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    In Situ High Voltage Electron Microscopy Investigation of Catastrophic Swelling in Uranium Intermetallic Fuels

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    Uranium-based intermetallics are being developed as high-density reactor fuel materials for low-enrichment and high power-density applications. However, the irradiation performance of several alloys with high uranium content, such as U3Si and U3Fe, suffers from the formation of extraordinarily large voids (on the order of several μm) at low or medium fuel burnup. This catastrophic swelling prohibits the use of the higher-density materials as reactor fuel. Similar alloys with slightly lower uranium content, such as U3Si2, develop a fine and stable dispersion of fission-gas bubbles (on the order of 10 nm) at low burnup, and catastrophic swelling does not occur. To investigate the mechanisms responsible for the breakaway swelling, and to define the range of applicability of intermetallic fuels with a high uranium content, the effects of burnup have been simulated by ion irradiation and implantation with 1 MeV krypton (Kr), and continuous in situ observations have been made by transmission electron microscopy (TEM).

    Below 550 K, crystalline U3Si becomes amorphous at very low Kr fluences. Irradiation of the amorphous U3Si at 470 K results in the rapid growth of the initial specimen perforation in a manner similar to the rapid swelling observed during reactor burnup. On the other hand, irradiation of the crystalline material at 620 K does not produce rapid growth. The rapid swelling at 470 K is not typical of growth caused by fission-gas bubbles; rather, it appears to be caused by the flow of the amorphous material away from the internal surfaces. Detailed examination during ion irradiation reveals that the resultant stresses produce microvoids and void coalescence in the amorphous phase. Flow does not occur in the crystalline phase, and thus may be typical of the radiation behavior of amorphous intermetallics.

    An unusual and important result is that once U3Si has been irradiated to a high fluence (above 2×1020Krm2), the irradiation behavior appropriate for the initial irradiation temperature is locked in, and that behavior persists even at irradiation temperatures that normally result in the opposite behavior. For example, after a 620-K irradiation, the crystalline state is retained during subsequent irradiation at 420 K to more than ten times the fluence required to amorphize unirradiated material at 420 K.


    high voltage electron microscopy, swelling, uranium silicide, amorphization, plastic flow, burnup, simulation

    Author Information:

    Birtcher, Robert C.
    Physicist, Argonne National Laboratory, Argonne, IL

    Allen, Charles W.
    Physicist, Argonne National Laboratory, Argonne, IL

    Hofman, Gerard L.
    Physicist, Argonne National Laboratory, Argonne, IL

    Rehn, Lynn E.
    Physicist, Argonne National Laboratory, Argonne, IL

    Committee/Subcommittee: E10.07

    DOI: 10.1520/STP49491S