Thermal-hydraulic analysis of mixed cores with optimized and standard fuel assemblies

Abstract

The thermal-hydraulic characteristics of the transition core was analyzed and the related thermal-hydraulic safety was investigated by evaluating DNBR at design overpower transient. In the course of this work, the establishment of the design DNBR limit using Improved Thermal Design Procedure(ITDP) was inevitable, so the DNBR sensitivity analysis was performed. The DNBR sensitivity factors were achieved with respect to core inlet mass flux, core inlet flow temperature, system pressure, core power level and nuclear enthalpy rise hot channel factor. Besides, flow redistribution effects on DNBR in the transition core were considered quantitatively by introducing the transition core DNBR penalty. COBRA-IV-I code was used for these calculations. In addition, R correlation was applied for the DNB calculation. From these calculations, it is found that the MDNBR values of the transition core through Standard Thermal Design Procedure(STDP) violate the safety limit and the possibility of DNB occurrence is higher in OFA than in STD fuel assembly. Exit void fraction of OFA is smaller than that of STD fuel assembly, implying less voiding in OFA, as experted. Meanwhile, the design DNBR limits of thimble cell and typicl cell are established for OFA and STD fuel assembly, respectively. And the transition core DNBR penalty to compensate the fact that OFA lose flow in the transition core is achieved. Finally, the fact that the MDNBR values of the transition core through ITDP satisfy the safety analysis DNBR limits is found. From above results, it can be concluded that firstly, the thermal-hydraulic characteristics of OFA slightly worse than that of STD fuel assembly, secondly, STDP has too excess conservatism in the DNB calculation compared to ITDP and finally, the transition core is confirmed to be safe with respect to the problem of the DNB.