Reading this news article piqued my interest. What I found interesting is not that Japan’s foreign minister proclaimed that Japan was capable of producing a nuclear weapon should it wish to, but that such a declaration was even necessary.
Japan has been this way for several decades, I would assume. As an engineer, the way I see it is this. In order to produce a nuclear weapon, you need the following:
1) A critical mass of fissionable material. The two most readily available materials are U-235 and Pu 239. For U-235 this requires about 52 kg of high purity (not a small amount by any means), since decreasing the purity greatly increases the total mass needed for critical mass. The other alternative is Pu-239, which requires about 10 kg of high purity material. Both are obviously difficult to obtain and purify, but since Pu-239 is already produced in fission reactors (of which Japan has many) and Japan has a fast breeder reactor which uses high-concentration fuel that could most easily and quickly be converted to a weapon.
Incidentally, Japan’s single fast breeder reactor at Monju is in Fukui prefecture near Tsuruga city. The reactor is about a 1-hour drive from my wife’s family home. In fact, her father took us there for a day trip where we went to the visitor center (tours of the actual plant are possible but require permission, which we didn’t have time to do…) There are actually 3 nuclear power plants on that one small peninsula (due to it being one of the most rural areas of Japan), but the Monju reactor is here.
Pretty much all that would be required for Japan to produce the critical mass would be to seperate the Uranium Oxide and Plutonium Oxide in the fuel pellets, and then reduce the Plutonium Oxide to metallic form. Both can be done chemically, and so can be done relatively quickly and cheaply.
2) Next you need to be able to build the actual bomb. For a Pu-239 weapon that Japan would most likely make, it would require the implosion design such as was used in Fat Man that hit Nagasaki. The gun-type design is considerably easier, but is not suited to a Pu-239 weapon as explained at the end of this article. However, Japan is one of the most technologically advanced nations in the world, and so the required explosives, electronics, etc. are certainly within Japan’s capability.
3) The final requirement is a delivery system. The two obvious delivery systems are dropping as a bomb from an airplane, and the other is mounting onto a ballistic missile. Although Japan has no bombers per se in the Japan Air Self-Defence Force, if Japan could produce a sufficiently small nuclear weapon, then delivery by fighter is a possibility, albeit with limited range (Japan does not have an aircraft carrier in its navy). Delivery by ballistic missile though, would be relatively easy for Japan to do. It already has a mature rocketry industry and regularly puts satellites in orbit. Putting a weapon system on such a missile would be quite simple.
However, there are several large difficulties that Japan would have to overcome. Even if East Asian stability eroded to the point where Japan felt it needed to produce a nuclear weapon, it would be met with strong internal resistance to the violation of the Three Non-Nuclear Principles (éžæ ¸ä¸‰åŽŸå‰‡), “Japan shall neither possess nor manufacture nuclear weapons, nor shall it permit their introduction into Japanese territory.” This policy was never formally made law, but has been a matter of national policy and more importantly, national consciousness since the end of WWII. This last fact makes it even more powerful than a law in many ways.
The other major difficulty would of course be fierce external opposition. I cringe to think what the reactions of nations such as China, South Korea, and North Korea would be if it were revealed that Japan was producing a nuclear weapon. At times it seems the only thing that North and South Korea really agree on is fear of an imperialistic Japan. I’ll leave that analysis to someone else though, such as Sayaka (her studies involve international security issues) . She could most certainly give a detailed description of the issues and likely outcomes, but I’m just covering the possibility from an engineering standpoint.