It isn't too big of an issue, rockets have to meet certain requirements to fly nuclear material. The flight profile is required to be over water, so intact material just gets diluted or sinks. There are some requirements about how the flight termination system works (eg it shouldn't spread payload debris over a large area) and general reliability considerations identical to those for human spaceflight.

Essentially, flying nuclear material is treated with the same level of basic care as flying humans.

Once the material is in space the regulations aren't as thorough, I imagine the most that NASA considers is for the launch to be into a direct injection orbit or at a sufficiently high parking orbit that there isn't an immediate risk of uncontrolled reentry in case of some failure.

https://space.stackexchange.com/questions/17518/what-does-it...

Basically zero. Unused nuclear fuel is not very radioactive. You can hold it in your hand with a glove. Once you split atoms it becomes more hazardous, but space nuclear doesn't start up until after it's been successfully lifted.