With this model, the minimum masses of Pu-239 required to obtain k-effective values of
0.95 and 1.0 in a polyethylene (60 vol.%) - water (40 vol.%) environment were determined from
calculations in which the mass of Pu-239 in a polyethylene - water reflected and moderated sphere
was varied parametrically between 300 and 425 g. For each Pu-239 mass value, the H/Pu ratio
was varied parametrically between 700 and 1100. The results of these calculations are
summarized in Table 3 and are shown graphically in Figure 1. As indicated from the results in
Figure 1, maximum calculated k-effective values occur for H/Pu ratios between 800 and 900. The
maximum calculated k-effective values from Figure 1 were used to plot maximum k-effective
versus Pu-239 mass and the results are shown in Figure 2. From Figure 2, the minimum masses
of Pu-239 that are required to obtain k-effective values of 0.95 and 1.0 are shown to be about 325
g and 405 g, respectively. Therefore, a value of 325 g was selected as the safety limit (k = 0.95)
and 160 g was very conservatively selected as the operating limit in order to be in compliance
with the requirements of both DOE-ID Order 5480.5A [mass limit < 45% of the failure mass
(k=1.0)] and 10 CFR Part 71 Section 71.61 (twice the number of allowed undamaged packages
are safely subcritical when optimally configured with close water reflection on all sides). As
indicated by the results of benchmark calculations reported in Section 5.0 and the conservative
nature of the evaluation, a bias is not warranted.
Thick steel and lead reflectors, when positioned tightly around fissile regions, are often
more effective than water reflectors. However, in a cask, these types of reflectors would not fit
tightly around the fissile material as does the polyethylene-water reflector used in the evaluation.
The thick, tight-fitting polyethylene-water reflector conservatively represents actual reflector
The materials to be transported in these casks is "waste" that will not normally contain
superior reflector materials such as beryllium. Waste containing more than trace amounts of
beryllium shall not be transported in these casks without additional review and approval.