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Two tests have been performed at the R2 reactor with the aim of demonstrating a technique for simulating a primary defect and the conditions leading to secondary failure of the cladding by hydriding. In the tests, steam was introduced into the rodlets at power. Upon detection of failure of the cladding, the linear heat rate was increased in two steps.
The two test rodlets were refabricated from a segment rod, which had obtained a burnup of around 25 MWd/kgU in the Ringhals 1 BWR plant. Neutron radiography and profilometry before and after irradiation were used in order to determine hydride blisters, defects and dimensional changes. The defects were further examined visually. Gamma scanning showed the distribution of fission products.
The two rodlets were operated at different linear heat ratings: 35 and 40 kW/m, respectively. Rodlet No. 1 failed at the bottom weld. The resulting crack had propagated in a direction which was mainly circumferential. Rodlet No. 2 failed with three axiaily oriented cracks emanating from local hydride concentrations. SEM examinations of fracture surfaces from both rodlets revealed a brittle fracture with almost no signs of ductile fracture. The cracks were seen to have propagated both radially and axiaily from the outside of the cladding. All macroscopic features seen in the examined axial crack of rodlet No. 2 have previously been observed in failed power reactor rods with long axial splits. The cracking mechanism of long splits in power reactor rods is therefore believed to have been duplicated in this R2 test.
Rodlet No. 2 yielded two specimens for metallographical examination. A transverse specimen was examined at successively ground and polished cross sections at the crack tip. No significant redistribution of the hydride was seen in the transverse sections. Hydride accumulation was observed at the crack tip of a tangential specimen with a crack. It was observed that crack propagation was accompanied by plastic deformation amounting to a wall thickness reduction of about 9%.
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Keywords: boiling water reactor, cladding, defect, elongation, hydriding, zircaloy.