[vectoral]

Sorry for the slow reply -- didn't realize there weren't notifications on HN.

U-235 (the fuel used in naval reactors) does undergo spontaneous fission, but not at a rate high enough so reach criticality. Like one of the other posters mentioned, you can make it easier to achieve and maintain criticality by changing the shape of the core (so that fewer neutrons leak out) but in general you do need a neutron source inside the core that is just always spitting out enough neutrons to help the reactor achieve criticality as the control rods are withdrawn.

Once the core has operated at power for long enough, some core materials become "active" (from irradiation) and may help contribute to the neutron source.

Straight U-235 is fairly safe (iirc), but even then I don't think you'd want to ship the fuel assemblies with any moderator as moderated neutrons are what make fission more likely.