It recently confirmed it had resumed the conversion of medium enriched uranium into oxide fuel to slow down growth of its stockpile.

BBC: The clock is ticking on Iran

The MOX facility will blend surplus weapon-grade plutonium with depleted uranium oxide to make mixed oxide fuel for use in existing nuclear power plants.

FORBES: Japanese Official: Most Dangerous Reactor May Have Ruptured, Leaking Radiation

The fuel in No. 3 is a blend of plutonium and reprocessed uranium, referred to as MOX (for mixed oxide) and manufactured by the French nuclear company AREVA. MOX fuel rods are also less stable than plutonium-free rods.

FORBES: Japanese Official: Most Dangerous Reactor May Have Ruptured, Leaking Radiation