This would be the update from the Nuclear Threat Initiative (NTI) on Japan's civilian highly-enriched uranium (HEU) programs.
Civilian HEU: Japan
Now this is the real deal. Plutonium is nasty stuff and difficult to transform into big bang weapons (just ask the North Koreans). HEU is nasty stuff and way too easy to transform into big bang weapons.
Where does Japan have its HEU? The update page lists four locations.
Yes, I too hope these sites are well guarded.