PANAMA : ein Rechenprogramm zur Vorhersage des Partikelbruchanteils von TRISO-Partikeln unter Störfallbedingungen

The computer code PANAMA and its underlying modeling assumptions are presented. The models are based on independent measurements of the properties of TRISO particles with a SiC interlayer. Essential features are the calculation of internal gas pressure, of coating strength and its decrease during irradiation and its weakening due to fission product interaction during accidents. At very high temperatures, particle life is determined by SiC thermal decomposition. Good comparison is obtained in the temperature range 1600 - 2500 ° C when applying PANAMA to a wide variation of existing accident simulation experiments with spherical fuel elements. At lower temperatures, PANAMA tends to be over-conservative. Predictions of particle failure during the depressurized accident sequence with the worst temperatures of the 200 MW$_{th}$ , side-by-side Modular Reactor System remain below the level of normal operations. The same holds true for the HTR-500 MW$_{e}$ accident sequence with the system under pressure. In the depressurized case, however, failure of all particles has to be expected after approximately 100 hours in the least favourable core position

Fission product release profiles from spherical HTR fuel elements at accident temperatures

With the construction of the cold finger apparatus, a new method has been developed to determine fission product release profiles during heating tests of irradiated spherical fuel elements. It is shown that this equipment works with high sensitivity and great precision for all important fission product nuclides up to 1800 °C.Together with the existing equipment, a total of 22 fuel elements with modern TRISO particles has been tested in the temperature range of 1500 - 2500 °C. In addition, experiments were done on seven UO$_{2}$ samples at 1400 to 1800 °C. For heating times up to 100 hours at the maximum temperature, the followingresults were obtained: silver is the only fission product to be released at 1200 - 1600 °C by diffusion through intact SiC, but is of low significance in accident scenarios; caesium, iodine, strontium and noble gas releases up to 1600 °C are solely due to various forms of contamination. At 1700 - 1800 °C, corrosion-induced SiC defects cause the release of Cs, Sr, I/Xe/Kr. Above 2000 °C, thermal decomposition of the silicon carbide layer sets in, while pyrocarbons still remain intact. Around 1600 °C, the accident specific contribution of caesium, strontium, iodine and noble gas release is negligible. This report is a translation of Jül-2091 published October 1986 in German