Critical dimensions of systems containing /sup 235/U, /sup 239/Pu, and /sup 233/U: 1986 Revision Page: 23 of 205
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A change in the density of a fissile sphere by the ratio p/p, leads to a changed critical
mass, rm,, that may be expressed as
Tme lmlo = ( p / po )
where n is constant over a considerable range of density ratios. In fact, where density of
both spherical core and reflector is changed by the same ratio and the ratio of reflector
thickness to core radius is maintained, then n = 2 (the value for an unreflected sphere).
Similarly, in the case of an infinite slab, the critical mass per unit area is necessarily
independent of p i.e., n = 0).
Where reflector characteristics remain constant, however, the value of n associated with
the density change of a spherical core depends considerably upon the system. Combined
Los Alamos, Livermore, and Rocky Flats data for U(93.5) metal and 6-phase plutonium
cores20,41,42 seem to follow a unique relation between the density exponent and the degree
of reflection (see Fig. 8). The scat er associated with subcritical plutonium measurements
would mask any small differences between the two fissile materials.
The experimental values of n (as determined by the UKAEA Atomic Weapons Research
Establishment at Aldermaston, ORNL, and Los Alamos) for near-equilateral nonmetal
cores are given in Table 6."3
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Paxton, H.C. & Pruvost, N.L. Critical dimensions of systems containing /sup 235/U, /sup 239/Pu, and /sup 233/U: 1986 Revision, report, July 1, 1987; New Mexico. (https://digital.library.unt.edu/ark:/67531/metadc1109879/m1/23/: accessed March 23, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.