The time it takes to create a nuclear weapon is almost proportional to the time it take to enrich enough fissile material to reach the desired weapon energy yield. Japan has the missile technology, but not the enriched fissile material which would likely take a great deal of time considering they don’t have the enriching facilities in place - at least officially.
There are always dirty bombs. Combine radioactive waste with an epoxy and explode it over an area. That would make that area inhabitable for years. They would have to go in and by hand scrape off every bit of epoxy.
I understand that they have plenty of material for “research” purposes.
AVLIS and MLIS plus old hat enrichment schemes.