Conventional light-water reactors use uranium and produce some plutonium during fission. Reprocessing creates an opportunity to reuse the spent fuel rather than storing it as waste, but the stockpiling of plutonium produced in the process raises concerns about nuclear proliferation.
Fast-breeder reactors are supposed to solve part of that problem. They run on both uranium and plutonium, and they can produce more fuel than they consume because they convert uranium isotopes that do not fission readily into plutonium. Several countries have developed or are building them, but none has succeeded in building one for commercial use. The United States, France and Germany have abandoned plans due to cost and safety concerns.
The prototype Monju fast-breeder reactor in western Japan had been in the works for nearly 50 years, but after repeated problems, authorities this summer pulled the plug, deeming the project unworkable and unsafe.
Monju successfully generated power using MOX in 1995, but months later, massive leakage of cooling sodium caused a fire. Monju had another test run in 2010 but stopped again after a fuel exchanger fell into the reactor vessel.
Some experts also suspect that the reactor sits on an active fault line. An independent team commissioned by the Nuclear Regulation Authority is set to inspect faults at Monju in early 2013.
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.
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