Volume 2, Issue 10 (November 2005)
The Effect of Duplex Cladding Outer Component Tin Content on Corrosion, Hydrogen Pick-up, and Hydride Distribution at Very High Burnup
The performance of two types of Duplex claddings irradiated to very high burnup in a German high duty pressurized water reactor (PWR) has been studied. Four fuel rods with Duplex cladding were moved from two discharged assemblies to a low-burnt assembly for further irradiation. These rods were finally unloaded after reaching a rod average burnup close to 70 MWd/kgU. The performance of these rods has been examined through pool-side inspection. Two of these rods have been studied further in a hot-cell post irradiation examination (PIE). The results of destructive and nondestructive examinations consistently show good corrosion resistance and low hydrogen pick-up for both Duplex variants. The hot-cell measurements reveal peak values of the oxide thickness of around 30 μm for both types of Duplex. Consequently, the claddings show low levels of absorbed hydrogen in the order of 300 ppm, as is indicated by metallographic cross-sections and the low axial growth of the rods. During the development of Duplex claddings, it was observed that the Sn content in the two components should not differ by more than about 0.7 wt%. A limited difference in Sn concentration implies that the two components have similar recrystallization temperatures. More important, hydriding studies of unirradiated Duplex claddings have shown that, by limiting the difference in Sn content, the amount of hydrides in the boundary zone between the inner component and the Duplex layer is reduced. There is a significant difference in the Sn concentration in the outer component of the two studied Duplex types. The effect of difference in Sn concentration on hydride distribution observed from studies of unirradiated claddings has, however, not been confirmed by the hot-cell studies presented here.