For instance, plutonium can be reprocessed into mixed oxide fuels and transmuted in standard reactors. However, this is limited by the accumulation of plutonium-240 in spent MOX fuel, which is neither particularly fertile (transmutation to fissile plutonium-241 does occur, but at lower rates than production of more plutonium-240 from neutron capture by plutonium-239) nor fissile with thermal neutrons. Even countries like France which practice nuclear reprocessing extensively, usually do not reuse the Plutonium content of used MOX-fuel. The heavier elements could be transmuted in fast reactors, but probably more effectively in a subcritical reactor which is sometimes known as an energy amplifier and which was devised by Carlo Rubbia. Fusion neutron sources have also been proposed as well suited.