This link has some details about what they were actually worried about - which appears to be that a nuclear weapon would trigger fusion of nitrogen nuclei:
That's fascinating - and, later, a somewhat similar calculation was done erroneously. The yield of the Castle Bravo bomb - the first hydrogen bomb to use lithium as its source of tritium - was 250% greater than predicted, on account of overlooking the prompt conversion of lithium-7 to tritium when bombarded by high-energy neutrons.
I'm sure I read that someone had done that calculation for Castle Bravo and was watching the test and for a few moments, as the fireball got bigger and bigger and bigger, that they had actually got the calculation wrong!
https://en.wikipedia.org/wiki/Castle_Bravo#High_yield