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

Abstract

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