The title "U235" refers to the fissile isotope of Uranium used in early nuclear weapons. The plot of the episode refers to both Uranium used as a power source (typically the isotope U-238 with a little U-235 mixed in) and a 1990s era nuclear warhead, which would use Plutonium (Pu-239) and the characters even refer to it as being Plutonium. Nowhere is U-235, the title of the episode, relevant.