That's not how shockwaves work. It's a compression effect, not combustion.
I'm aware of how nuclear explosions work, they are well explained in the Vault Dweller Survival Guide (the Fallout 1 manual
- actually the description is quite scientific)
http://fallout.wikia.com/wiki/Vault_Dweller%27s_Survival_Guide#Nuclear_blast_effectsThe displacement wave of a kiloton bomb would be of about 1km, and with increased power, the blast increases, which would also apply in space. so while the yield of the harbinger is 5000Mt, its blast wave is compressed. Thermal radiation (heat) increases parallel to the bomb's power, but the blast does not. to quote the "Survivor's guide" I linked (again, the explanations are rather backed by scientific fact, its just an easy source of info XD )
"A 20 megaton bomb can cause potentially fatal third degree burns at a range of 40km, where the blast can do little more than break windows and cause superficial cuts. A convenient rule of thumb for estimating the short-term fatalities from all causes due to a nuclear attack is to count everyone inside the 5 psi blast overpressure contour around the hypocenter as a fatality
A 5000Mt bomb would annihilate a continent not because of the size of the explosion but the sheer magnitude of the thermal radiation and blast power, you'd crack the planet's crust on the impact site at that power level.
