Applicability of Core Thermal-Hydraulic Models in REFLA Code to 17 × 17 Type Fuel Assembly of PWR

Abstract

The applicability of core thermal-hydraulic models in REFLA code was evaluated to a 17×17 type fuel assembly of PWR, the models which have been developed for the reflood phase of LOCA of PWR with a 15×15 type fuel assembly. The two assemblies are different each other in respect of (1) the rod bundle configuration (fuel rod diameter and pitch) and (2) the grid spacer type (existence of mixing vane and interval between spacers). The effects of (1) and (2) were investigated experimentally and the applicability was assessed with the data.
The film boiling heat transfer and void fraction models in REFLA code could apply to the 17×17 type fuel assembly within the error band of each model (±30%). The difference of grid spacer type did not affect the turnaround temperature which is the maximum clad temperature at each elevation but quench velocity. The quench velocity became lower in the case of grid spacer type of 17×17 type fuel assembly and the increase of heat transfer coefficient was delayed. From the analyses by the REFLA code with adjusted quench velocity correlation, the delay of increase of heat transfer coefficient was found to give no significant effects on the peak clad temperature even under the condition calculated by evaluation model code of reactor safety analysis.