[acidburnNSA]

One advantage submarine reactors have is that they can use fully enriched uranium (bomb-quality, >90% U-235). You can still make pretty small reactors with non-weapons uranium (defined as <20% enriched) but they are not as performant or long-lasting as the pure U-235 submarines.

[otp124]

Interesting. So can the core of uranium weapons be re-used for nuclear reactors? (Assuming there is a big arms reduction event)

[cperciva]

Yes. Plutonium from disassembled nuclear weapons is also used this way, under the name "mixed oxide fuel". Some reactor designs are better at burning "MOX" than others; CANDU reactors are particularly good at this, and in the early post-cold-war years there was a very active US-Canada-Russia programme to use Canadian reactors to burn surplus Russian fuel, funded by the USA (which was very concerned about the potential for fuel from decommissioned Russian weapons to get onto the black market).

[acidburnNSA]

Absolutely. In fact, from 1993 to 2013, fully 10% of the electricity in the USA came from dismantled ex-Soviet nuclear bombs through just such a arms reduction effort [1]. The material was downblended with U-238 to take it from weapons-grade to reactor-grade. The bombs that were once pointed at American cities ended up powering them. It was glorious.

[1] https://en.wikipedia.org/wiki/Megatons_to_Megawatts_Program