Johannes,

If you understand mathematics, use it to show how negligible your effect is. If you don't, then don't pretend you do.

The high energy transmitted to any particle by nuclear decay will be absorbed and thermalised after a few mean free paths.

if you don't understand what this means, then either shut up, or ask politely for help with basic physics.

If you do understand, then you can calculate that molecules at the top of the stratosphere don't care whether 10,000 Chernobyls, and/or 1,000 Hiroshimas per year, are blasting out on the surface. They only feel the average global atmospheric temperature.

Greenhouse effects on the whole atmosphere are more important in determining molecular loss than are radioactive effects at the base of the atmosphere. (And contrary to any "obvious intuition" greenhouse warming of the surface may well involve upper atmosphere cooling!)