Note: This drawing is schematic only. The actual towers were much taller
and the observed arch of destruction of the energy-directed thermonuclear
device was correspondingly more narrow.

 The Ground Zero here is in the original sense of word, a nuclear blast site. The thermal energy may absorb heat at a rate of 10 E 23 ergs / cm2 sec and near the bomb all surfaces may heat to 4000 °C or 7200 °F igniting or vapourizing violently. Source: US Department of Defense & US Department of Energy, Glasstone - Dolan: 'The Effects of Nuclear Weapons' (1980).

The thermonuclear bomb used was a 'pure' hydrogen bomb, so no uranium or plutonium at all. The basic nuclear reaction is Deuterium + Tritium Alpha + n. The ignition of this is the fine part, either with a powerful beam array or antimatter (a very certain way to get the necessary effect of directed energy in order not to level the adjacent blocks of high-rise buildings, as well).