>a bit sketchy at that (a hypothetical of the challenger event spreading radiation across the upper atmosphere of our globe, and what those ramifications would be comes to mind).

Except that things like RTG's for space use are designed to be strong enough that a Challenger-type event wouldn't result in the release of any radioactive materials. The RTG's themselves and the General Purpose Heat Sources (GPHS) inside them are designed to withstand re-entry from low Earth orbit without any release of radioactive material.

There is no reason why a spacecraft propelled by a nuclear thermal rocket in space couldn't be designed in a similar fashion, such that the heating elements are protected until they are deployed in the engine, which wouldn't happen until the spacecraft is actually in orbit.

Or maybe just send the engine up by itself, fully assembled but inactive and in a protective case designed to permit a safe reentry, and mate it with the rest of the spacecraft once in orbit.