The major fear that comes up with these is: what if the missile blows up while still in earth? How are these risks managed?
Pu-238 has a specific activity of 634 billion Bq/g, nearly 8 million times that of the Kilopower reactor's U-235 fuel (80 thousand Bq/g). Once the reactor attains criticality, it produces fission products that have even higher specific activity than Pu-238. But the reactor can remain safely inactive until the risky launch phase is over. There is no way to inactivate the decay of Pu-238 for the launch phase.
The risks of RTG launch are handled by using designs with high mechanical/thermal robustness to encapsulate the plutonium ceramic. I think that they were already safe enough. But the Kilopower reactor is inherently low-radiotoxicity before criticality, which makes it safer yet during the launch phase.
The new fuel rods going into it are totally safe from a radioactivity perspective. Here's a picture of a man holding a fuel rod bundle with nothing more than gloves on. http://nuclearstreet.com/images/img/dw037.jpg
https://www.iaea.org/newscenter/multimedia/photoessays/train...
At all stages the material can be handled with no more protection than gloves. It is roughly as dangerous as handling lead fishing weights until the fuel actually attains criticality.
See https://en.wikipedia.org/wiki/Kosmos_954 and https://en.wikipedia.org/wiki/US-A
https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_ge...