Coupled Thermal-hydraulics And Neutronics Simulation For Natural Circulation System

Natural circulation system has attracted a wide attention for many advantages,such as simplicity in structure,inherent safety and low noise level.Meanwhile,reactivity feedback influences power change and then affects the variation of thermal-hydraulic parameters.The study of reactivity feedback in natural circulation system provides strong support to reactor design and safe operation.Considerable calculations and theory analysis of natural circulation system in steady state and transient state are conducted in this paper.The heating section is divided into three sections,including single phase,subcooled boiling section and saturated boiling.Separated-phase model is adopted in the calculation for two phase flow.The point reactor neutron kinetic equations with six-group delayed neutrons are used to get natural circulation transient power during physical process.Complicated coupled relationship between thermal parameters and core reactivity is established on the analysis of effects of fuel temperature,coolant temperature and void fraction on reactivity.The conservation equations are used to get flow rate during thermal process.MATLAB software is chosen for self-programming,including steady state program for natural circulation,simple transient state program and nuclear coupled program in transient state.RELAP5/MOD3.4 is applied to verify the effectiveness and rationality of previous results.The results indicate that steady natural circulation flow rate ascends with the same trend of the increasing power or system pressure.It reaches the maximum and tends to decline.The steady flow rate could decrease because of the increasing inlet subcooled temperature or the decreasing diameter of heating section.Flow excursion is suppressed by decreasing power,increasing system pressure and increasing diameter of heating section.In transient natural circulation without nuclear coupled section,raising system pressure,loop diameter and the length of riser make the system more stable.Subcooling exposes non-linear effect on system instability.Flow instability only occurs if inlet subcooling varies in specific range.When power or flow climbs up,the frequency of flow pulsation rises as well.In the transient analysis of the natural circulation system with reactivity feedback,if the natural circulation system only has coolant or heating test section temperature feedback,it can be more stable.However,if the natural circulation system only has void feedback,it can be more unstable.Comprehensive analysis suggests that the nuclear coupled mode is more stable than the pure thermo-hydraulic mode.Larger absolute value of coolant temperature,heating test section temperature or void feedback coefficient makes the amplitude of reactivity and heating power increase.When the change of the load is small or a small reactivity is introduced,the system can return to stability.Natural circulation system operation has a self-stability and load follow feature to some extent with negative reactivity coefficient.Comparing the results of RELAP5 and MATLAB self-program,it is concluded that self-compiling program is accurate and reasonable.Stability boundary is obtained in the typical conditions.