[WildTurkey]

By grabbing electrons from other atoms. Many of the fission product nuclei are halogens (I-131 for example), and the final preferred charge state of any halogen to be negatively charged.

I-131 forms a diatomic molecule via a covalent bond. It is not "negatively" charged.

[Wonder Warthog]

"I-131 forms a diatomic molecule via a covalent bond. It is not "negatively" charged."

Eventually, yes. But it's native preference is to have an extra electron. Until it slows down enough to actually FORM that covalent bond, the probability is high that it will be negatively charged.