Fluid Retention Effect Of VCT And Its Impact On RCS Boron Concentration Dynamic Response

Control rods and boron are widely used to regulate reactivity in PWR nuclear power plants.Boric acid is added to the coolant as a neutron poison to compensate for excess reactivity in pressurized water reactor nuclear power plants.The uniform distribution of boric acid within the Reactor Coolant System(RCS)is critical for reactivity control and safe operation.During the adjustment operation of boron,RCS is charged with demineralized water or concentrated boric acid through the Chemical and Volume Control System(RCV)in conjunction with the Reactor Boron and Water Makeup System(RBM).During this process,the Volume Control Tank(VCT),an important piece of equipment in the RCV,has a serious retention effect on the fluid resulting in a very slow outlet concentration-response.The retention effect will lead to the RCS boron concentration level being unable to reach the target value for a long time.It would affect the timeliness of reactivity control and eventually endanger the safe operation of the unit.During boron adjustment operation,the VCT is essentially a tank with a horizontal buoyant jet.The outlet concentration response characteristics of the tank with a horizontal buoyant jet are studied.The mechanisms and characteristics of mixing and fluid retention inside the tank are analyzed.A multi-dimensional coupled thermalhydraulic analysis model considering the retention effect of VCT for predicting the boron transport and diffusion process during the boron adjustment operation is established and employed to study the evolution and characteristics of the RCS concentration dynamic response.This work has important theoretical significance for assessing the boron transport and diffusion in nuclear power plants and has great application value for the optimization and upgrading of our country’s third-generation nuclear power technology and for improving the safety of existing nuclear power units.In this thesis,an experimental bench is designed and built to study the outlet concentration response characteristics of the horizontal buoyant jet mixing tank.The effects of inlet flow rate,initial concentration difference,and nozzle size on the evolution of the outlet concentration response and response time of the horizontal buoyant jet mixing tank were investigated.In addition,experimental correlations are provided for predicting the response time of the horizontal buoyant jet mixing tank.The main outcomes are as follows.(1)Increasing the inlet flow rate significantly shortens the response time of both the horizontal positively and the negatively buoyant jet mixing tank.(2)For the positively buoyant jet mixing tank,increasing the initial concentration difference or nozzle size can reduce response time.(3)For the negatively buoyant jet mixing tank,the response time increases with the initial concentration difference and nozzle size when the initial jet Richardson number(Rijet)is less than 0.067,and decreases with the increase of concentration difference and nozzle size when Rijet is greater than 0.067.In this thesis,a numerical study of mixing and fluid retention in a horizontal buoyant jet mixing tank is carried out.First,the stratification criteria for the tank with a horizontal buoyant jet is proposed.Second,the mechanism that how jet entrainment and stratification entrainment influence the mixing and fluid retention in a horizontal buoyant jet mixing tank was revealed.Third,the effects of the jet inlet flow rate,initial concentration difference,and nozzle size on mixing and retention were studied.The main outcomes are as follows.(1)Mixing and fluid retention are affected by jet entrainment and stratification entrainment when stratification exists inside the tank in the initial state.When there is no stratification in the tank at the initial stage,the mixing and fluid retention in the tank are only affected by jet entrainment.(2)For the horizontal positively buoyant jet mixing tank,when stratification exists inside the tank in the initial state,the degree of the internal mixing driven by jet and stratification entrainments is positively correlated with the flow rate and negatively correlated with the initial concentration difference and nozzle size at the same dimensionless time.The fluid retention effect is negatively correlated with the flow rate and positively correlated with the initial concentration difference and nozzle size in the declining phase.(3)For the horizontal negatively buoyant jet mixing tank,the degrees of internal mixing and fluid retention are positively correlated with the flow rate,and negatively correlated with the initial concentration difference and nozzle size.(4)When there is no stratification in the tank in the initial state,the effects of the inlet flow rate,initial concentration difference,and nozzle size on mixing and fluid retention are negligible.(5)For a tank with a horizontal positively buoyant jet,when Rijet is between 8.26×10-4 and 6.64×102,the degree of fluid retention is positively correlated with Rijet.Otherwise,it is almost unaffected by Rijet.(6)For a tank with a horizontal negatively buoyant jet,when Rijet is between 7.45×10-4 and 0.221,the degree of fluid retention is negatively correlated with Rijet.Otherwise,it is almost unaffected by Rijet.Based on the above study on the mixing and fluid retention phenomena in the tank with a horizontal buoyant jet,a multi-dimensional coupled thermal-hydraulic analysis model for simulating the boron transport and diffusion process during the boron adjustment operation is further developed in this paper.Based on the numerical model,the interaction between boron transport and mixing within VCT and RCS boron concentration response was investigated.On this basis,the effects of VCT inlet flow rate,VCT initial concentration difference,VCT initial temperature difference,and VCT nozzle size on the RCS boron concentration dynamics response during dilution and boration were studied respectively,and the optimization measures for the boron adjustment process were proposed.The main outcomes are as follows.(1)Increasing the VCT inlet flow rate can significantly reduce the completion time of both the dilution and boration processes.However,the absolute value of the difference between the final concentration and the target concentration(|εft|)has the risk of exceeding the threshold.(2)During the dilution process,increasing the VCT initial concentration difference can reduce completion time but rise the |εft|.During the boration process,increasing the VCT initial concentration difference can increase |εft| and completion time.(3)To minimize the completion time,it should be ensured that the temperature of the demineralized water in the demineralized water storage tank is always kept at a low level,and the temperature level of the solution in the boric acid tank should be increased appropriately.(4)The optimal VCT nozzle sizes for dilution and boration processes are different.Two inlets with different nozzle sizes can be designed for VCT to suit the requirements of the dilution and boration processes respectively.