A nuclear powered engine on a plane which has a sole purpose of delivering H-bombs probably did not cause too much environmental concerns back in those days.
The H-bombs the plane was delivering would cause far more environmental damage than a crash and breach of the reactor powering the plane would have.
That being said, if they ever did get to test flying such a plane, I hope they would have done it only over oceans far from any populated areas.
Now the Russians are trying to develop a nuclear powered ICBM. We have seen they are having problem with it.