GETTERING OF HYDROGEN FROM ZR-2.5NB PRESSURE TUBES

Yttrium is being investigated as a hydrogen getter to prevent delayed hydride cracking in Zr-2.5Nb pressure tubes in CANDU nuclear power reactors. Yttrium strips have been encapsulated in zirconium alloy and attached to the ends of hydrided pressure tube sections to determine the effect of the degree of contact between the yttrium and the encapsulation on the gettering rate. Rates for strips hot isostatically pressed into the encapsulation were in good agreement with diffusion model predictions assuming complete contact. Rates for strips brought into contact by cold rolling were slightly lower than those for the hot-pressed strips, while little gettering was observed for loose strips sealed in the encapsulation by tungsten-inert gas welding. The effect of hydrogen flux rate to the yttrium on gettering was determined at 313-degrees-C for hydrogen fluxes from three to nine times those predicted in reactor. It was found that these fluxes did not affect the gettering rate for hydrogen concentrations up to 58 at.% in the hot isostatically pressed yttrium inserts. Inserts that were thermally cycled and inserts that had not been hot pressed achieved similar gettering rates.