What the scientists didn’t know at the time was that the “dry” lithium deuteride had much more explosive potential when the neutrons it released split the atoms in the uranium-238 “jacket” much higher than anticipated. That’s why the original yield estimate was around 6 MT, when in reality it came out to 15 MT.
The scientists learned that tritium effect could allow more yield from less core materials which would eventually would lead to the ability to have the MIRV warheads on the large land- and sea-based missiles. The size reduction of the actual warhead was also enhanced by the use of smaller electronic components.
Yeah, that, and one of the lithium isotopes they thought would be "inert" wasn't -- it fused, too. I think it was Li-7, IIRC.
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.
That turned out to be an incorrect hypothetical construct. Former member of AFSWP What the physicists KNEW was that u-238 Was NOT fissionable.