This is a portion from Carey Sublette's excellent Nuclear Weapons Frequently Asked Questions, it is hosted here with the author's permission.
By the year 2000 Japan will have an inventory of about 55 tonnes of separated reactor grade plutonium. It should be noted that this is enough plutonium to manufacture ~10,000 warheads, more than the combined nominal arsenals of the U.S. and Russia combined under START II.
Japan has a very aggressive nuclear power program, and is developing plutonium as a reactor fuel in a big way. Japan is maintains an active breeder reactor program and expects to institute a plutonium energy economy with full reprocessing after the year 2000.
Overall Japan has an extremely advanced civilian scientific and engineering infrastructure capable of supporting nuclear weapons development and production. Japan has indigenously developed some uranium enrichment processes (e.g. the Ashi chemical exchange process), and has the technical means to deploy other processes if it chooses to do so. As one of the two leading manufacturing nations for computers (especially supercomputers), and has the second most advanced inertial confinement fusion program in the world, Japan is well positioned to quickly develop thermonuclear weapons.
In 1989 Japan produced 28% of its electricity (30500 MW) from 39 nuclear power plants, but had 26 more plants under construction or on the planning board. This would bring its nuclear power production to 57000 MW, over 50% of its total. In 1995 it had 50 reactors operating, providing 31% of its electricity. Japan plans eventually to generate all of its base load electricity from nuclear power.
Japan has an active breeder reactor development program, and operates the Monju fast breeder reactor. Japan has a limited plutonium reprocessing pilot plant at Tokai, and has contracts with Britain and France for several tons of reprocessed plutonium, although tens of tons are expected in the future.
The Rokkasho separation plant, under construction by Japan Nuclear Fuels Ltd since 1993, will have a capacity of 800 tonnes/yr of heavy metal. Safety upgrades have delayed its completion until 2003, at a cost of US$15 billion. Reprocessing costs are expected to be 40% higher than currently incurred for reprocessing in Europe.
At the end of 1994 Japan possessed 13 tonnes of separated plutonium.
Of this, 4352 kg was held domestically:
at reprocessing plants 836 kg,
at fuel fabrication facilities 3018 kg, at reactors and R&D facilities
498 kg; and 8720 kg was held overseas:
in UK 1412 kg
in France 7308 kg.
Japan has used plutonium in mixed oxide fuel for light water reactors and for fast neutron reactors over some 15 years. In 1994, 323 kg of plutonium was used in Monju, Joyo and Fugen reactors, and 111 kg was recovered from reprocessing spent fuel in Japan.
By the year 2000 Japan will have an inventory of about 55 tonnes of separated reactor grade plutonium. It should be noted that this is enough plutonium to manufacture ~10,000 warheads, more than the combined nominal arsenals of the U.S. and Russia combined under START II.
Although hard information about this is lacking, it is likely that Japan has undertaken advanced design work on a full range of nuclear weapon types. As noted at the beginning of this sub-section, this would be almost mandatory for national security reasons if only to create a base of expertise for conducting intelligence assessments of the nuclear programs of other nations. In contrast with Germany, Japan is in a relatively exposed position to potential threats with a long-term trend that is decidely negative due to the rapid growth of China's strength. It may also be argued that the lack of NATO memebership for Japan makes the U.S. nuclear umbrella somewhat more tenuous. These factors give Japan greater incentive to maintain a latent nuclear weapons capability. Should Japan decide to do so, it is likely that emergency capability nuclear weapons could be deployed by Japan within a few months of a decision to produce them.
According to proliferation assessments made by the U.S. government, no nonnuclear country is as well positioned to "break-out" and develop advanced nuclear weapons than Japan.