Seven Years of Uranium Alloy Development at Weldon Spring, 1959/1966. Page: 15 of 47
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Before undertaking an explicit description of the events beginning in 1959, it.
may be of assistance in examining the entire subject to outline in rather simple
fashion some of the problems which had been experienced in irradiation at the
reactor sites and the targets in fuel element characteristics which had been
viewed as essential.
During the early nineteen fifties there was considerable pressure on the feed
material chain as well as on the reactor sites to operate at maximum pile
efficiency in order to achieve as completely as possible the plutonium
production schedules which had been layed down. Because of this demand for
production,.any interference with irradiation.was a cause for serious concern
and reactor shutdowns for removal and replacement of ruptured fuel slugs were
viewed with considerable disapproval. In most cases the contributing cause of
such ruptures was. known even though the means of curing the cause may not have
been understood. One general category of fuel element failure arose (and still
does arise in similar circumstances) from corrosion.of the aluminum Jacketwal
because of local overheating and thereby accelerated -corrosion from the cooling,
water. Such overheating could be attributed .in many cases to distortion of the
fuel core which had diminished the size of the water cooling passage in the
annular channel surrounding the fuel slug, thus restricting the possible
cooling of the slug.surface. One source of such distortion is simple warpage
of the uranium arising either from the presence of residual internal stresses
or from unfavorable alignment of the crystal structure leading to dimensional
growth under irradiation. A second form of distortion has been a roughening
(sometimes called bumping or pimpling).of the fuel surface during irradiation.
This has been associated with uranium structures that are grossly coarse in
grain size and which apparently possess domains of an order of magnitude larger
in size than the apparent grain dimensions, in which the component grains
possess a very similar orientation. This roughening can also reduce the
effective cooling-channel annulus and thereby the local flow of cooling water.
A further cause of overheating is concerned perhaps less with the metallurgical
characteristics of the fuel core as with the method and effectiveness of
cladding the core with its protective jacket. In cases where a.metallurgical
bond has not been established between the core and the clad, there can be local
areas of poor heat transfer and, as a result, surrounding local hot spots.
Here again, the higher temperature of the jacket leads to corrosion by the
cooling water and ultimate penetration with the release of fission products and
the need for reactor shutdown to discharge the slug in question.
The other major source of fuel element failure had been that of rupture of the
uranium itself which usually resulted in local tearing of the aluminum clad and
thereby a release of fission products. Such ruptures had been difficult to
diagnose since in some cases they, had resulted from the formation of uranium
oxide by reaction with .water which had penetrated the clad, perhaps through a
defective weld. In other circumstances, they apparently have been associated
with temperatures at the center of the uranium section which has led to internal
stresses which the metal has been unable to withstand. In recent years, such
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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/m1/15/: accessed March 18, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.