There is very little point in using a fission rocket low in the atmosphere (wouldn't play to it's strengths, would not be much better than chemical rockets), bu

There is very little point in using a fission rocket low in the atmosphere (wouldn't play to it's strengths, would not be much better than chemical rockets), but there is no problem in lifting fission rocket engines to orbit, because a fission reactor made out of U-235 is not significantly radioactive before it's turned on. So you can ship it into orbit in some configuration where it cannot turn on by accident (half of fuel removed and in storage or on a different rocket), and then only turn it on when you are in a safe orbit pointing away from earth.