Japan also burned MOX in four conventional reactors beginning in 2009. Conventional reactors can use MOX for up to a third of their fuel, but that makes the fuel riskier because the plutonium is easier to heat up.
Three of the conventional reactors that used MOX were shut down for regular inspections around the time three Fukushima Dai-ichi reactors exploded and melted down following the March 201l earthquake and tsunami. The fourth reactor that used MOX was among the reactors that melted down. Plant and government officials deny that the reactor explosion was related to MOX.
Japan hopes to use MOX fuel in as many as 18 reactors by 2015, according to a Rokkasho brochure produced last month by the operator. But even conventionally powered nuclear reactors are unpopular in Japan, and using MOX would raise even more concerns.
When launched, Rokkasho could reprocess 800 tons of spent fuel per year, producing about 5 tons of plutonium and 130 tons of MOX per year, becoming the world's No. 2 MOX fabrication plant after France's Areva, according to Rokkasho's operator.
The government and the nuclear industry hope to use much of the plutonium at Oma's advanced plant, which could use three times more plutonium than a conventional reactor.
Meanwhile, the plutonium stockpile grows. Including the amount not yet separated from spent fuel, Japan has nearly 160 tons. Few countries have more, though the U.S., Russia and Great Britain have substantially more.
"Our plutonium storage is strictly controlled, and it is extremely important for us to burn it as MOX fuel so we don't possess excess plutonium stockpile," said Kazuo Sakai, senior executive director of Rokkasho's operator, JNFL, a joint venture of nine Japanese nuclear plant owners.
Rokkasho's reprocessing plant extracted about 2 tons of plutonium from 2006 to 2010, but it has been plagued with mechanical problems, and its commercial launch has been delayed for years. The operator most recently delayed the official launch of its plutonium-extracting unit until next year.
The extracted plutonium will sit there for at least three more years until Rokkasho's MOX fabrication starts up.
Giving up on using plutonium for power would cause Japan to break its international pledge not to possess excess plutonium not designated for power generation. That's why Japan's nuclear phase-out plan drew concern from Washington; the country would end up with tons of plutonium left over. To reassure Japan's allies, government officials said the plan was only a goal, not a commitment.
Japan is the only nation without nuclear weapons that is allowed under international law to enrich uranium and extract plutonium without much scrutiny. Government officials say they should keep the privilege. They also want to hold on to nuclear power and reprocessing technology so they can export that expertise to emerging economies.
Many officials also want to keep Rokkasho going, especially those in its prefecture (state) of Aomori. Residents don't want to lose funding and jobs, though they fear their home state may become a waste dump.
Rokkasho Mayor Kenji Furukawa said the plant, its affiliates and related businesses provide most of the jobs in his village of 11,000.
"Without the plant, this is going to be a marginal place," he said.
But Rokkasho farmer Keiko Kikukawa says her neighbors should stop relying on nuclear money.
"It's so unfair that Rokkasho is stuck with the nuclear garbage from all over Japan," she said, walking through a field where she had harvested organic rhubarb. "... We're dumping it all onto our offspring to take care of."
Nearly 17,000 tons of spent fuel are stored at power plants nationwide, almost entirely in spent fuel pools. Their storage space is 70 percent filled on average. Most pools would max out within several years if Rokkasho were to close down, forcing spent fuel to be returned, according to estimates by a government fuel-cycle panel.
Rokkasho alone won't be able to handle all the spent fuel coming out once approved reactors go back online, and the clock is ticking for operators to take steps to create extra space for spent fuel at each plant, Nuclear Regulation Authority Chairman Shunichi Tanaka said.
"Even if we operate Rokkasho, there is more spent fuel coming out than it can process. It's just out of balance," he told the AP.
A more permanent solution — an underground repository that could keep nuclear waste safe for tens of thousands of years — seems unlikely, if not impossible.
The government has been drilling a test hole since 2000 in central Japan to monitor impact from underground water and conduct other studies needed to develop a potential disposal facility. But no municipality in Japan has been willing to accept a long-term disposal site.
"There is too much risk to keep highly radioactive waste 300 meters (1,000 feet) underground anywhere in Japan for thousands or tens of thousands of years," said Takatoshi Imada, a professor at Tokyo Technical University's Decision Science and Technology department.