Looking it up in a reference, natural lead is about 1% Pb-204, 24% Pb-206, 22% Pb-207, and 53% Pb-208. These are all of the stable isotopes. If Pb-204 absorbs

Looking it up in a reference, natural lead is about 1% Pb-204, 24% Pb-206, 22% Pb-207, and 53% Pb-208. These are all of the stable isotopes. If Pb-204 absorbs a neutron it turns into Pb-205, which isn't stable but its half life is ten million years so it's only very weakly radioactive. The -206 and -207 turn into other stable isotopes, of course. So those don't matter so much: about half of your neutrons enter lead nuclei which can safely absorb a neutron. The other half, the Pb-208, turns into Pb-209. Pb-209 is radioactive with a surprisingly short half-life of 3.25 hours, so that's gone in a matter of weeks. The nasty stuff is the lead atoms which happen to absorb two different neutrons during the explosion. This becomes Pb-210, which has a half life of 22 years: short enough that it's going to be emitting a bunch of radiation, long enough that it won't be gone after a few weeks. So the answer is probably "it's very rare in these sorts of explosions to have two neutrons hitting a single lead nucleus." There are other possibilities (maybe they got the lead from spent nuclear fuel or centrifuges or something and it's all Pb-206) but that sounds like it's the most plausible.