"Reliable" in nuclear technology means something very different from "reliable" in space, not least because the failure modes are different.

If a Falcon Heavy with ordinary cargo fails, a few fish will die on impact, but otherwise the damage to the ocean and the atmosphere is not that great and lasting.

"Seeding around" tons of nuclear waste into the atmosphere and into the ocean water would be a major disaster. This stuff will circulate for decades or even thousands of years, depending on its half-time.