Firstly, Japan currently lacks the technology to produce the highly enriched plutonium required to complete nuclear warheads. Its plutonium programme is for peaceful purposes to enable a country with few natural resources to secure a stable supply of energy. The purity of plutonium used in nuclear power reactors is said to be too low for military use. Although detailed information about the design of nuclear warheads is classified, it is generally believed that plutonium 239 required to produce such warheads must be enriched to 93 percent or purer, while reprocessing of nuclear reactor fuel yields material enriched only to about 60 percent [19].
In September 1994 the international environmental group Greenpeace, issued a report which alleged that the United States, in violation of its law designed to halt the spread of materials to make nuclear weapons, has been providing secret nuclear technology and technical data to Japan to help it build a reprocessing plant at the Tokai nuclear power complex [20]. Japan's Science and Technology Agency immediately denied the report [21]. Although the report implied that the purity of the extracted reactor-level plutonium is sufficiently high to be used in nuclear warheads, many specialists disagree [22]. It is illogical that the U.S. government, whose official policy has been to discourage Japan from developing weapons system not complementary to the U.S. system [23], would transfer any kind of technology or technical data that could help Japan to become a nuclear power. In support of this, on 29 December 1994, the U.S. Department of Energy concluded that there was no case of illegal nuclear technology or technical data transfer to Japan [24].
Japan has opened all of its atomic power activities to IAEA verification. Since 1994, in order to further improve the transparency of its nuclear fuel recycling program it has started to disclose specific data detailing its quantities of plutonium stock.
Secondly, Japan lacks the technology to produce highly enriched uranium to be used in uranium bombs and used as the initiator of thermonuclear bombs. It is generally believed that uranium 235 must be enriched to 93 percent or purer to be used in nuclear warheads, while commercial light water-reactors require fuel material that is only slightly enriched to about 3 percent [25]. This means that Japan currently does not possess the technology to produce warheads to complete ICBMs and submarine-launched ballistic missiles (SLBMs). In other words, credible nuclear deterrence is practically beyond the reach of Japan. To arm itself with nuclear weapons but not acquire credible deterrence would be a totally irrational act.
Some American experts claim, however, that it is possible to produce nuclear weapons with reactor-grade plutonium [26]. Suppose, for the sake of argument, those experts are right and Japan indeed possesses the capability to produce nuclear warheads. In order to achieve a credible nuclear deterrent capability, Japan must further acquire a means to deliver such warheads. Here lies a third technological hurdle Japan must clear before it can possess a useable and strategically meaningful nuclear capability. Civilian rocket technology and a military missile technology are not the same. Japan's H-2 rocket technology cannot readily be converted into a ballistic missiles technology [27]. In addition, Japan would have to develop the accurate inertial guidance system necessary for ballistic missiles.
Finally, even if Japan could produce ballistic missiles, they would inevitably be vulnerable to pre-emptive destruction, because geographically Japan is a narrow country with few suitable sites. To possess an invulnerable strategic arsenal Japan would have to deploy SLBMs. Japan would first need nuclear submarines to carry them, and an extensive terrestrial or satellite communications grid to support their activities. Japan has done nothing in the way of research on nuclear engines required to build nuclear submarines [28], or on such an extensive communications system.
It would not, of course, be technically impossible for Japan to clear these technological hurdles. It would, however, surely take time and oblige the Japanese people to bear heavy costs for research and development. Such developments would moreover inevitably become transparent.
Underlying Logic
Will Japan Go Nuclear - Myth and Reality
Japanese Attitudes