Maximum irradiation temperatures ranged from 260 to over 1000°C. Surface roughening occurred in the alloys contain¬ing 25 and31 w/o uranium. Volume increases at any given temperature for all alloys were linear with increasing burnup. The rate of volume increase for all alloys rose from approximately l%per a/o burnup at the lower tem¬peratures to a value of 2.5 at650°C. Thereafter the swelling rate increased somewhat, reaching a value of 6% volume increase per a/o burnup at800°C. The rates of volume increase under irradiation of thorium-uranium alloys in the entire temperature range studied were significantly less than those reported for the best uranium-base alloys. It is suggested that the excel¬lent resistance to high-temperature swelling of the cast thorium-uranium alloys resulted from the fact that a dispersion of very thin uranium particles was obtained. A high probability, therefore, existed for fission recoils to escape from the uranium particles into the isotropic and less densely packed thorium matrix.
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