Well, let's see what would be consistent with what we know? So far as we've seen, D&D magic doesn't create radioactivity. There are no Uranium Elementals, or whatnot. To me, this seems like the key point...
So, assuming that what you have really is radioactivity, and not some other energy that might be magical, we have to think about what it is, in game terms. There are three basic choices -
1)Radioactivity is as open to magical manipulation as any other energy, it just hasn't been done before. Then this is just another big magical event. Treat it as all the others.
2)Radioactivity is orthogonal to magic. The two don't interact directly at all. radioactive power and events aren't influenced by magic, and vice versa. Again, this means little for the Weave directly (but may indirectly, if it is larage scale). But folks dying from radiation poisoning may not be helped by magic. Magic wont' make affected land fertile again, because the magic can't touch the radioactivity. Maybe radioactivity becomes a new source of power for people/monsters, and again, they don't interact. Dispel Magic won't stop a radiation powered effect, and a radioactive "dampen power" ability won't do jack against a fireball.
3)Radioactivity is antithetical to magic. Radioactivity creates "dead magic" zones, etc. We've seen something very much like this before in FR, in the Time of Troubles.