The lattice parameters of the cubic solid solutions from the system (Ul-,Pu,)N were measured at 25°C. The lattice constant, uo, of UN was 4.88918a0.00017 A and that of PUN was 4.90476~0.00008 A. The parameter of the solid solution does not depend linearly on the composition parameter, x. The X-ray specimens contained from 600 to 1200 ppm total O+C as impurities; thus, these samples had much lower impurity levels than those of previous investigators. The compositions of the X-ray specimens were corroborated by quantitative chemical analysis. The significant deviation from Vegard's relation and the small variation observed for Q in this system are discussed in terms of the strongly hybridized electron f-d bands that have been calculated for these compounds.
Progress is reported in several areas of development of fueled graphite containing coated particles for nonpurged gas-cooled reactor systems. Hie sol-gel process has been modified for making spherical particles of both thorium-uranium carbide and thorlum-uranium oxide suitable for coating. Equipment has been assembled and methods have been developed for deposition of pyrolytic-carbon coatings under wellcontrolled conditions. Damage to coated particles during fabrication into a graphite matrix depends on the molding pressure and the volumet ric consent of coated particles. Vendor-supplied coated particles and fueled graphite spheres have been evaluated extensively in both inand out-of-reactor tests.Duplexand triplex-coated particles have excellent fission-gas retention at 2050°F to burnups of 15 at. %, and one triplex sample has also shown good retention at 25OO°F to 20 at. $ burnup. Fueled graphite spheres containing coated particles have good Irradiation performance, but the fission-gas release rates are somewhat higher than for unsupported coated particles. Fueled graphite spheres react with water vapor about as rapidly as do Speer Mod-2 and ATJ grades of graphite. The diffusion rates in pyrolytic carbon are the same for uranium, thorium, and protactinium. The diffusion rates in the direc tion parallel to the deposition plane are much higher than those in the perpendicular direction.
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