Generally the sort of lightweight reactors NASA is looking at for space power use highly enriched uranium. U234 isn't particularly radioactive (it's lasted since the Earth was formed) and far less toxic than the hydrazine propellant our ships carry but it's a significant proliferation risk if it should all into the wrong hands.

But yeah, it's not dangerous like the P238 in a radioisotope thermal generator (RTG). To put off enough heat to power a spacecraft just through natural decay you need something ferociously radioactive.