The calculated bare sphere critical masses are: Cf-249 5.90 kilograms Cf-251 1.94 kilograms Cf-252 2.73 kilograms By using a thick beryllium reflector the critical masses can be reduced to 40% or so of their bare value so that the most fissile of these isotopes, Cf-251, would have a reflected critical mass of 780 grams.
(The calculated bare sphere critical masses; are; Cf-249 5.90 kilograms Cf-251 1.94 kilograms Cf-252 2.73 kilograms) (the critical masses; can be reduced; to)
The calculated bare sphere critical masses the critical masses
are can be reduced
Cf-249 5.90 kilograms Cf-251 1.94 kilograms Cf-252 2.73 kilograms
to