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Japan’s Prime Minister Shinzo Abe, center, arrives at Schiphol Amsterdam airport, Netherlands, Sunday, March 23, 2014 to attend the March 24-25 Nuclear Security Summit in The Hague. (Toussaint Kluiters, Pool/AP)

The Japanese government confirmed Monday it will ship hundreds of pounds of nuclear explosive materials back to the United States, after using it for decades to conduct research on nuclear reactor fuels.

Japan’s expected, public pledge came in a joint statement with Washington that was timed to coincide with the first day of a U.S.-led, international Nuclear Security Summit on March 24-25 in the Netherlands.

The plutonium and enriched uranium to be returned are stored at Japan’s Tokai nuclear research complex on the Pacific coast about 65 miles from Tokyo, a facility that U.S. officials long complained was poorly protected from thefts or terrorist assaults.

The materials — consisting of pocket-sized wafers of metallic plutonium and uranium — could in theory be used to build an arsenal of 68 bombs equal in power to the weapon that destroyed much of Nagasaki in 1945.

But Japan will still have plenty of such materials in its holdings once the repatriation is completed. The plutonium alone is 3.5 percent of what Japan has in its own warehouses, and less than one percent of the country’s total holdings (some of it is stored outside the country). It also represents just 4 percent of what the country can produce in a year at its new plutonium factory, now scheduled for completion in October in the village of Rokkasho.

The deal comes amid heightened U.S. pressure on Japan to reduce or limit the size of its plutonium stockpile, mostly to reduce the risk of its theft by terrorists, and a global drive by Washington to convince other nations to do the same.

As part of its effort to prevent any thefts of other dangerous radioactive materials, such as medical isotopes, that aren’t explosives but could be used to sicken or kill, the United States also pledged with 22 other nations attending the summit to try to secure all radiological substances within their borders by 2016.

Update, March 25 at 11:22am: The final 2014 Nuclear Security Summit communique also included language that, for the first time, urged states to keep stockpiles of plutonium to a “minimum level.” While the statement was hedged — saying that stocks of the nuclear explosive should be minimized “consistent with national requirements” — it nevertheless marked a milestone in the biennial summit process, which began in 2010. Previous summits focused on reducing stockpiles of the other main nuclear explosive, highly-enriched uranium, and did not call for limits on plutonium stockpiles, due to opposition from countries like Russia and India that are seeking to develop commercial plutonium nuclear programs.

The joint U.S.-Japanese statement, released by President Obama and Japanese prime minister Shinzo Abe, said: “This effort involves the elimination of hundreds of kilograms of nuclear material, furthering our mutual goal of minimizing stocks of HEU [highly-enriched uranium] and separated plutonium worldwide, which will help prevent unauthorized actors, criminals or terrorists from acquiring such materials.”

The main elements of the deal were reported on March 11 by the Center for Public Integrity, in a series of articles detailing recent friction between the two countries about Japan’s plutonium programs.

Monday’s joint statement didn’t provide any figures, but according to a decade-old U.S. government report, the Fast Critical Assembly research facility at Tokai has about 730 pounds of separated plutonium. It also has about 1,210 pounds of enriched uranium, capable of use in weapons.

The statement gave no timetable for the transfer of the materials, and did not mention the total size of Japan’s stockpiles or refer to growing tensions between the two countries over Japan’s plan to operate the Rokkasho factory under what U.S. officials consider to be inadequate security.

The square-mile facility, located on a stretch of Pacific 1,000 miles north of Tokyo, is capable of producing 8 tons of plutonium each year. But its workers have not been subjected to background checks, most of its guards are not armed, and its security forces do not routinely engage in realistic training exercises, according to U.S. officials.

While Japan has said it plans to use the newly-made plutonium as commercial reactor fuel, no reactors capable of burning it are now in operation, due to the March 2011 Fukushima disaster.

Officials in Abe’s government have privately told American experts they want to reopen a half dozen or so idled reactors by the end of this year, and more in the coming years.

Nobuyasu Abe, a former U.N. undersecretary general for disarmament who was recently named to the nation’s Atomic Energy Commission, said March 15 at a Brookings Institution symposium about Rokkasho that once the plant is finished, Japan will probably operate it at “partial capacity.”

“That is necessary in order to keep the plutonium balance at least equal, not increasing,” he said — referring to the government’s evident desire to consume the plutonium-based fuel as it is being created.

Kenneth Luongo, a former Department of Energy policy advisor and current president of the Partnership for Global Security, called Monday’s announcement of the return of the weapons materials from Japan “a positive step forward,” especially in East Asia, where Japan is just one of a number of countries trying to decide the shape of their nuclear programs.

“But Rokkasho is going to be a gigantic plutonium factor, so that is a major concern,” Luongo said.

The Fast Critical Assembly, which has operated since the 1967, was damaged by the same March 2011 earthquake that hit Fukushima and is currently undergoing repairs.

During a visit to the research reactor in November, the Center for Public Integrity found aging infrastructure — including unpainted walls and old equipment — as well as relatively light security, given the weapons materials used there.

According to the White House, once the Tokai uranium arrives in the United States, it will be diluted or “down-blended” into low-enriched uranium for use in commercial reactors or other civilian purposes.

The plutonium meanwhile will be stored pending a United States decision on how to get rid of its excess plutonium.

The statement also said the United States has agreed to help Japan “design new enhancements” to the research facility at Tokai, including converting the test reactor there to use non-explosive fuel.

The summit’s agreement regarding radiological materials, including isotopes used for medical purposes, is meant to keep these materials from being used to make a dirty bomb — a conventional explosive device salted with highly radioactive material that, while it can’t produce a nuclear blast, can produce a cloud of dangerously radioactive debris.

The 23 countries — out of 53 attending the summit in The Hague — that signed the pledge include Algeria, Georgia, Kazakhstan, Morocco, Turkey and the United Arab Emirates, as well as a number of Western industrialized nations. Conspicuous by their absence from the list are the nuclear powers France, China and Russia.

The signers of the deal also pledged to follow the International Atomic Energy Agency’s guidelines for security for radiological materials.

A March 2014 report by the Center for Nonproliferation Studies found that last year alone, there were 153 cases where authorities in 30 countries lost control of some of their radiological and nuclear materials.

The vast majority of these cases, a total of 141, involved materials that are dangerously radioactive but not usable in nuclear weapons. In about half of cases, the report blamed the loss of the materials on “negligence” by the people handling them. In about a third of the cases, the materials were lost or stolen during transit.

In one high-profile incident, thieves stole a truck outside Mexico City in early December that was hauling cobalt-60 from a hospital to a radioactive waste storage center, according to news accounts. The truck and its contents were recovered not far from where it was taken three days later, but only after the thieves were exposed to dangerous radiation.

The United States may find it hard to meet the new pledge. Last year, the Energy Department acknowledged that 1,500 U.S. hospitals use radiological sources that could be turned into dirty bombs, and warned that it could take until 2025 to improve security for all those sources, at a cost of hundreds of millions of dollars.

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