Chain Reactions
For U-238 using fast neutrons, we have for Pf:
Pf = ?f / [?f + ?absorption + ?escape] , and with a large enough mass of uranium, ?escape = 0,
so at best, Pf-U238 = 0.5 / [0.5 + 2.0 + 0] = 0.2, which is less than the required 0.4, so
U-238 cannot be used for a bomb.
However, U-235 and Pu-239 can be. (See their values on the data sheet.)