The effect of oxidized UO₂ on iodine induced stress corrosion cracking of fuel sheathing

Abstract

The effects of UO_2+x, oxygen and graphite on iodine induced stress corrosion cracking of Zircaloy sheath specimens were investigated. In these tests, axially slotted Zircaloy rings were prepared from two types of sheathing: current CANDU-6 37 element fuel sheathing manufactured by Cameco and an older Pickering MLI-790 sheathing. The specimens were pre-stressed with a Zircaloy wedge of two types; a static wedge and a sliding wedge where the later allowed for in-situ tightening. The specimens were installed in a glass blown tube with an iodine filled breakable goose neck glass vial. Iodine quantities used per total Zircaloy surface area ranged from 4.25 mg cm^-2 to 35.85 mg cm^-2. To prevent specimen oxidation by air the glass tube was evacuated to pressures of 8e-6 to 3e-5 torr, which was then installed into a tube furnace at a temperature of 350°C to simulate the fuel temperature near the fuel-to-sheath gap in a fuel element. An orange-red residue consisting mainly of zirconium di-iodide (ZrI₂) was found deposited on the glass tube internal walls as confirmed by neutron activation analysis. Post examination was performed to inspect for signs of cracking in the specimens. For intact specimens, a specially designed ring deflection tester was used to assess the degree of weakening. Current sheathing with no prism planes of (1,1,-2,0) in their texture did not crack although some surface erosion was observed. After exposure to iodine these specimens deflected, on average, 0.98±0.09 mm. However, specimens that included an oxygen source deflected somewhat less at 0.80±0.07 mm on average. With the older Pickering type specimens, which had prism planes in their texture, a number of failures occurred. It was further shown that oxidized fuel with dried graphite (also known as CANLUB) prevented surface erosion of these specimens; however, iodine gas (I₂) persisted longer. This work suggests that failure is more likely caused by iodine gas compared to a zirconium iodide gas. It was also seen that with in-situ stress, corrosion was enhanced when the natural and brittle surface zirconium oxide was disturbed. These preliminary test results suggest that oxygen from oxidized fuel, or reduced with dried graphite at 350°C to produce CO_x, provide a thin ZrO₂ protective surface layer on the sheathing internal surface. The later may partly explains the chemical benefits of graphite.