Spherical Fuel Research Articles

Migration Behavior of Fission Products in and from Spherical High-Temperature Reactor Fuel Elements

Diffusion behavior of some metallic fission products in high-temperature reactor fuel elements, which had been irradiated in an in-pile gas loop (Saphir) installed in the Pegase reactor (France), was studied. Diffusion coefficients of cesium and silver in hightemperature isotropic pyrolytic carbon and graphite matrix under in-pile conditions were obtained by analyzing the concentration profiles of the fission products in the fuel elements, which had been measured by postirradiation examination. Although ruthenium profiles were measured, analysis of the diffusion coefficients could not be carried out because of the virtually flat distributions. By comparing the concentrations of the cesium isotopes in the fuel-free zone of the elements, it was found that TUCs behaved anomalously in the graphite matrix, which was, probably, caused by activation of an undetectable amount of TTCs impurity involved in the matrix. For the extremely high concentration of these fission products, which had been observed near the surface of the element, two causes, the uranium contamination concentrating there and the trapping effect in the defects introduced by fission of the locally concentrated uranium, were considered, although these high concentrations of the fission products were neglected in the analysis. Furthermore, Transport behavior of the fission products through the gas gapmore » from the fuel element to the graphite tube containing the elements was studied by measuring the concentration profiles in the tube. It was concluded that ruthenium transport occurred by direct fission recoil from the surface uranium contamination, whereas that of cesium, by desorption from the surface.« less

Fission Product Release from Coated Particles Embedded in Spherical Fuel Elements for High-Temperature Reactors

The results of irradiation experiments on spherical fuel elements in the Studsvik R2 and the Juelich FR12 reactors are used to derive release data for both gaseous and metallic fission products. Fission gas release follows the well-known dependence on the square root of half-life showing that diffusion is the controlling release process for both intact and failed particles. The release of /sup 110m/Ag was generally about two orders of magnitude higher than that of /sup 137/Cs. Profile measurements indicate good retention of strontium. Results of the concentration profile and in-pile release measurements allow verification of data from out-of-pile experiments.