Fabrication experience and integrity confirmation tests of the first-loading-fuel of the HTTR

The first-loading core of the High Temperature Engineering Test Reactor (HTTR) consists of 150 fuel assemblies. An HTTR fuel assembly is so-called a pin-in-block type of hexagonal graphite block. A fuel rod consists of a graphite sleeve and of 14 fuel compacts. In a fuel compact, about 13,000 TRISO coated fuel panicles are dispersed densely. The coated fuel panicle is TRISO (Tri-isotropic) type with four coating layers. The fuel kernel is low-enriched (average 6wt%) UO2. The fabrication of the first-loading fuel started from June 1995. A total of 4,770 fuel rods were successfully produced and transferred to the reactor building of the HTTR. Finally, in the reactor building, the fuel rods were inserted to the graphite blocks to form fuel assemblies. On December 1997, 150 fuel assemblies were completely formed and were stored in new fuel storage cells. The cells were filled with helium gas to keep the fuel blocks in dry condition. Fabrication technology of the HTTR fuel was established through a lot of R and D activities and fabrication experiences of irradiation examination samples spread over about 30 years. High quality and production efficiency of fuel were achieved by the development of the fuel kernel process using the vibration dropping technology, the continuous 4-layer coating process and optimization of the compaction conditions. In the safety design of HTGR fuel, it is important to retain fission products within the coated fuel panicles so that their release to the primary coolant may not exceed an acceptable level. From this point of view, as-fabricated failure fraction is important. In the specification, SiC-failure and exposed uranium fractions were determined to be less than 1.5x10-3 and l.5x10-4, respectively. The quality of the first loading fuel fully satisfied the design specifications for the fuel. The fuel compacts contained almost no through-coatings failed particles and few SiC-defective particles. Average through-coatings and SiC defective fractions were 2x10-6 and 8x10-5 respectively. In parallel with the fabrication of the first-loading fuel, JAERI carried out integrity confirmation tests of the fuel. The tests were (1) independent as-fabricated SiC defective fraction measurement, (2) acceleration irradiation test and (3) high-temperature heat up test of irradiated fuel compact. Through the tests, integrity of the first-loading fuel of the HTTR was finally confirmed. (author)