[Fred Hayek]

What was not accounted for was lithium 7 being involved in the reaction. When the Li-7 was cooked, it changed to Li-6, which acted like the rest of the Li-6. The lighter isotope provided the tritium to fuse with the deuterium. Thus the runaway. There were more neutrons available to fast fission the U-238. The latter provided much of the yield as well as the fallout.

[RayChuang88]

What was even scarier was there were test technicians on the far side of the island where there detonated the Castle Bravo bomb who should have been safe if the yield was only 6 MT--but with the 15 MT "runaway," the technicians had to stay in the sealed blockhouse for much longer until the detonation effects dissipated.