FISSION-GAS INDUCED CLADDING DEFORMATION OF LWR FUEL-RODS UNDER REACTIVITY INITIATED ACCIDENT CONDITIONS

Behavior of irradiated fuel rods under power burst conditions by accid ental reactivity insertion in light water reactors (LWRs) has been stu died in the Nuclear Safety Research Reactor (NSRR). In the experiments , cladding hoop deformation, which reached up to about 10%, was much l arger than that of the fresh rods. The current LWR fuel behavior analy sis codes, which only take account of the thermal expansion of the fue l pellets for the deformation calculation, under predicted the plastic deformation of the cladding to be less than about 1%. Fission gas rel ease during the pulse irradiation tests reached as high as 22% in the NSRR irradiated fuel tests. In order to describe these test results, a model of grain boundary fission gases to cause the cladding deformati on has been developed and installed in a fuel behavior simulation code , FRAP-T6. In the model, the over-pressurized gases by the pulse irrad iation cause grain boundary separation and stress the cladding during the tests. The model assumes that the gases remain in the fuel during the early part of pulse irradiation and are released to the open volum e in the rod after the cladding deformation. The model, in combination with a fuel thermal expansion model, GAPCON, which was validated thro ugh fresh fuel tests, reproduces the NSRR test results reasonably well .