Research On The Numerical Models And Approach For The Prediction Of CHF In A PWR Rod Bundle With Spacer Grid

Accurate prediction of the critical heat flux(CHF)in a PWR fuel rod bundle with spacer grid is of great importance for the design and screening of nuclear fuel assembly.Recently there is little research on the prediction of CHF in a rod bundle with spacer grid through numerical approach.In this paper,a new model for active nucleation site density is developed through theoretical analysis and experimental data.On the basis of the new model,two-phase CFD models are developed for high pressure conditions in which PWR operates.Based on two-phase CFD simulation,combining with the feature of rod bundle and principle of CHF prediction,numerical models and method for the prediction of CHF in PWR rod bundle with spacer grid are proposed.This work provides a new assistant method for the design of spacer grid.PWR usually operates in high pressure conditions,in which region the existing models for active nucleation site density have some disadvantage.Thus in this paper,a new model for active nucleation site density is developed based on theoretical analysis and experimental data.The new model takes the influence of pressure,contact angel and wall superheat into consideration.The new model is proved accurate through comparison with the experimental data,including high pressure conditions.Sensitivity analysis on the submodels of two-phase CFD simulation is performed to understand their influence on two-phase distribution.Then through comparing with the experimental data,the submodels and their coefficients are screened to develop suitable numerical models for the simulation of subcooled boiling flow in PWR high pressure conditions.The new model for active nucleation site density and a model for bubble departure diameter that involves the effect of high pressures are employed.Based on the accurate prediction of two-phase distribution,boiling curves are proposed to predict the CHF.For uniformly heated tubes,a wide range of operation conditions are tested and good agreement is obtained for most of the cases except high quality,low mass flow and low pressure conditions.Nonuniformly heated tubes are also tested and some differences are found between uniform and nonuniform cases.The CHF is also predicted by the observation of near wall void fraction and wall temperature.Both the CHF and their locations are well captured.Based on the developed numerical models and CHF prediction method,simulation is performed for PSBT subchannel test.The calculated void fraction and predicted CHF agree well with the experimental data,proving that the developed CHF prediction method can be used in subchannels.Considering the features of rod bundle with spacer grid,the numerical models are adaptively adjusted and then used in the simulation of boiling two-phase flow in PSBT rod bundle with spacer grid.In high void region,in which DNB often occurs,the predicted results consist well with the experimental data.At last,CHF prediction method for rod bundle with spacer grid is proposed on the basis of two-phase distribution.