[sfifs]

No it's because marine reactors require highly enriched uranium to get the power density to weight ratio. https://en.wikipedia.org/wiki/Nuclear_marine_propulsion#Diff...

As you might guess, highly enriched uranium is trivial to make "dirty bombs" out of and a small step away from actual fission bombs. So it's avoided like plague in nuclear engineering unless it's absolutely performance critical and will be used in a highly secure place - like naval submarines & aircraft carriers.

[alephnil]

Not only dirty bombs, but actual nuclear weapons. With the more than 90% highly enriched uranium found in submarines, you can make a Hiroshima-type gun assembly device without much technical sophistication.

[adrianN]

I'm no nuclear engineer, but I assume that it's not too hard to make the reactors slightly bigger and refuel them more often for civilian purposes.

[sfifs]

Wonder if you've read the famous Paper Reactors Real Reactors paper:

http://ecolo.org/documents/documents_in_english/Rickover.pdf