The one-group calculation method consistently underestimates the true critical mass - primarily because it does not take into account the effects of inelastic scattering in softening the neutron spectrum. The one-group calculated critical mass estimates are thus lower bounds on the true value. Comparison between the one-group calculations and the actual values for the highly fissile isotopes for which good experimental data is available (U-233, U-235, Pu-239, and Pu-241) shows a consistent underestimate of 70-75% of the true value. For less fissile isotopes, where critical mass estimates have been offered by others (these are mostly calculated estimates also, but with more sophisticated models), the underestimates are more severe (at worst 22-29% of the 'true' value for Pu-242). This too is to be expected because the effects of inelastic scattering is relatively greater in less fissile materials. On the other hand, the estimates for extremely fissile transuranics like californium isotopes should be quite good.
Uhm, wow. So just to review what we're messing with here:
|Boom! By ESA/Hubble, CC BY 4.0, https://commons.wikimedia.org/w/index.php?curid=35306789|
Due to an unsuspected lithium-7 fusion pathway, the 1952
Castle Bravo test firing produced a 15Mt explosion vice a
planned 5Mt. Fallout reached a Japanese fishing boat, the
Daigo Fukuryū Maru. Classy as always, Lewis Strauss
accused the crew of being within the restricted zone.
|Brigitte Bardot in a bikin|
Because Bikini Atoll, that's why.