The half-life of fuel for these reactors is very long - over 100,000 years. So for all practical biological effects they are basically stable. If it exploded before it was activated (far into space), it would be about the same danger to people and the environment as the rest of the exploding metal the rocket was made of.

Pretty much any uncontrolled explosion propels highly toxic material over a large area (rocket fuel is not lickable). I don't see how alpha radiation is inherently worse than hydrazine.

For that case there are launch escape systems typically used for crewed missions.

The alternatives still seem worse.