When the nuclear bomb is ignited, the liquid hydrogen in the tank would evaporate instantly. The evaporation heat of liquid hydrogen of a tank of any reasonable size is negligible compared to the energy released even by a small (Hiroshima) 15kT TNT fission bomb.

Note after comment by @rob: That there won't be any significant fusion of ordinary hydrogen with hydrogen has been answered here: https://physics.stackexchange.com/q/87972/129209