Some assess that the scale and sophistication of Japan's nuclear infrastructure would enable it to build a nuclear weapon in a matter of months, should the unlikely political decision be taken to do so.
Strategic rival China has sought to draw attention to this fact, issuing loud warnings over Japan's stocks of nuclear material, for example.
But it should be noted that under the terms of the Non-Proliferation Treaty -- which Japan ratified in 1976 -- states are entitled to peaceful nuclear technology for energy purposes if they forswear nuclear weapons.