The plutonium Japan has stockpiled for decades was supposed to be a smart energy source for the resource-poor nation.

But last year’s earthquake-triggered nuclear accident, along with growing global anxiety about extremists gaining access to radioactive material, is turning a potential asset into a liability.

Speaking at the Nuclear Security Summit here Tuesday, Japanese Prime Minister Yoshihiko Noda described a “myth of safety” that, he said, had lulled Japan into a false sense of security about its nuclear facilities.

He said disasters such as the rupture of holding tanks at the Fukushima Daiichi nuclear plant pose a real threat. But, he added, “the man-caused act of sabotage will test our imaginations far more than any natural disaster.”

Because opposition to nuclear energy has spiraled in Japan since the Fukushima disaster, the country is operating just one of its 54 reactors and has scant hope of activating more anytime soon. That means there is little use for the country’s stockpiles of separated plutonium and few easy options for reducing the supply.

At the summit, the White House announced a meeting in Japan next year at which five countries — Japan, South Korea, the United States, Britain and France — will discuss ways to improve the security of radioactive materials when they are being transported.

President Obama noted Monday that the “smallest amount of plutonium — about the size of an apple — could kill hundreds of thousands and spark a global crisis.”

“We simply can’t go on accumulating huge amounts of the very material, like separated plutonium, that we’re trying to keep away from terrorists,” Obama said in a speech at Seoul’s Hankuk University of Foreign Studies.

The 9/11 Commission reported that al-Qaeda had attempted to acquire nuclear materials, and other terrorist groups such as Aum Shinrikyo — the Japanese cult group that staged a sarin gas attack on the Tokyo subway system in 1995 — have also reportedly tried to obtain radioactive materials.

According to the International Panel on Fissile Materials, there are about 500 metric tons of plutonium in the world. (There are also about 1,440 tons of highly enriched uranium.)

Japan has more plutonium on its hands than any other non-weapons state, according to a 2011 report from the panel. Three-quarters of Japan’s plutonium is stored in other countries, but Japan holds nearly 10 tons, enough for more than 1,000 weapons, in various storage facilities and nuclear reactors across the country.

It obtained all that plutonium because it had one of the world’s most ambitious non-weapons nuclear programs. Japan not only wanted to use nuclear reactors to supply half of its energy but also wanted to reprocess the plutonium from the spent fuel produced by those reactors so the material could be used in the reactors again.

Most countries choose simply to store their spent fuel — a less expensive option, but one that invites its own controversies and considerations about handling of nuclear waste.

Japan, even before the Fukushima accident, had faced numerous technical problems that slowed or delayed its plans to reuse the separated plutonium as reactor fuel.

The country drafted its first plans for reprocessing plutonium several decades ago, when most scientists believed that the planet had a thin supply of mineable uranium, which is fed into reactors. Separated plutonium represented an alternative source of fuel.

But scientists have learned that the world supply of mineable uranium is much more abundant than thought, undermining any justification for stockpiling plutonium.

“These were visions that made sense 30 to 40 years ago, when we thought there was little uranium in the world,” said Laura Holgate, an Obama senior adviser on weapons of mass destruction and nuclear threats. “But now we know that the shortage concept is antiquated. We also know more about how vulnerable separated plutonium can be from a terrorist point of view.”