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    Oxidation and Deuterium Uptake of Zr-2.5Nb Pressure Tubes in CANDU-PHW Reactors

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    Oxidation and deuterium uptake in Zr-2.5Nb pressure tubes are being monitored by destructive examination of tubes removed from commercial Canadian Deuterium Uranium Pressurized Heavy-Water (CANDU-PHW) stations and by analyses of microsamples, obtained in-situ, from the inside surface of tubes in the reactor. Unlike Zircaloy-2, there is no evidence for any acceleration in the oxidation rate for exposures up to about 4500 effective full power days. Changes towards a more equilibrium microstructure during irradiation may be partly responsible for maintaining the low oxidation rate, since thermal aging treatments, producing similar microstructural changes in initially cold worked tubes, were found to improve out-reactor corrosion resistance in 589 K water.

    With one exception, the deuterium uptake in Zr-2.5Nb tubes has been remarkably low and no greater than 3-mg/kg deuterium per year (0.39 mg/dm2 hydrogen per year). The exception is the most recent surveillance tube removed from Pickering (NGS) Unit 3, which had a deuterium content near the outlet end about five times higher than that seen in the previous tube examined. Current investigations suggest that most of the uptake in that tube may have come from the gas annulus surrounding the tube where deuterium exists as an impurity, and oxidation has been insufficient to maintain a protective oxide film.

    Results from weight gain measurements, chemical analyses, metallography, scanning electron microscopy, and transmission electron microscopy of irradiated pressure tubes and of small coupons exposed out reactor are presented and discussed with respect to the observed corrosion and hydriding behavior of CANDU-PHW pressure tubes.


    corrosion, hydriding, pressure tubes, Zr-2.5Nb, Zircaloy-2, out reactor, irradiation, microstructure, precipitation, percent theoretical uptake

    Author Information:

    Urbanic, VF
    Research scientist, Atomic Energy of Canada Ltd., Chalk River Nuclear Laboratories, Chalk River, Ontario

    Warr, BD
    Research scientists, Ontario Hydro Research, Toronto, Ontario

    Manolescu, A
    Research scientists, Ontario Hydro Research, Toronto, Ontario

    Chow, CK
    Research scientist, Atomic Energy of Canada Ltd., Whiteshell Nuclear Research Establishment, Pinawa, Manitoba

    Shanahan, MW
    Technical superintendent, Ontario Hydro, Central Nuclear Services, Toronto, Ontario

    Committee/Subcommittee: B10.05

    DOI: 10.1520/STP18855S