Flying a reactor to the moon has the same problem as launching toxic waste into the sun. What happens if that flight explodes in the atmosphere? The risk is intolerable.
First of all flying things into the sun is incredibly power intensive and expensive. You are comparing truck loads of 'toxic waste' to a tiny amount of nuclear fuel.
What if the flight explodes in the atmosphere, basically nothing. The reactor is enclosed in steel and will fall into the ocean. At that point in time the reactor is not active.
The reactor is basically just a few pallets of U-235 and U-238, those falling into the ocean isn't a big deal.
https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_ge...