Constrained by limited experimental data, development of CHF correlation for PWR fuel assemblies under transient and accidental conditions at low pressure levels (2–10 MPa) is a typical statistical problem with small sample amounts, but simultaneously has requirements of high prediction accuracy. In this study, stepwise regression method was used to develop a new CHF correlation for application in PWR under low pressure conditions. First, several essential thermal-hydraulic parameters which might influence CHF were selected based on consensus characteristics of DNB phenomenon. With stepwise regression, the form and coefficients of the proposed CHF correlation were optimized in a dynamic manner. Compared to currently available CHF correlations, represented by the Westinghouse W-3 correlation, the CHF correlation obtained by stepwise regression has a much simpler form and matches also well with the experimental data.
The CHF in PWR fuel assemblies is usually predicted by the local flow correlation approach based on subchannel analysis while the effects of spacer grids, cold walls, non-uniform heat flux, etc are investigated. By using the subchannel code ATHAS to calculate each set of bundle CHF data, the local thermal-hydraulic parameters at DNB occurrence point were obtained. In present study, the minimum DNBR point method was applied to develop a new CHF correlation for PWR fuel assemblies. The so-called “three-step method” and “magnitude analysis method” were used to determine the shape and the expression of each item, respectively and the least square method was applied to determine the coefficients of the correlation. Based on the large database of CHF tests, the CHF correlation named ACC correlation has been developed to calculate the risk of DNB. The analysis and assessment results indicate that the ACC correlation can fit the experimental data well with high prediction accuracy and correct parametric trends. Coupled with subchannel code ATHAS, this correlation can simulate the thermal-hydraulics performances of PWR fuel assemblies exactly.
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