An extensive experimental program was conducted on the fuel and cladding for the EGCR fuel elements to establish the adequacy of the design. Methods of inspecting the components and manufacturing the fuel assemblies were developed or perfected. Studies were also conducted on the physical and mechanical properties of the type 304 stainless steel cladding and the UO2 fuel pellets to provide a basis for predicting the behavior of the elements in service. Data from heat treating of neutron-activated fuel showed that fission-gas release will not be a limiting factor in this design. Out-of-pile thermal-cycling studies on simulated fuel elements established that axial growth of fuel elements at a predictable rate can be expected after the cladding collapses and contacts the fuel pellets, and that characteristic cracking of the UO2 pellets occurs without displacement of fuel. Impurities in the helium coolant were observed to cause oxidation and carburization of the cladding, but the extent of these reactions can be controlled by regulating the ratios of the impurities. Creep tests at several temperatures indicate that, with the exception of hydrogen, the various impurities will have little detrimental effect on the stress-rupture strength of the cladding. Tube-burst tests in air yielded data that permit a prediction of the life of the fuel element in case of loss of coolant pressure and when compared with results of similar in-pile experiments indicate that irradiation effects reduce the stress-rupture strength of the cladding by as much as 25%.