The highest density sources of energy aside from antimatter are fusion and fission . Fusion includes energy from the sun which will be available for billions of years ( in the form of sunlight ) but so far ( 2018 ) , sustained fusion power production continues to be elusive . Power from fission of uranium and thorium in nuclear power plants will be available for many decades or even centuries because of the plentiful supply of the elements on earth , though the full potential of this source can only be realised through breeder reactors , which are , apart from the BN-600 reactor , not yet used commercially . Coal , gas , and petroleum are the current primary energy sources in the U.S. but have a much lower energy density . Burning local biomass fuels supplies household energy needs ( cooking fires , oil lamps , etc. ) worldwide . The density of thermal energy contained in the core of a light water reactor ( PWR or BWR ) of typically 1 GWe ( 1 000 MW electrical corresponding to ~ 3 000 MW thermal ) is in the range of 10 to 100 MW of thermal energy per cubic meter of cooling water depending on the location considered in the system ( the core itself ( ~ 30 m3 ) , the reactor pressure vessel ( ~ 50 m3 ) , or the whole primary circuit ( ~ 300 m3 ) ) . This represents a considerable density of energy which requires under all circumstances a continuous water flow at high velocity in order to be able to remove the heat from the core , even after an emergency shutdown of the reactor . The incapacity to cool the cores of three boiling water reactors ( BWR ) at Fukushima in 2011 after the tsunami and the resulting loss of the external electrical power and of the cold source was the cause of the meltdown of the three cores in only a few hours , even though the three reactors were correctly shut down just after the Tōhoku earthquake . This extremely high power density distinguishes nuclear power plants ( NPP 's ) from any thermal power plants ( burning coal , fuel or gas ) or any chemical plants and explains the large redundancy required to permanently control the neutron reactivity and to remove the residual heat from the core of NPP 's
Tōhoku earthquake
2011 Tōhoku earthquake and tsunami
Energy density