Actually spreading it out over a large area is much safer. What you don't want is a big hunk of highly enriched uranium landing somewhere. Not that it is very likely to harm anyone, but it becomes quite a nightmare to deal with it.
Any loss of containment is not going to play well in the news media.
We saw the hyperreactivity over Fukushima. I even know some very educated people who should know better like not wanting to eat any seafood caught in the Pacific.
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.
This is only true if the fission reactor's fuel isn't scattered over square kilometers after a launch failure.