The Japan Times has published Brad Glosserman's very competent review of Japan's need for a discussion on nuclear weapons, "Japan Peers into the Abyss", on the opinion page of this morning's print edition.
Unfortunately, The Japan Times has the storage capacity of a thimble, so the above link may soon go dark. The essay will probably be available on a more permanent basis on the Pacific Forum CSIS website later this month.
One of the fun parlor games that Glosserman avoids indulging in is the always amusing, "How long would it take for Japan to become a nuclear power?" Up until a few months ago I had always thought that it would take a number of months or even years for Japan to arm itself with nuclear weapons, based upon the amount of time it would take to perfect an implosion device for a plutonium weapon.
In a humbling exchange on Dr. Jeffrey Lewis' Arms Control Wonk blog, I learned something important that I really should have known: that Japan has 2 tons of highly enriched uranium (HEU) on its soil, much of it at the Fast Critical Assembly (FCA) reactor in Tokai Mura.
....which is really important to know...because according to slide #12 of this helpful presentation, (thanks Japan Atomic Energy Commission!) 60 kgs of HEU would be sufficient for a simple gun-type fission weapon.
So if the Japanese government, or even a cabal within the Japanese government, felt pressed to respond to the threat posed by the many nuclear weapons states in the area--then with the H2A rockets all ready tested and ready to go, the amount of time required to "go nuclear" would likely be brief.