Fuel-element performance in operating pressurized-water reactors has been excellent. Over 250 000 fuel elements containing UO2, UO2-ThO2, or UO2-PuO2 fuel clad with iron- or zirconium-base alloys have performed reliably over a broad range of operating conditions. Experimental evidence to date suggests that fuel elements for pressurized-water reactors can be subjected to more demanding conditions.