Abstract
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)O2±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.
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