Unit I-1 was designed in 1967 and went online in 1970. From September 2010, unit I-3, which went online in 1976, has been fueled by MOX fuel.
Mixed oxide (MOX) fuel contains plutonium recovered from spent nuclear fuel (Source: World Nuclear News and The Japan Times). So far, media has not reported about the use of MOX fuel, possibly because it could add to concern and confusion. I have tried to access Tepco's website but it is not responding (I get an error message) possibly due to too much activity.
Fortunately, Units I-4, I-5, and I-6 were shut down for maintenance at the time of the earthquake/tsunami on March 11, 2011.
Green Action, an independent NGO, is carefully monitoring the developments at the different nuclear reactors along the coast in north-eastern Japan.
From The Japan Times:
Monday, Aug. 23, 2010
Fukushima reactor receives MOX
FUKUSHIMA (Kyodo) Tokyo Electric Power Co. on Saturday loaded a nuclear reactor in Fukushima Prefecture with MOX, a controversial fuel made with reprocessed plutonium and uranium oxides, as it prepares to become the leading power utility's first facility to go pluthermal.
The No. 3 reactor at Tepco's Fukushima No. 1 plant will be the nation's third pluthermal facility, but only the first to be refurbished since the plant was built 34 years ago.
Tokyo Electric plans to activate the reactor on Sept. 18 and let it start generating electricity on Sept. 23.