Tangential, but: U-238 is fissionable but not fissile; no amount of U-238 will give you a massive explosion if you bang it together. It's U-235 that's the fissile isotope.

(Even banging that together by hand won't give you a massive explosion, though it will give you a moderately large explosion and an extremely lethal dose of radiation: the jargon is "predetonation" or "fizzle". You need to bring a critical mass into existence hard and fast, e.g. by imploding a hollow sphere with explosive lenses, or a partial reaction will blow the pieces apart before criticality really has a chance to get going.)