[vilhelm_s]

"Days" seems way too short. I Googled a bit, and the pages that came up (see below) suggest between 6 months and 10 years.

The estimates vary depending on how many and what kind of weapons they would build. Apparently Japan has a small stockpile of weapon-grade plutonium, and a huge stockpile of less refined plutonium. So they could build a small number of bombs quickly (the 6 month estimate), but building more would require either designing the weapon around less refined plutonium (which increases the risk that it would not work) or building a refinement plant (several years). They would also need some way to deliver the weapons---the most difficult option is ballistic missiles, which gives the 10-year estimate, while cruise missiles on submarines would be faster.

http://dspace.mit.edu/handle/1721.1/85865 (see footnote 35 on page 11)

http://thediplomat.com/2012/10/japan-joining-the-nuclear-wea...

http://muse.jhu.edu/journals/washington_quarterly/v026/26.1k...

[Dylan16807]

How about the traditional nuclear bombing method of a plane?

[vilhelm_s]

The second link says

> A nuclear triad — land- and sea-based missiles combined with weapons delivered by manned bombers — holds little promise in light of Japan's lack of geographic depth and the vulnerability of surface ships and aircraft to enemy action. That means fielding an undersea deterrent would be Tokyo's best nuclear option.