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    The Role of Lithium and Boron on the Corrosion of Zircaloy-4 Under Demanding PWR-Type Conditions

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    We report here the investigations on the corrosion of Zircaloy-4 cladding in out-of-pile and in-pile loops. Tests were conducted on the cladding in the fresh, preoxidized, and preirradiated conditions. The exposures were in low and high lithium coolant, with and without boron, and under single-phase and two-phase heat transfer conditions.

    In the out-of-pile loop tests, under single-phase heat transfer condition, acceleration was observed when lithium was at 70 ppm and higher. Boron abated the lithium effect. Under two-phase heat transfer conditions, increased corrosion was seen with 10 ppm lithium and 100 W/cm2 only at void fractions >30%. Boron, even at 100 ppm, had an ameliorating effect. In the n-pile tests, with 4 and 10 ppm Li and 1000 ppm B for a total exposure of 240d, the exit void fraction was low (∼5%), compared to the out-of-pile tests (34%). The prefilmed oxide thickness varied from 5 to 50 μm, compared to 5 μm in the out-of-pile tests. However, an enhanced corrosion due to lithium was not observed.

    Thin pretransition films, grown at locations corresponding to a zero void fraction, showed hundreds of ppm lithium in the case of oxides grown in high lithium coolant and tens of ppm lithium in the case of oxides grown in low lithium coolant. Addition of boron to the high lithium coolant reduced the lithium pickup by nearly an order of magnitude. In the case of thick post-transition oxides, the lithium depth profiles showed marked differences for films grown under single-phase (with high lithium in the coolant) and two-phase (with low lithium in the coolant) heat transfer conditions. In the former case, the lithium concentration was highand the profiles were flat. In the latter case, the lithium concentration was high only in the near surface layers and was one to two orders of magnitude less in the bulk of the oxide. It appears that lithium became concentrated in the liquid layer on the surface of the cladding, under conditions of boiling and high void fraction (>30%), and resulted in rapid corrosion; subsequently, further corrosion had occurred in steam containing volatilized lithium hydroxide.

    While some uncertainties regarding the lithium effect in-reactor remain from a mechanistic point of view, we have concluded that a lithium effect increasing rapidly the corrosion rate even towards the end of a fuel cycle, when the boron level is expected to be low, is highly unlikely. The heat flux and the void fraction, even under nucleate boiling conditions, would be quite low. In addition, with the lithium at 2 ppm and still some boron left in the coolant, the thermal hydraulics-water chemistry combination is far removed from the conditions of 10 ppm lithium, 100 W/cm2, and >30% void fraction required to bring about an enhanced corrosion in ∼26d.


    Zircaloy-4, corrosion, heat flux, irradiation, void fraction, lithium effects

    Author Information:

    Billot, P
    CEA, Saclay, Gif-sur-Yvette,

    Yagnik, S
    EPRI, Palo Alto, CA

    Ramasubramanian, N
    ECCATEC Inc., Saclay, Gif-sur-Yvette,

    Peybernes, J
    CEA, Cadarache, St. Paul-lez-Durance,

    Pêcheur, D
    CEA, Cadarache, St. Paul-lez-Durance,

    Committee/Subcommittee: B10.02

    DOI: 10.1520/STP11389S