Everything on the military is classified. Basic ass radios from the 80s are top secret, classified just means they don’t want enemies to know the exact specifications of their equipment.

There’s plenty of insurers not in America…

A nuclear reactor isn’t actually a very complicated machine, in a sense. Put enough nuclear fuel in one place and it gets hot. Then, drive a heat engine with it. Usually one based on steam, although closed-cycle gas turbines, sterling engines and airbreathing jet engines have all been experimented with.

It’s just that you have to keep track of neutron moderation and cross sections, half lives of thousands of isotopes, thermal changes, non-constant demand and the possibility of point failures, all under the condition that you can’t let anything escape. That makes it complicated, but then again each individual part on that list can be learned from open-source materials.

It’s even known what general kinds of reactors are on various military nuclear submarines. For example, the earlier Soviet designs used a liquid lead-bismuth cooled fast neutron design, which is why the Russians have so much polonium, while the modern designs use a pressurised water coolant.