Fitting Of Critical Heat Flux Correlation For Fuel Elements

AP1000 is one of the GEN-? nuclear power technologies developed from the traditional PWR.The passive design is introduced to provide a revolutionary safety system.Passive measures are introduced to prevent and relieve severe accidents.The safety system and relevant assistant system are simplified.Therefore this technology has lots of advantages such as safer design,simplified system,less construction and shorter period for emergency response.The most outstanding feature is its passive safety system,which greatly reduces the possibility of man-made mistake,improves the safety and enhances the economic competence.Critical heat flux(CHF)means the critical point above which lots of bubble would form and cover the heated surface,leading to rapid increase of surface temperature and burn of heated surface due to the lower heat transfer coefficient of gas.Among the studies on nuclear reactors,CHF is one of the most concerned issues.It is one of the key points in reactor design to predict the threshold value of heat flux by CHF correlations,which is also the necessary measure to ensure the operation safety.There are two ways to develop CHF correlations for bundles.The first one is based on the CHF correlations for circular pipes,adding correction factors considering the CHF mechanism and effects of grids and cold wall in bundles.The second one is directly making use of the CHF experimental data in bundles.In this paper,previous researches on CHF for low pressure and low mass flux conditions are presented.CHF correlations for both the circular pipe and bundle are summarized.Based on the summarization,effects of parameters on the CHF values are calculated and analyzed using various CHF correlations for bundles.Making use of the CHF experimental data of AP1000 obtained by Westinghouse,the coefficients of the CHF correlation are produced by the least square method with the help of the AP1000 software VIPRE-W.Subsequently a new correlation available for low pressure and low mass flux conditions is proposed.After developing the correlation,the M/P data are checked for normal distribution.The effects of parameters on CHF and the DNBR threshold value are evaluated by the correlation,the results of which show that the parametric effects agree with the physical mechanism and phenomena,and the DNBR threshold value is capable of ensuring the possibility of nonoccurrence of DNB in 95%confidence interval is 95%in fuel assemblies for both normal and anticipated transient operational occurrences.This paper presents a method of developing CHF correlation in bundles,and provides a correlation of relatively high accuracy for predicting the CHF in bundles.The results can be the foundation of theoretical CHF analysis,and provides help for developing other CHF correlations.