Fuel elements for nuclear reactors consist of a core of fissionable material-uranium—and usually source materials or diluents surrounded by an inert corrosion-resistant jacket or cladding. The cladding material stops fission products from escaping, eliminates any reaction or solution between the fuel and coolant, restrains the fuel to a limited extent from warping and surface roughening, and reduces the danger of toxic materials being dispersed in the coolant system.