An Evaluation of the Thermal-Hydraulic and Fuel Rod Thermal and Mechanical Deformation Behavior During the First Power Burst Facility Nuclear Blowdown Tests

A preliminary evaluation was made of the results from the LOC-11 nuclear blowdown tests conducted in the Power Burst Facility. The objective of the LOC-11 tests was to measure the thermal and mechanical deformation behavior of pressurized and unpressurized fuel rods exposed to a blowdown (coolant depressurization) similar to that expected in a pressurized water reactor (PWR) during a hypothesized double-ended cold-leg break. Incipient fuel rod cladding collapse and swelling occurred during Test LOC-11C, when the two unpressurized and two pressurized PWR-type rods were exposed to a system blowdown with measured cladding temperatures increasing to 1030 K. To better understand the test results and to evaluate prediction capability, RELAP4-calculated coolant thermal-hydraulic and fuel rod thermal behavior and FRAP-T4-calculated fuel rod mechanical deformation behavior were compared with the test LOC-11C data. The RELAP4 posttest calculations of coolant behavior generally agreed well with the measured coolant behavior; however, the calculated cladding surface temperatures were ∼50 K greater than measured. The FRAP-T4 calculations of cladding deformation using “best-estimate” models slightly overpredicted the observed ballooning of the pressurized rods and underpredicted the collapse of the unpressurized rods.