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Irradiation experiments with certain low-enrichment, high-density, uranium-base intermetallic alloys that are candidate reactor fuel materials, such as U3Si and U6Fe, have revealed extraordinarily large voids at low and medium fuel burnup. This phenomenon of breakaway swelling does not occur in other fuel types, such as U3Si2 and UAl3, where a distribution of relatively small and stable fission gas bubbles forms. In situ transmission electron microscope observations of ion radiation-induced rapid swelling of intermetallic materials are consistent with growth by plastic flow. Large radiation enhancement of plastic flow in amorphous materials has been observed in several independent experiments and is thought to be a general materials phenomenon. The basis for a microscopic theory of fission gas bubble behavior in irradiated amorphous compounds has been formulated. The assumption underlying the overall theory is that the evolution of the porosity from that observed in the crystalline material to that observed in irradiated amorphous U3Si as a function of fluence is due to a softening of the irradiated amorphous material. Bubble growth in the low-viscosity material has been approximated by an effective enhanced diffusivity. Mechanisms are included for the radiation-induced softening of the amorphous material, and for a relation between gas atom mobilities and radiation-induced (defect-generated) changes in the material. Results of the analysis indicate that the observed rapid swelling in U3Si arises directly from enhanced bubble migration and coalescence due to plastic flow. 34 refs., 11 figs.
The swelling of uranium and of a few selected uranium alloys on post-irradiation annealing was investigated by utilizing density measurements in conjunction with the observation of pores in the microstructures of annealed specimens. Specimens were irradiated to about 0.3 at.% burnup in a constrained condition at approximately 275 deg C and were subsequently pulse annealed. The amount of swelling was found to be less than 1% for U specimens that were pulse annealed up to 75 hr at temperatures below 550 deg C; the amount of swelling, however, increased considerably on annealing at temperatures between 550 and 650 deg C. Specimens pulse annealed up to 75 hr at 618 deg C decreased in density by approximately 18%. The swelling was accompanied by the formation of bubbles on grain boundaries in recrystallized regions. The observations suggest that recrystallization is a necessary prerequisite for pronounced swelling in the alpha phase.
In this work the photographic emulsion technique has been applied to the study of the long range particles given off during neutron irradiation of uranium. Alvarez first observed these particles with an ion chamber and linear amplifier. Segre, et al (LA-87) studied these particles with an ion chamber which gave coincidences between these particles and fission. They found them to be coincident with fission within the time constant of their circuit (approx 5u sec). They were also able to identify these particles as alpha particles from the density of ionization near the end of the tracks. Hughes has observed a number of these alpha particles in the cloud chamber, but his setup did not permit him to observe the associated fission track. The cloud chamber or photographic emulsion technique permits information to be obtained about the alpha particles and their association with the fission process that cannot be got by the ion chamber method. The photographic emulsion technique has been used in this case because it lends itself very well to a study of rare events of this type.
The swelling of intermetallic materials depends upon the crystalline or amorphous state of the material. When U3Si is irradiated at temperatures above its amorphization limit, it remains crystalline and does not suffer extraordinary swelling. However, when it is irradiated at temperatures below its amorphization limit, body forces associated with the irradiation cause any internal free volume to suffer a rapid growth, and the material swells at an anomalously fast rate. As has been previously noted, fission-gas bubbles are not directly responsible for this swelling; however, once formed, the bubble volume, as any interior volume, may increase rapidly. An unusual and important result is that once U3Si has been irradiated to a high fluence, (above 2 x 102° Kr m/sup /minus/2/), the irradiation behavior appropriate for the initial irradiation temperature is locked in, at least temporarily, 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. 19 refs., 3 figs.