I'd like to see some numbers on that.
The Hawaii test was about 1.5 megatons, which is not exactly the kind of thing you're going to toss behind a pusher plate.
Certainly charged particles from such a bomb would collect in the radiation belts, but I'd have to see how it would compare to the crap the Sun dumps into those belts every day.
In any case, I don't see Orions as a routine method of transportation, but rather a method of getting huge payloads someplace a few times.
I would have no real objection to restricting such vehicles to launching from cislunar space or the Moon.