Coupled Neutronics And Thermal-Hydraulics Research Of Multi-Channel Liquid-Fueled Molten Salt Reactor

As one of the fourth generation advanced nuclear energy systems,the liquid-fueled molten salt reactor(MSR)has the characteristics of high temperature,low pressure,high chemical stability,etc.It is suitable for efficient utilization of thorium resources and miniaturization,and can be used for power generation in remote and isolated areas,seawater desalination,high-temperature electrolysis hydrogen production,etc.In January 2011,dedicated to the efficient utilization of thorium resources and the comprehensive application of nuclear energy,the Chinese Academy of Sciences launched the Future Advanced Nuclear Fission Energy-Thorium Molten Salt Reactor Nuclear Energy System project.In the channel-type liquid-fueled MSRs,the fuel salt flows through the graphite channel,and acts as both fuel and coolant.Due to the flow effect of the fuel,the neutron physical and thermal-hydraulics features are different from those of traditional solid fuel reactors.Delayed neutron precursors(DNP)circulate with the fuel molten salt in the primary loop,causing a loss of reactivity.Most of the energy generated by nuclear fission is directly released into the molten fuel salt,and another part of the energy released in the graphite is also taken out of the core by the fuel salt through convection heat transfer.The neutron flux density,the concentration of DNP,the temperature field,and the flow field are strongly coupled in liquid-fueled MSRs.The physical models and calculation code of traditional solid fuel reactors are not suitable for liquid-fueled MSRs.Because of this,the following research work is carried out in this paper.The mathematical model and numerical method are deduced first.Based on the exponential transformation method and the second-order nodal expansion method,the spatio-temporal multi-group neutron diffusion equation of liquid-fueled MSRs is solved,and the one-dimensional DNP transport equation is solved by the method of characteristic and the finite difference method.Finally,the neutron dynamics model of the liquid-fueled MSR under Cartesian geometry and hexagonal geometry is constructed.The neutron adjoint equation and the adjoint DNP equation are solved in the same way.The few group homogenization parameter required for the neutron physics calculation is obtained by using lattice code or Monte Carlo code to generate data at different temperature points and then calculated by interpolation.The three conservation equations of one-dimensional single-phase flow are solved using the method of characteristic and the finite difference method.The thermal conductivity equation of the graphite is deduced with cylindrical approximation,as well as the corresponding analytical solution,the finite difference method and the layered analytical method.Then the multi-channel thermal fluid model of the liquid-fueled MSR core is established.Based on the energy conservation equation of the molten salt in the loop,as well as the finite difference method and the method of characteristic for solving the equation,the thermal fluid model of the loops is established.According to the mathematical model and numerical method established above,three calculation modules of neutron dynamics,core multi-channel thermal fluid,and loop system were developed.Through the modular implicit internal coupling method,a neutronics and thermal-hydraulics coupling code Thor CORE3 D for channel-type liquid-fueled MSRs is constructed.The independent validation of the modules and the validation of the coupled code are carried out using the Molten Salt Reactor Experiment(MSRE)experimental value and the single-tube analytical solution.The results show that the Thor CORE3 D neutronics and thermalhydraulics coupling code is suitable for steady-state simulation and transient analysis of channel-type liquid-fueled MSRs.To deeply analyze the neutronics and thermal-hydraulics coupling effect of liquidfueled MSR,the steady-state and transient models of the MSRE are established using Thor CORE3 D.First,a DNP steady-state effect analysis is carried out.The results show that the primary loop recirculation time,the fuel salt flow rate and the distribution of the fuel salt flow field will significantly affect the steady-state effective delayed neutron fraction(DNF)loss of the liquid-fueled MSR,and the effect on DNF of different DNP groups is different.Both DNP out-of-core decay and DNP core distribution can cause loss of DNF.Then,the neutronics and thermal-hydraulics coupling effect in MSRE reactivity insertion transient,overcooling and overheating transients,and lose of primary flow transient are analyzed.The results show that due to the different initial DNFs under different flow field states,the power and temperature responses of the system are different in the reactivity insertion transient,and the power and temperature can eventually be stabilized in the same state due to the negative feedback effect.MSRE can shutdown in a short time in overheating transient,which confirms the safety of liquid-fueled MSRs.In the overcooling transient,the reactor power increases rapidly,the final power value is positively related to the initial power.In the overcooling transient,the core power variation is affected by the combined effect of DNP flow effect and temperature negative feedback effect.When the initial power is low,the DNF loss reduction effect dominates,and the power will increase first.When the initial power is high,the temperature negative feedback effect dominates,and the core power directly decreases.Through the analysis and comparison of different fuels,it was found that although the DNF of U-233 fuel is lower,it still has good safety due to the higher temperature reactivity coefficient.Based on the above,according to the design and transient analysis requirements of liquid fuel molten salt reactor,and according to the characteristics of channel-type liquid-fueled MSRs,the neutronics and thermal-hydraulics coupling model of liquidfueled MSR including loop system is established,and the coupling code Thor CORE3 D is developed.Then,through the steady-state and transient neutronics and thermalhydraulics coupling analysis of MSRE,the important properties of the liquid-fueled MSR are revealed,which provides a powerful tool and practical reference for the design and transient analysis of liquid-fueled MSRs.