The United States believed in 1977 that nuclear weapons could be produced from plutonium extracted from spent nuclear fuel at Japan’s light water reactors and conveyed its view to Tokyo, according to Japanese diplomatic records declassified Wednesday.
A U.S. arms control official told a Japanese diplomat based in Austria that the common belief that reactor-grade plutonium is unfit for weapons production is wrong, according to a cable sent by then-Ambassador to the U.S. Fumihiko Togo to the Foreign Ministry in Tokyo on Feb. 23, 1977.
At the time, the administration of U.S. President Jimmy Carter was pursuing nonproliferation following a nuclear test carried out by India in 1974.
Nuclear experts in Japan, including Ryukichi Imai, a former adviser to the governmental Japan Atomic Energy Commission, long believed that plutonium extracted from reprocessed fuel was not suitable for atomic bomb production.
Washington was opposed to Tokyo’s plan to open the country’s first nuclear reprocessing plant in Ibaraki Prefecture.
The U.S. arms control official also told the diplomat from the Japanese Embassy in Austria that compared with high-purity weapons-grade plutonium, reactor-grade plutonium is not optimum for arms production because of its relatively low explosive power.
The official did not want the information made public but reiterated that nuclear weapons could be made from plutonium extracted from spent fuel.
The diplomatic record also revealed that the U.S. pressured South Korea into abandoning its plan to purchase a nuclear reprocessing facility in the mid-1970s.
Resource-poor Japan promoted a policy from the late 1950s of recycling nuclear fuel by extracting plutonium from spent fuel. The Carter administration later endorsed a plan by Tokyo to open the reprocessing plant in Ibaraki that limited the amount of reprocessed fuel.
The Nuclear Non-Proliferation Treaty does not clearly ban nonnuclear states from acquiring nuclear fuel reprocessing technology that can produce weapons-grade plutonium. At present, Japan keeps 44 tons of plutonium.