The CRP-6 benchmark on HTGR fuel behavior under accident conditions

National engagement as well as bilateral or multi-national cooperation in HTGR fuel development is ongoing and is expected to further improve fuel performance and the ability to make reliable predictions. The accident condition benchmark exercise, one of the key elements within the sixth IAEA-directed Coordinated Research Project (CRP) on 'Advances in HTGR Fuel Technology Development', has successfully demonstrated to be a useful basis for verification and validation in establishing the reliability of code predictions. Participants in the accident condition benchmark included France, Germany, Russia, South Africa, Korea, and the United States applying a total of eight models to all or a part of the 24 proposed benchmark cases. The benchmark consisted of three parts, a sensitivity study to examine fission product release from a fuel particle, the postcalculation of well documented irradiation and heating experiments, and finally some predictive calculations. In the sensitivity study, most codes have shown good agreement among each other. Differences can be explained by different assumptions for input data or boundary conditions. In comparison with the numerical procedure of the diffusion calculation for the kernel, the application of the analytical solution offered by the Booth model appears to be more accurate method. Time step length may also influence the calculational results. From the postcalculations of heating tests, it appears that the diffusion coefficient for cesium in silicon carbide is still varying over a broad range. In particular, strontium release data are obviously largely overpredicted and should undergo a thorough review. Silver release measurement results are often unexpected and inconsistent, and therefore extremely difficult for postcalculation. One of the most recent heating experiments, HFR-K6/3, has shown surprisingly low krypton and cesium release values, which are largely overpredicted by the model calculations. This extremely good accident condition performance of the very latest manufacture of German TRISO fuels should be the starting point of further fuel studies. Additional irradiation and postirradiation examination will be required for newly manufactured fuel, in order to improve the existing statistical data base for fuel performance analysis. (author)