Japan has had a space program launching planetary exploration packages for some time. The largest issue with using rocketry for strategic military purposes is downsizing a payload to available launch technology.
Keep in mind that Japan had in it’s possession +331 kilograms weapons grade plutonium on loan from the US until this past May. Nikki Haley had a cow that the material was shipped to the Savannah facility.
They have over 10 tonnes of at least reactor grade in country, and additional amounts totaling ~38 tonnes at various reprocessing operations out of country. What research could they have possibly been doing requiring weapons grade material in their possession for decades?
1.) It will not take 10 years.
2.) The first we will know of it is when the CIA scratches its collective head and asks "where did that shit come from?"