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TOKAI, Japan — Wedged in a corner of a pine-scented, campus-like research center here is the so-called Fast Critical Assembly, which — except for a squat reactor containment dome — looks more like a suburban elementary school than a nuclear laboratory handling some of the world’s most dangerous materials.

About a dozen researchers work at the FCA, which began operations in 1967. They study plutonium and weapons-grade uranium, the principal building blocks of nuclear arms, to design fuel for Japan’s future breeder reactors, a key to the country’s ambition to increase its energy independence.

But their work will soon be altered, because Japan — after years of resistance — has finally agreed to return its stock of plutonium at the FCA to the United States, in the form of 5,451 square metal wafers, according to officials in both countries. The agreement is to be announced at a U.S.-led international summit to promote the security of such fissile materials, beginning March 24 in the Netherlands.

The Obama administration will hail the deal as a crucial step in its global campaign to ensure that terrorists cannot obtain such explosive materials, authorities say. U.S. officials say they have been worried the materials here have been casually guarded, and are concerned that that there is no federal standard requiring workers at such plants to be subjected to formal, detailed background checks like those for nuclear workers in the United States.

Some nonproliferation experts say the withdrawal of the explosives from Japan, while a step in the right direction, represents little more than window dressing, however, since the country has 9.3 metric tons of additional plutonium stored at other locations domestically and plans to start producing more at its new Rokkasho factory later this year.

“The first place the Japanese would go if they wanted to make nuclear weapons would be to [this] stockpile, because it’s the best plutonium they have for making weapons,” said Thomas Cochran, a Natural Resources Defense Council physicist who has spent much of his career opposing breeder reactor programs like Japan’s. “But they have plenty of other plutonium. If they wanted nuclear weapons, this [shipment] wouldn’t reduce the risk of [their] acquiring them.”

Nor would the return of the materials, by itself, reduce the risk of terrorists stealing similar materials stored elsewhere in Japan, including at Rokkasho, Cochran said.

About 50 researchers and staff presently work at the research facility here, formally known as the Tokai Research and Development Center and located about 70 miles north of Tokyo. It includes a storage building holding a total of 730 pounds of plutonium and 660 pounds of weapons grade uranium. More than 90 percent of the plutonium is the isotope plutonium-239, the kind favored by professional nuclear weapons designers.

Both the plutonium and the weapons grade uranium were provided by the United States and Britain to Japan’s civil nuclear program starting in the 1960s. And the Obama administration has been asking for the materials since before the president convened the first international Nuclear Security Summit in 2010, partly because U.S. officials were alarmed by how casually the explosives have been protected there.

The stocks of both explosive materials here could provide sufficient fuel for roughly 375 bombs with the explosive force of 20 kilotons, or 20,000 tons of TNT. That was the power of the bomb that exploded over Nagasaki in 1945.

Like the security guards stationed at most Japanese nuclear facilities, none at the Fast Critical Assembly facility are armed, although local police are stationed nearby in the larger research campus. Japanese officials have acknowledged to visiting Westerners that if the guards ever faced attacking terrorists, their instructions are to flee.

To U.S. officials, this approach is almost unthinkable — as if the U.S. had decided it couldn’t or wouldn’t post armed guards with security clearances at the sites in Texas, New Mexico, and California where most of its bomb fuels are stored.

With the encouragement of the United States, Japan since 1978 has shut down a few research reactors that used such risky fissile fuels, sometimes converting them to use low-enriched uranium. But Japanese officials have until now resisted relinquishing the fuel at the Tokai facility, the largest research center of its kind in the world, because it will be difficult to replace.

Tsuyoshi Yamane, a silver-haired senior engineer at the site, told the Center for Public Integrity during a visit in November that there has been increasing “pressure imposed” by the international community, including the United States, for Tokai to halt its work with nuclear explosives.

He acknowledged that the facility is “very, very old” and that a room adjacent to the reactor in which fuel is assembled had cracked during the March 2011 earthquake. “Luckily, that was a day when there was no actual test or experimenting,” he said.

But Yamane said then that the fuel development done at Tokai could not be conducted using other, less dangerous materials. “The research groups who work here have said they need to use the system as it is, in order to carry out experiments and tests,” he said.

Two-thirds of that research, he said, is aimed at designing the most efficient fuels for fast breeders, a type of reactor that both consumes and produces plutonium. The United States, France and Britain all launched commercial fast breeder programs, only to abandon them in the 1980s and 1990s. Only Japan, Russia and India are still developing fast breeder reactors.

A Japanese official, who spoke on condition he not be identified, said a key issue in recent negotiations with Washington over the materials’ return was whether alternate fuels might be used for research at Tokai, possibly with U.S. or British help.

During a recent visit, Yamane led his entourage down a corridor lined with dingy beige walls, with linoleum peeling off floors beneath fluorescent lights. After the visitors picked up dosimeter badges and put on white overalls, slippers, and hats, they moved through an airlock into the reactor workroom. A construction crew and foreman were working inside the room, repairing the building and replacing an overhead crane. One worker was seen carrying a heavy tool bag through the main airlock without any escort.

Under current Japanese practices, these subsidiary workers also are not subjected to detailed background checks.

The Fast Critical Assembly at present consists of two steel honeycombs, about eight feet high and eight feet wide, facing one another. To conduct the fuel studies, wafers of plutonium or uranium 2 inches by 2 inches or 1½ inches by 1½ inches are assembled by hand in trays resembling those that hold library catalogue cards and inserted into the cells. The low-power reactor goes critical — the nuclear chain reaction begins — when the two sides of the apparatus are pushed together.

The direct involvement of the researchers in manipulating the disks greatly increases the risk of theft or diversion, according to U.S. experts who have visited the plant.

No diversion has ever been documented. But another facility at the Tokai research center — the Plutonium Fuel Production Facility — has acknowledged difficulties in keeping track of its plutonium. It opened in October 1988 to develop fuels for a small experimental breeder reactor. But by 1994 about 152 pounds of plutonium — enough for about 23 bombs — was missing, a fact first reported by the late Paul Leventhal of the Nuclear Control Institute of Washington.

As a result, the International Atomic Energy Agency asked Tokai’s operators to shut down the fuel plant, cut up the equipment and measure the plutonium stuck inside it. Officials spent two years and $100 million on the effort, and whittled the “missing” plutonium down to about 22 pounds — still about three bombs’ worth. That was the best they said they could do.

Jonathan Lalley, assistant press secretary for national security at the White House, said return of the plutonium from the Fast Critical Assembly was the subject of an “ongoing negotiation.” But other informed sources in both countries said the deal was done.

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