The reactor will be launched in a "cold" state, it will also be in a robust structure (though not as strong as RTGs, though that's unnecessary). The absolute worst case scenario is that you get a release of U-235. That's not great but on the scale of radiological hazards it's comparatively very minor. The reactor doesn't start generating fission products (aka "fallout" isotopes) until it's in space and operating, presumably with no chance of returning to Earth.