The big problem is that most people don't realize that isn't a problem at all.
We've been launching radiation-powered devices into space for decades in the form of RPS's, Radioisotope-powered systems. In one launch, the rocket exploded and even then the radioactive parts were unharmed[0]
And 'accidentally reenters the atmosphere' isn't really a thing. Once an object is in orbit above a certain height, it takes a lot of energy to get it out of orbit. The only alternative is waiting for the very faint amount of atmosphere to drag on it and bring it down- and anything about 1000km, that would be decades of waiting.
We launch the radioactive components safely, as we have lots of experience doing. We then assemble the nuclear engine in-orbit at an orbit above 1000km. Then we go to Mars!
Maybe we should just blow up a nuclear bomb in space just to make sure there isn’t any weird interaction of physical forces we didn’t anticipate for an explosion of that magnitude in space
A "simple" nuclear engine might just contaminate a huge area in such a scenario. What does this new engine do, to prevent that from happening?