Producing Nuclear weapons takes alot of money. Japan is in a decade long recession with no end in sight. Japan's economy could not survive the high taxes required to finance a nuclear weapon program.
The Japanese would never go for it, unless they thought we would no longer provide their defense.
Well - we need to encourage them to go ahead and work to improve their defenses - because we have our own economic problems. Perhaps there are some in our government not yet sold out to the communist Chinese who can get that point across. We need all the friends against the Communist Chinese government that we can get!