Numerical Simulation Of Flow Distribution At The Entrance Of The Reactor Core

Coolant mass flow distribution at the bottom of the reactor core is important because it directly affects the critical heat flux(CHF)and hot channel factor which has significant effects on thermal-hydraulic behavior of core and,consequently,the operating limits of the nuclear power plant(NPP).To achieve the higher nuclear safety standards that were improved after Fukushima nuclear accident,the PWR design studied in this paper has improvement and optimization in many key aspects.The new design gets better flow characteristics by simplification for lower plenum internals and improvement of the flow distribution devices.To verify the rationality of the reactor design,the analysis for coolant flow distribution at the bottom end of the reactor core and internal flow field is needed.Computational fluid dynamics codes were used to study the flow characteristics of the downcomer and lower plenum of PWR through analysis of established model and the computational data.Sensitivity of the internals was also analyzed.On the basic of above analysis,studies for flow distribution at the bottom end of the reactor core and internal flow field are implemented under main pump rotor seizure accident and main pump lose power accident.The calculation results show that,coolant distribution is uniform at the entrance of reactor core components,as the maximum value of normalized coolant flow is 1.070 while the minimum value is 0.885.Coolant flow is fairly uniform in the central region and slightly decreased in the marginal region.Regarding the flow path of coolant,coolant flow distribution has large gradient at the outlet of downcomer and finally gets good performance with the help of lower plenum internals.Flow resistance gradually decreased in following geometric construction: flow equalization plate,downcomer,core support plate,support component.The pressure value at the lower support plate and the bottom of channel head are highest.The flow equalization plate is efficient and can inhibit the development of eddy current,but it produces greater flow resistance.Support component has small impact on flow distribution,but it can efficiently inhibit the development of eddy current with eddy current suppression plate.As the flow equalization plate,the lower support plate is efficient and has greater flow resistance.Stronger and more complicated eddy currents are found under main pump rotor seizure accident,however flow distribution finally gets uniform with action of the internals..The phases of main pump lose power accident have little impact on the flow distribution at the entrance of the reactor core.