NACA Technical Memorandums

This report was made to study the velocity distribution in an open, in a partially open, and in a partially expanding jet. The open-jet observations reveal minor systematic discrepancies from Tollmien's theoretical velocity distribution. The shearing-stress distribution for the partially open jet was determined. The value derived for the ratio of mixing distance to jet width was found to be in close agreement with the corresponding value for the open-jet boundary

Transportverhalten von Plutonium und Americium in niedrig angereicherten beschichteten Brennstoffteilchen bei hohenBestrahlungstemperaturen

Low enriched coated fuel particles with different kernel composition (oxide, carbide, oxicarbide) were irradiated at high temperatures in the range of 1100-1600°C reaching burnup values of 11-12 % FIMA. By mechanical separation of kernel and coating of single irradiated particles followed by chemical separation and alphaspectrometric determination of plutonium and americium, the internal release of both transuranium elements was measured. In coated particles with U0$_{2}$ kernels the amount of Pu and Am in the coating was the saure as the amount of uranium before irradiation : any internal release of Pu and Am from UO$_{2}$ kernels could not be observed. From UC$_{2}$ and UCO kernels both transuranium elements were released, the fractional release of americium was always higher than that of plutonium. Effective diffusion coefficients of the transuranium elements describing their release behaviour from UC$_{2}$ kernels were found to be in the range of (2,0$\pm$0,8)-10$^{-14}$ cm$^{2}$s$^{-1}$ (plutonium) and (2,4$\pm$0,8)-10$^{-13}$ cm$^{2}$ (americium) at an average irradiation temperature of about 1350°C. Using UO$_{2}$ kernels these coefficients are surely below 2,5 $\cdot$ 10$^{-18}$ cm$^{2}$s$^{-1}$. Plutonium and americium diffused through pyrocarbon coatings nearly equally with average diffusion coefficients of 1,6$\cdot$10$^{-11}$ cm$^{2}$s$^{-1}$ (plutonium) and 2,3$\cdot$10$^{-11}$ cm$^{2}$s$^{-1}$ (americium). These data agree within their standard deviation with earlier published diffusion coefficients of plutonium in pyrocarbon coatings

Improvement in Retention of Solid Fission Products in HTGR Fuel Particlesby Ceramic Kernel Additives

Increased requirements concerning the retention of long-lived solid fission products in fuel elements for use in advanced High Temperature Gas-cooled Reactors led to the development of coated particles with improved fission product retention of the kernel, which represent an alternative to silicon carbide-coated fuel particles. Two irradiation experiments have shown that the release of strontium, barium, and caesium from pyrocarbon-coated particles can be reduced by orders of magnitude if the oxide kernel contains alumina-silica additives. It was detected by electron microprobe analysis that the improved retention of the mentioned fission products in the fuel kernel is caused by formation of the stable aluminosilicates SrAl$_{2}$Si$_{2}$O$_{8}$, BaAl$_{2}$Si$_{2}$O$_{8}$ and CsAlSi$_{2}$O$_{6}$ in the additional alumina-silica phase of the kernel