(CNBC) No nation has suffered more in the nuclear age than Japan, where atomic bombs flattened two cities in World War II and three reactors melted down at Fukushima just three years ago.

But government officials and proliferation experts say Japan is happy to let neighbors like China and North Korea believe it is part of the nuclear club, because it has a “bomb in the basement” -– the material and the means to produce nuclear weapons within six months, according to some estimates. And with tensions rising in the region, China’s belief in the “bomb in the basement” is strong enough that it has demanded Japan get rid of its massive stockpile of plutonium and drop plans to open a new breeder reactor this fall.

Japan signed the international Nuclear Non-Proliferation Treaty, which bans it from developing nuclear weapons, more than 40 years ago. But according to a senior Japanese government official deeply involved in the country’s nuclear energy program, Japan has been able to build nuclear weapons ever since it launched a plutonium breeder reactor and a uranium enrichment plant 30 years ago.

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