Heat transfer considerations for the first nuclear blowdowns. [PWR]

The first nuclear blowdowns were carried out in the Power Burst Facility at the Idaho National Engineering Laboratory as the LOC-11 series of experiments. This test series was designed to simulate a blowdown transient in a pressurized water reactor (PWR) so that nuclear fuel performance could be investigated under conditions representative of the PWR 15 x 15 fuel element design. Post-test calculations using the RELAP4 computer program were performed for the LOC-11B and LOC-11C tests. Comparisons between calculations and experimental data revealed that the ability to accurately model (1) critical heat flux (CHF) during low core flow conditions, (2) initial stored energy in the fuel rods, and (3) radiative heat transfer between fuel rods and shrouds, was required to adequately represent the fuel rod thermal behavior. Pre-test calculations performed using RELAP4 with licensing-type heat transfer and fuel rod models resulted in peak cladding temperatures several hundred K higher than measured, thus providing further evidence of the need to accurately model heat transfer and fuel rod behavior.