STP681

    Interaction of Oxidation and Creep in Zircaloy-2

    Published: Jan 1979


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    Abstract

    In the event of a loss-of-coolant accident in water-cooled reactors, the Zircaloy-2 cladding is subjected to a rapid increase in temperature and stress in an oxidizing environment. Accurate numerical values of creep parameters are thus required for use in tube deformation models that predict fuel cladding behavior during such an accident. Also, it is necessary to know quantitatively the effect of oxidation on creep behavior.

    Creep tests have been performed in the temperature range 1000 to 1925 K on Zircaloy-2. In this temperature range, the material undergoes a phase transformation from α to β phase at ∼1100 K. Tensile creep results on these two phases have been performed in vacuum. The α-phase material was tested in strip form in a conventional creep machine, although to obtain data at high temperatures in the β phase, resistance-heated wire specimens were used. The creep of both the α and β phases of Zircaloy in the unoxidized condition can be satisfactorily represented by the Dorn equation for dislocation creep controlled by lattice diffusion: ε˙i=AiGibikT(σGi)niDi where i = α, β and the creep parameters Ai and ni take the values: Aα=1.67×1011Aβ=0.6nα=5.36nβ=3.7

    Further experiments are reported on the influence of oxygen on creep. The first series of experiments on material with oxygen in uniform solid solution indicate that oxygen has no effect on the creep of β-phase material until the solubility limit of oxygen is reached. However, the creep of α-phase material is significantly modified by the presence of dissolved oxygen. Creep rate is shown to decrease according to the expression ε˙=ε˙0exp(βC) where ε˙0 is the creep rate of Zircaloy-2 containing no dissolved oxygen, β is a constant at a given stress and temperature, and C is the oxygen concentration. Numerical values of β are discussed in the text.

    Creep of duplex α + β phase is also investigated using techniques both of equilibrating the structure at test temperature before testing and also admitting measured quantities of oxygen during test. Using these techniques combined with experiments at different temperatures permits a wide range of structures to be obtained and transient effects to be observed. The relationships between the different morphologies of the duplex material, the relative proportions of the two phases, and creep behavior are discussed.

    Grain boundary sliding in β-phase Zircaloy-2 is also investigated. The rate of sliding is shown to vary, with stress raised to a power slightly greater than unity with an activation energy close to that for lattice diffusion, and results are interpreted in terms of sliding controlled by diffusion between undulations in the boundary geometry. The geometry required to satisfy the observed rates of sliding is estimated. It is shown that oxygen has no effect on sliding, although nitriding inhibits sliding.

    Keywords:

    Zircaloy-2, α and β phase, creep, Dorn equation, oxidation, bamboo structures, grain boundary sliding


    Author Information:

    Burton, B
    Research officers, Central Electricity Generating Board, Berkeley Nuclear Laboratories, Berkeley, Gloucestershire,

    Donaldson, AT
    Research officers, Central Electricity Generating Board, Berkeley Nuclear Laboratories, Berkeley, Gloucestershire,

    Reynolds, GL
    Research officers, Central Electricity Generating Board, Berkeley Nuclear Laboratories, Berkeley, Gloucestershire,


    Paper ID: STP36701S

    Committee/Subcommittee: B10.02

    DOI: 10.1520/STP36701S


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