Kyushu Electric Power Co. said workers fired up the No. 3 reactor at its Genkai plant in the southern prefecture of Saga using MOX fuel - a mixture of plutonium oxide and uranium oxide.
The reactor is scheduled to start generating electricity Monday for a monthlong test run, and then begin full-fledged operations after a final government inspection and approval in early December, company official Futoshi Kai said.
The Genkai plant marks the beginning of Japan's use of MOX fuel for so-called "pluthermal" power generation, approved by the Cabinet more than a decade ago.
MOX fuel is a central element of Japan's plans to reduce its dependence on energy imports. Supporters say nuclear power is a viable clean energy that will support global efforts against climate change because it is essentially carbon-free.
Critics say MOX is too volatile and produces highly radioactive waste. Dozens of protesters rallied outside the government office in Saga on Thursday, raising safety concerns about the use of plutonium-based fuel.
Saga Gov. Yasushi Furukawa said he reminded Kyushu Electric to give safety top priority, and promised local residents that officials would closely monitor plant operations.
Japan, which now relies on nuclear plants for a third of its electricity needs, aims to raise that to nearly 40 percent in 2010. The government has said it hopes to convert as many as 18 nuclear reactors that now use more common uranium to those that use MOX.
The Japanese public, however, has grown increasingly wary of the nuclear power industry following a spate of safety problems, shutdowns and cover-ups.