Critical dimensions of systems containing /sup 235/U, /sup 239/Pu, and /sup 233/U: 1986 Revision Page: 46 of 205
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Instead of being listed in Table 10, data from Los Alamos for unmoderated uranium metal
at various enrichments appear in Fig. 19.6"-11 Because the curves for unreflected uranium
and uranium with a natural uraniun reflector appear to be parallel, it is assumed that.
the curve for water reflection would also he parallel as indicated in Fig. 19. Exponential
experiments, which supplement critical experiments, indicate that unmoderated uranium
cannot become critical if the "U content is below 5 or 6 wt%. A cooperative European
reactor physics program has narrowed this limit ing critical enrichmient to 5.56 0.02 at%
2351, i.e. U(5.49).72
This 23511 enrichment limit applies to single undiluted uranium units. As shown by the
following Oak Ridge critical dat a, it does not apply to clusters of metal units in water. For
example, ten 442-kg 11(1.95) metal annuli were critical as a triangular h-ttice with 2.54-cm
opt imtnn separat ion of annuli.7 74 The dimensions of these units were 18.3-cm-o.d., 6.60-
cm-i.d. and 101.6-cm-long. The critical nass. 86 kg 2311I compares with 54 kg 235U in a
similar 9-unit critical array of 11(1.951 annuli with the outside diameter reduced to 15.7
cm and other dimensions the same.
Optimum critical lattices of massive 1(3.85) annuli, and rods consisting of annuli with
water-tilled interiors, again were triangular with 2.5-cm separation of units. As examples,
the interpolated critical mass of 332-kg annuli, 18.3-cm-o.d., 6.6-cm-i.d. and 76-cm-long,
was 57-kg 23511, and that of 380-kg rods of the same outside diameter and length was 90
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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/m1/46/: accessed March 21, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.