Though I’m no expert on nuclear fission I would imagine that any atom can be split including Hydrogen (though then we would get into the components of protons and such which is above my pay grade). The question would be is it worthwile as far as yeild to do that.
I do know this. When we did our surface nuclear tests in Nevada it was pretty hard on sheep flocks in Utah. They were getting exposed to a radioactive form of Iodine (which was very bad) as well as a radioactive form of Ceisium (which gets mistaken for Calcium and stored in bones which is very bad long term). Niether of these elements are components of a standard atomic or thermonuclear weapon so they had to be created by the blast.
Secondary radioactive isotopes are created by nuclear explosions through nuclear spallation, basically random rejumbling of atoms; the nuclear version of free radicals. And you certainly can split every atom except hydrogen, since it only has one proton. And btw I’m no expert either, I’ve just done my own research.