product. The void fraction under the rated condition is assumed to be as low as 0.2 %. However, as the fuel flow rate decreases, the void fraction increases inversely proportional to the flow rate and must be considered in any transient analysis. Therefore, analyses with a point reactor kinetics and thermal-hydraulics coupling model have been performed to account for void flow using the CFD code. Comparison between the transient under two-phase flow and the transient under single-phase flow in which the effect of reactivity due to voids is taken into account with the CFD analysis, shows almost the same results, although there are slight differences due to the difference in fuel volume corresponding to the void fraction. When analyzing transients with a system code that has difficulty handling two-component two-phase flow, the void effect has been considered using the above-mentioned single-phase flow analysis. In this way, the entire plant is analyzed using the RELAP5-3D code and the core inlet conditions are passed to the FLUENT code to analyze the transient behavior. The transients analyzed are UTOP, USBO, ULOFF, and ULOHS under the assumption of bypassing the safety protection system. In addition, a transient assuming the loss of helium bubble supply under the decay heat power conditions is analyzed and discussed. The results of the analysis are compared with the results of the same event without voids analyzed using RELAP5-3D. In the case of single-phase flow, the reactor power changes mainly due to temperature feedback. Although the two types of transients have similar appearance, the breakdown of reactivity is quite different. Relating the comparison between the calculated result using the system code and CFD code shows that for the short time behavior until the fuel flow rate shifts to low flow rate, the results of the CFD code, which can analyze the behavior of the helium voids, are more appropriate. However, there are no major problems in the same transient calculation using RELAP5-3D, which calculates the entire plant system.