As we discussed, each $235$U atom that undergoes fission releases approximately $200$ MeV of energy. The energy of a nuclear weapon is expressed in terms of an equivalent amount of TNT $(2,4,6-$trinitrotoluene$),$ for example a $20$ mega$-$ton warhead. TNT contains $4\mathrm{.}7$ megajoules per kilogram and a ton of TNT is a unit of energy equal to $4\mathrm{.}184$ gigajoules. The Little Boy atomic bomb dropped on Hiroshima on August $6,1945,$ exploded with an energy of about $15$ kilotons of TNT$.$ The W$88$ is a modern United States thermonuclear warhead, with an estimated yield of $475$ kiloton, and is small enough that the Trident II submarine$-$launched ballistic missile can be armed with up to $8$ W$88$ warheads.