Seven Years of Uranium Alloy Development at Weldon Spring, 1959/1966. Page: 21 of 47
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Hanford in adopting the same addition to 0.95% enriched metal processed from
UFe secured from the cascade system. It had been found that the metal
processed from this source possessed particular purity and, as might be
expected, had experienced difficulties in irradiation through the surface
roughening called bumping because of the very large grain size. The
iron-silicon addition to the enriched grade of fuel cores therefore became a
standard and staisfactory insurance of fine grain size.
MECHANISMS OF ALLOY CONTROL OF URANIUM .GRAIN SIZE
The experience with alloy additions to dingot metal generated greater
interest in the choice of alloys which had been observed in work with
uranium fuel in England and in France. Taking the experience in, all three
areas into account, it appeared that the elements of greatest effectiveness
included iron, silicon, aluminum, chromium, and perhaps molybdenum. The
phase diagrams for the systems were not known in any detail,. even for the
binaries; and there was relatively little information available concerning the
limits of solid solubility in uranium in any of the three allotropic forms..
There was also little basis for decision as to whether one additive element:
might be more effective than another either by reason .of its atomic size
relative to the uranium lattice or because of its ability to form some
particular compound. From this point of view, for instance, iron might be:
assumed to be more effective if a precipitate were to be formed since one
atom of iron would tie up six uranium atoms in the formula UeFe; whereas
silicon in U3Si would involve only three uranium atoms. At about this time,
however, certain experimental heat treatments (not directly related to alloy
control) were observed to indicate an unusual behavior. It was discovered
that an alloyed dingot, if annealed at a temperature below 1000 F. and then-
beta heat treated, would possess a very fine grain size; whereas the same
material annealed above 1000 F. and then beta treated would exhibit a
considerably coarser structure. These observations were strongly suggestive
of the existence of a precipitate that could be influenced by heat
treatment. Accordingly, an intensive search for the existence of precipitates
and their possible identities was undertaken.
Careful examination of the standard alloy in the as-rolled condition revealed
no trace of precipitates at magnifications of up to 1000 X.. Laboratory-
sized dingots, however, prepared to compositions in the range of 300-500 ppm
Fe and 200-300 ppm Si, did reveal the presence of an array of precipitates in
the uranium matrix. Annealing these alloys in the high alpha temperature range
caused these precipitates to agglomerate with a reduction in the number of
particles as the average size of particle increased.9,49 It was then
determined that if the standard alloy of 150 Fe - 100 Si was annealed for at
least 8 hours at 1020*F., a faint trace of precipitates: could be seen at
1000:X and with an increase in annealing temperature to-1190*F., the same -
influence of agglomeration could be observed. The inter-relation between
annealing temperature and grain size after subsequent heat treatment was once
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Fellows, J. A. Seven Years of Uranium Alloy Development at Weldon Spring, 1959/1966., report, January 1, 1966; Weldon Spring, Missouri. (https://digital.library.unt.edu/ark:/67531/metadc1033773/m1/21/: accessed March 25, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.