The United States in 1977 assessed that plutonium Japan produced for its nuclear-energy program could be used to produce weapons and expressed that belief to the Japanese government, Kyodo News reported on Wednesday, citing recently declassified diplomatic documents.
A U.S. nonproliferation official informed a Japanese envoy based in Austria the then-prevalent theory that reactor-grade plutonium could not be used to fuel nuclear warheads was incorrect, according to a February 1977 diplomatic missive sent from the Japanese embassy in the United States to the Japanese Foreign Ministry.
Japanese atomic specialists at the time did not view plutonium withdrawn from used reactor fuel to be suited for use in making warheads.
The United States back then was against a Japanese government plan to begin operating the nation's first spent fuel reprocessing facility. Today, Japan's multiple reprocessing plants have recovered enough plutonium to fuel hundreds of weapons. The country has so much of the material it cannot store it all on its own territory. There are currently some nine tons of plutonium in Japan while the United Kingdom and France are holding an additional 35 tons of the Japanese-produced proliferation-sensitive substance.
Japan has long-been considered to be a nuclear-breakout state, meaning it has the technical know-how and resources to develop a warhead should the government make the political decision to do. However the island nation has forsworn nuclear arms production since coming under attack by U.S. atomic bombs in World War II. The growing threat posed by North Korea's nuclear weapons program as well as China's increasingly willingness to assert its territorial claims in the region has spurred some limited talk inside Japan about whether the country should develop a nuclear military capability.