Reexamining the 1968 Report on Japan’s Nuclear Weapons Capability: From a Comparative Perspective

Abstract

In the late 1960s, the Japanese government’s Cabinet Research Office secretly investigated Japan’s nuclear weapons capability and then produced a report in 1968. From a technological and financial standpoint, the report concluded that Japan could build a small number of nuclear bombs without difficulty. Meanwhile Prime Minister Eisaku Sato had announced the “Three Non-Nuclear Principles” in December 1967, but the Japanese government policy did not explicitly prohibit Japan from possessing a nuclear weapons capability. Relying on the 1968 report and other materials, some published works insisted that the Sato government sought to maintain Japan’s nuclear weapons capability. This article questions the validity of this argument by reexamining the 1968 report and Japan’s atomic energy development in comparison with a U.S. government study from the mid-1960s on nuclear non-proliferation policy toward Japan.

Comparison of the analyses of the 1968 report and the U.S. study on Japan’s fissile material production capability reveals that the latter was based on a more realistic scenario of Japan’s nuclear armament than the former. The 1968 report assessed that Japan could build nuclear bombs by using the plutonium produced by a modified Calder Hall reactor purchased from the U.K. because Japan would obtain reprocessing capability in the early 1970s. To do so, however, required Japan to refuse the safeguards stipulated in the 1958 Japan—U.K. atomic energy agreement. The 1968 report found that it would be damaging and unlikely for Japan to consider such a course of action. In contrast, the U.S. study, which also concluded that Japan had the ability to manufacture plutonium bombs, assumed that Japan would construct a heavy-water moderated reactor using safeguards-free natural uranium to evade international safeguards. Unlike the U.S. study, the 1968 report did not explore feasible measures for Japan to build nuclear bombs.

This article also argues that the Sato government lacked political determination to develop and maintain Japan’s nuclear weapons capability. In the late 1960s, Japan was acquiring a nuclear weapons capability as a result of its atomic energy development, which did not follow the scenario in the aforementioned U.S. study. The delay in the construction of a reprocessing plant in Tokai Mura illustrated that the Japanese government did not prioritize the development of Japan’s nuclear weapons capability. Moreover, it became more difficult for Japan to go nuclear against the will of the U.S. because in the late 1960s the former deepened its dependence on the latter for atomic energy development. Nevertheless, Japan’s atomic energy complex and national security circles had a common interest in promoting Japan’s atomic energy development as a national policy, and consequently Japan retained its nuclear weapons capability.