Performance of HTR fuel samples under high irradiation and accident simulation conditions, with emphasis on test capsules HFR-P4 and SL-P1

The central aspect of the safety philosophy for a small HTR is the retention of fission products, particularly those of the iodine nuclides, in the fuel elements during operation and accidents. The HFR-P4 and SL-P1 experiments were intended to test the performance limits of the LEU TRISO particle during irradiation. Due to the low heavy metal contamination of modern fuel elements, fission gas or iodine release is dominated by the number of defective coated particles. For this reason, thedetermination of the number of damaged particles was the central objective of measuring the fission gas releases in the reactor and in the extensive post-irradiation examinations. Although the values for burnup, fast neutron fluence and irradiation temperature envisaged in an HTR-MODUL were clearly exceeded, it became apparent that of a total of 78,400 particles employed, only one was defective (i.e. not gastight) during irradiation. In which connection, the damage probably occurred before irradiation, at the end of pre-activation treatment. Two further incidences of particle damage probably occurred during shutdown of the HFR-P4 experiment. During the 300 h heating experiments at 1600°C and 1700° C, no particle defects occurred in seven compacts within the temperature/time conditions characteristic of a MODUL depressurization accident. In the case of the compacts with 14 % burnup at 1600°C and 10 or 11 % burnup at 1700°C, the release of fission gas started to rise after 47 to 135 hours to about 0.01 % at the end of the experiment, and in one case to 0.1 % as the consequence of one failed particle. Only in the 1800°C experiment did the Kr release rise continuously, but still only reached 0.01 % after 150 hours. During the heating experiments the fission products were released in sequence fission gases, iodine, strontium, caesium and, with the highest proportion, silver, which is of little significance for accident consequences. Due to their transport and deposition behaviour in the core, caesium and strontium release, is insignificant for MODUL accident impacts. After 300 hours at 1600°C, more than 99 % of the caesium is still in the coated particles. However, considerable release begins with increasing heating temperature amounting to about 50 % from the compact at 1800°C after 280 h. Strontium is more effectively retained in the fuel kernels and matrix graphite at 1600°C than caesium. At 1700 and 1800°C, the strontium release approaches that of caesium. All these tests, containing in total the equivalent number of particles of 5 fuel elements, show that - even after more severe irradiation conditions than those envisaged during HTR MODUL operation - no particle failures occured either during operation or during severe accident conditions