Evaluation of ECR and PCT in balloon region under LOCA conditions of the Halden IFA 650 tests (single rod, tests)

摘要

During a postulated loss - of - coolant accident (LOCA), the reactor coolant pressure may drop below the internal fuel rod gas pressure causing the fuel cladding to swell (balloon) and, under some conditions, rupture. A ballooned region may be filled partially or totally with fuel fragments, depending on the fuel burn - up. Fuel relocation consists in the accumulation offuel debris in the ballooned region of the burst cladding which results from small fuel fragments slumping from upper locations and eventually fuel dispersal from rod. This process is initiated at the cladding burst time and is driven by both gravity and pressure difference between the rod upper plenum and primary coolant. Due to the accumulation offuelfragments in the ballooned region, local power is increased and consequently the peak cladding temperature (PCT) and the equivalent cladding reacted (ECR) may increase too. This phenomenon may be compensated by the increase of the heat transfer due to the increased cladding area in the ballooned region. Both phenomenon are linked and cannot be taking into account separately. The aim of this paper is to evaluate iffuel relocation impact on ECR and PCT could be compensated by the benefit of considering the increased heat transfer area (IHTA) of the ballooned region. For this purpose, experimental results from HALDENLOCA tests IFA 650. 2, IFA 650. 4 and IFA 650. 7 have been analyzed. Experimental data have been compared with calculated data obtained from FRAP - T6 calculations. This code is used by IBERDROLA in the LOCA Licensing Analysis Methodology based on relaxed Appendix K approach, and has been modified and validated by IBERDROLA, based on the Limerick and Halden experimental tests. This evaluation has been performed to determine if new requirements for balloon area should be needed in the USNRC LOCA regulation (IOCFR50. 46).