Numerical Analysis Of Critical Heat Flux In Small Modular Reactors Fuel Assembly Under Natural Circulation

Critical heat flux(CHF)is a heat transfer phenomenon characterized by the departure from nucleate boiling or the dryout of the liquid film on a heated surface causing the heat transfer deterioration and a sudden increase of surface temperature.CHF is an important thermal hydraulic parameter.For most CHF studies,they usually focus on the conditions of the high pressure and high flow.With the development of the small modular reactors(SMR),the interest on the CHF at low pressure and low flow natural circulation conditions has been continuously increased.In this paper,the numerical analysis of CHF under natural circulation conditions in SMR is carried out against the experimental study of the uranium hydrogen zirconium type TRIGA(Training,Research,Isotopes,and General Atomics)reactors.The RELAP5/MOD3.3 and TRACE thermal hydraulic codes are used to model and analyze the single-bundle,three-bundle and four-bundle test facility.CHF is simulated within the experiment conditions:mass flux varying from 0 to 400 kg/m~2s,pressure varying from 110to 270 kPa,inlet subcooling varying from 10 to 90 K.The CHF valua predicted by codes are compared with the TRIGA experimental data.The main research results are as follows:(1)The trends of CHF predicted by RELAP5 and TRACE codes with mass flow,pressure and subcooling are obtained,and the influence of each parameter on CHF value is analyzed.(2)The error range between the predicted CHF and the experimental value are analyzed.The calculation error of RELAP5 code is within±55%and the error of TRACE is within±40%.On one hand,the error may partly come from that the CHF models used in the codes are not applicable for the conditions in the current experiment.On the other hand,the codes have some limitations in simulating the complex three-dimensional flow channels.(3)Comparing the predicted values of the two codes with the experimental values,and combining the trend and error of the predicted values with the experimental values,it is found that for CHF numerical simulation of the TRIGA experimental facility under natural circulation conditions,the simulation results of the TRACE code are better.Finally,the existing CHF empirical correlations whose application ranges are close to the low flow and low pressure conditions are selected for calculation and comparison.And based on the existing CHF correlation,an improved correlation is proposed based on the experimental data.It is found that compared with other selected correlations,the El-Genk correlation used for slug-churn and churn-annular flow transitions and the improved correlation based on the Yuzhou Chen correlation have a smaller relative error between the calculated value and the experimental value.And these two correlations are more suitable for the prediction of CHF value under the low pressure and low flow natural circulation conditions.The research in this thesis can be used to evaluate the accuracy of CHF prediction model in RELAP5 and TRACE codes under low pressure and low flow conditions,and to evaluate the applicability of CHF empirical correlations under natural circulation conditions.The study in this thesis has a reference significance for the CHF prediction in SMRs under natural circulation conditions.