HBK project high temperature reactor fuel cycle. Retention of solid fission products in oxide HTR fuel

Low- and medium enriched UO₂ HTR fuel has been assessed for irradiation performance and retention of solid fission products. Work concentrated on cesium and silver and results were compared to those obtained with high-enriched (Th,U)O₂ fuel particle kernels for the German THTR. Evaluation is presented of the Dragon rig test HTR 3 (in the OSIRIS reactor, Saclay) and of the KFA DR-K4 experiment, which was a directly cooled test with 25 stacked spherical fuel elements in the Dragon reactor. The measurement technique involves mechanical separation of fueled kernel and coatings. Thereby one particle each is inserted into a glass capillary tube and held between two steel pistons, which are carefully and slowly driven towards each other by a micrometer screw. After cracking the particle the kernel and coating fragments can easily be recovered and are separately r-counted. By this method the release from kernels can be established. Results, combined with actual irradiation time allow to deduce an effective 'reduced diffusion coefficient in terms of the equivalent sphere model.