The Fukuoka High Court on Monday upheld a lower court decision rejecting a group's demand that fuel containing plutonium should not to be used in a currently idled reactor.

Kyushu Electric Power Co. is seeking approval to restart the No. 3 reactor at the Genkai plant in Saga Prefecture and use plutonium-uranium mixed oxide fuel, commonly known as MOX fuel.

MOX fuel is created using plutonium and uranium extracted from spent nuclear fuel. It is a key component in the nuclear fuel cycle pursued by the nuclear power industry and the government.

The Genkai No. 3 reactor, designed to run on both MOX fuel and uranium fuel, became the first in Japan in 2009 to use MOX.

It was taken offline in 2010 for a regular checkup and has remained idle as the operator sought to clear the stricter safety requirements introduced following the March 2011 Fukushima crisis.

The earlier ruling in March 2015 by the Saga District Court was the first over the legality of MOX use at a reactor. The court said then there was no evidence proving that using the fuel raises the possibility of a serious accident.

The plaintiffs, which included both local residents and anti-nuclear activists, argued that a gap could develop between the MOX fuel and its container tube, and that this gap would cause a cooling failure and lead to a major accident such as a meltdown.