A nuclear fuel reprocessing plant in northern Japan (search) started tests with depleted uranium (search) Tuesday, taking a major step in the country's closely watched efforts to use experimental reprocessed fuel to boost its energy self-sufficiency.

The test at Rokkasho, about 580 kilometers (360 miles) northeast of Tokyo (search), marks the first use of radioactive materials at the plant, said Masanori Hiroo, a spokesman for plant operator Japan Nuclear Fuel Ltd.

The plant is at the center of Japan's hopes of using a reprocessed fuel called mixed oxide, or MOX, in n dealing with a list of possible problems and developing troubleshooting measures.

Plant workers on Tuesday transported some 53 tons of depleted uranium, which contains lower level of radioactivity than ordinary uranium, into the plant to produce liquid uranium.

The 2.1 trillion yen (US$20 billion; euro14.95 billion) Rokkasho plant began operating in the early 1990s as a fuel storage site, and is expected to hold fuel and waste for up to 50 years. Since opening, it has taken in 779 tons (857 short tons) of spent fuel, more than a quarter of its capacity.

The reprocessed fuel the plant is expected to eventually make could be used in reactors that burn a mixture of uranium and plutonium, or more advanced fast-breeder reactors, which use plutonium instead of uranium and produce more plutonium for use as fuel.

Nuclear power is at the center of resource-poor Japan's plans to become more energy independent. Japan's 52 active nuclear power plants supply more than one-third of its energy and government plans call for more production.