The Preliminary Analysis On Reactivity Initiated Transient Of Molten Salt Reactor

Molten salt reactor (MSR) is a type of liquid fuel reactor, in which the fuel is dissolved in afluoride salt. The advantages of MSR such as the good neutron economy, inherent safety, onlinereprocessing, less radioactive waste, sustainable development and nuclear nonproliferation makeit a popular reactor. It then becomes one of the six candidates of the Generation IV Reactor in theGeneration IV International Forum. The liquid thorium-molten salt reactor (TMSR) is launchedby the Chinese Academy of Sciences as a leading science and technology projects. The maingoal of this project is to develop the fourth generation advanced nuclear energy technology.From the experiences of nuclear power development, the reactivity initiated transient analysis isalways the important part of the reactor safety and there is no exception in the MSR. As noted,the fuel of MSR is fluid and the sytem of MSR is in high temperature, low pressure and strongcorrosive. These make the MSR different from the traditional solid fuel reactor. Thus reactivityinitiated transient analysis of MSR is also difference to other kinds of solid fuel reactor. In thisstudy, we have analyzed the types of initial events and the accident in TMSR. The researchmethod of reactivity initiated transient analysis is set up, and the transient behaviors ofLiF-BeF2-ZrF4-UF4with no-thorium fuel and LiF-BeF2-ZrF4-ThF4-UF4with thorium fuel arepresented. The parameters of reactivity and effective delayed neutron are also showed in thisstudy. Finally, we analyzed the power and temperature under transient conditions such as for thestep reactivity initiated event, ramp reactivity initiated event, overcooling event and loss of flowevent. This study enables us to understand the transient of power and temperature of TMSR byreactivity initiated event. Thus the work helps us to get better understanding the safety of reactor.In this research, we firstly analyzed the accident classification of PWR, HTGR and SFR.And then, we studied the initial events and accident types of MSR based on the accidentclassification method in the criterion of HAFs, HADs and IAEA reports. Six types initiatedevents such as reactivity initiated, increase in heat removal by the secondary side, decrease inheat removal by the secondary side, radioactive release from a subsystem or component,anticipated transients without scram, and external events are provided in MSR. According to theconsequence of radioactive release by events or accidents, the events of MSR are divided intofour classifications. These are normal operation, anticipated operation event, infrequent incident,and limited events.The temperature coefficients of reactivity are important for the analysis on reactivityinitiated transient. In this paper, the MCNP5software was applied to the temperature coefficients of fuel and graphite in the TMSR with no-thorium and thorium fuel. We also used the empiricalformula method to analyze the power coefficients of reactivity. The results showed that thetemperature coefficients and power coefficients were negative, which demonstrated the safetyfeatures of TMSR.A part of delayed neutrons precursors drift out of the reactor and decay in the loop, whichwill lead to the variety of reactivity. Based on the neutron diffusion model of MSR, anapproximate method is introduced to study the delayed neutron under different position ofreactor core and various fluid velocity of TMSR. The distribution of delayed neutron of staticmolten salt is also analyzed. From these results, we can find that the varied trend of delayedneutron is consistent in the TMSR with no-thorium and thorium fuel, the delayed neutron sourcedistributions increased in the decrease of the mass flow rate of molten salt, and the effect of shorthalf-time of delayed neutron group was small in the various of mass flow rate.Unlike the solid fuel reactor, the fuel of MSR is fluid. As a result, a part of delayed neutronsprecursors drift out of the reactor and decay in the loop. So, the neutron diffusion model of liquidfuel was deduced from neutron transport model of reactor according to conservation principles ofneutron number. From the neutron diffusion model, the point neutronics kinetic model couplingheat transport model was founded. The solution of the model was solved by using dde23solver,which is the algorithm variable step length of Runge-Kutta method in the Matlab2009software.Finally, we applied this model for benchmarks of MSRE experiment. The loss in the delayedneutron precursor fraction due to fuel motion, the transient under pump start-up and pumpcoast-down was calculated in the benchmarks. The results of this model were compared to thevalue of MSRE experiment and other models, which demonstrated the reliability and accuracy.Finally, the reactivity initiated transient was preliminary analyzed in the paper. The powerand temperature transient were studied in the step reactivity initiated event, ramp reactivityinitiated event, overcooling event and loss of flow event. In the transient of TMSR withno-thorium fuel under full-power, the maxium value of step reactivity initiated event is0.5%k/k,the maxium velocity of ramp reactivity initiated event is0.06%k/k/sec for10seconds, thedecreased temperature of overcooling event in the inlet fuel is125℃.But in the transient ofTMSR withthorium fuel under full-power, the maxium value of step reactivity initiated event is0.4%k/k, the maxium velocity of ramp reactivity initiated event is0.06%k/k/sec for10seconds, the decreased temperature of overcooling event in the inlet fuel is100℃.