I may not be an expert nor even considered highly knowledgeable on the subject. But I do know enough to be very, very dangerous!
I see that - or in this case not dangerous. U-235 is the fuel that burns in a uranium reactor. There have been a few Plutonium fueled reactors - and given the thousands of tons of Plutonium on the planet that may become normal (Japan has bought most of it - I think for power plants - but it also threatened China it would "field more than a thousand nuclear weapons" if it didn't stop threatening its neighbors). And there are experiments with U-233 - which is very attractive - and quite superior reactor fuel - but it is not economic to start a fuel cycle. India and Israel are most likely to start that one day - because it allows thorium to be a source of fuel. Uranium is not abundent in South and East Asia - one of Japan's problems.
Anyway, uranium and plutonium is not really suitable for radiological weapon use BEFORE you put them in a reactor (after is a different subject - but that is not exactly a good application - there are better ways to get there). Further, radiological materials are problematical: they decay - they don't sit well on a shelf without losing potency - they are not very effective - are subject to conditions like wind - and they cost MORE per casualty (by far) than using the same reactor time to make atom bomb fuel. Which is why they did not catch on after the US and USSR did lots of tests on the Axis idea in the 1950s. [I have ALL the US test reports - if anyone wants to see them.]
U-233 is pretty hot - or rather it comes with other isotopes that are pretty hot - so it is almost like radiological material even if intended as bomb fuel. In fact, the best way to use U-233 is something like a CANDU reactor: the Canadian system makes plutonium in a reactor and just lets it sit there - becoming fuel. You can put thorium in the core and it will convert to U-233 - and that then becomes more reactor fuel - replacing the "burned" U-235. And that - by the way - is a WWII era Japanese idea - one that one day may turn out to matter: they may make plutonium fired power reactors and put thorium in there - generating U-233 to replace the plutonium. When they run out of plutonium, they can probably keep the cycle going with U-233 alone. But the wartime Japanese program had these machines:
a sub critical reactor experiment (similar to ours with graphite, this used heavy water) - so they could design a reactor knowing critical mass requirements with their chosen moderator - this is described in the report by Dr Nagano at the Navy Park meeting in July 1942
a prototype reactor - it used the fuel and heavy water from the previous experiment in part - at Konan - this reactor was used until 1948 by the Russians with some of its Korean staff - and it was first described open source in Japan's Secret War (now in rewrite - and I am helping the author with the technical interpretation of archival materials he and others have found)
a first generation submarine reactor design - 12,000 hp more or less - and no less than three were built - but only one likely ever got fueled. The third was not far along, but the second was siezed in 1945 and sent to Russia - which interestingly adopted this same size (12,500 hp) for its first generation submarine reactors - but that was a different technology.
The Japanese tried to make natural uranium reactors work - only France ever tried that for a submarine outside Japan. Sub reactors otherwise use very highly enriched U-235 - higher than bomb grade - to achieve high power density and low cost (by not needing heavy water by the ton).
There is some evidence they got one to work - for a short period. At that time materials could not work with high temperatures found in modern sub reactors with highly enriched fuel - so they needed a lower pressure lower temperature design. But they DID trade with Germany in zirconium - an alloy ONLY used for reactor fuel elements - and something still used ONLY that way. IF Germany and Japan didn't need this alloy - why did they invent it and trade in it at great diffiiculty during the war (see MAGIC intercepts tracking it)? If they did need it - what for?
