2. Nuclear Power Reactors and Their Fuels

Ceramic oxide fuels, in particular, uranium dioxide (UO₂) in the form cylindrical pellets, with zircaloy as cladding are used in the majority of operating nuclear power reactors. However, some reactors employ mixed uranium-plutonium oxide (mixed oxide, MOX²; (U,Pu)O_2±x). The two major isotopes of natural uranium are ²³⁵U (0.711 wt%) and ²³⁸U 99.284 wt% with traces of ²³⁴U isotope. In the light water-cooled reactors (LWR), including pressurized water (PWR), boiling water (BWR) and water-cooled and water-moderated (VVER) power reactors, the fuel is enriched to increase the abundance of fissile ²³⁵U in the naturally occurring uranium up to slightly less than 5 wt% because the main contributor to fissioning is ²³⁵U. In the pressurized heavy³ water reactors (PHWR), excluding the fuel used in the German pressurized vessel design in Atucha in Argentina, natural uranium is used in fabricating the UO₂ pellets. Due to its lower neutron absorption, heavy water (D₂O) is one of the most efficient moderators to be used with natural uranium. For LWR applications, fuel needs to be enriched (up to 4.9 wt% ²³⁵U in U) because light water (H₂O) not only scatters but absorbs many neutrons to be an effective moderator to be used with natural uranium.