U238 is fissionable only with fast neutron, however, fast neutron is hard to come by since in ordinary temperature, you can't accelerate neutron. That's why fission bomb doesn't use U238. However, fission-fusion-fission bomb does use U238 as temper. When fusion generates a large among of fast neutrons, U238 starts fission and increase the yield of the nuclear weapon. This is how the modern nuclear weapon is designed.