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[Supplementary explanation] High Temperature Gas-Cooled Reactor Fuel In the high temperature gas-cooled reactor, coated fuel particles (CFPs) are employed as fuel to permit high outlet coolant temperature. In the High Temperature Engineering Test Reactor (HTTR), Tri-isotropic (TRISO)-CFPs are employed as fuel. The TRISO coatings consist of a low density, porous pyrolytic carbon (PyC) buffer layer adjacent to the spherical fuel kernel, followed by an isotropic PyC layer (inner PyC; IPyC), a silicon carbide (SiC) layer and a final PyC (outer PyC; OPyC) layer as shown in Fig. 1. In safety design of the HTGR fuels, it is important to retain fission products within particles. High quality and production efficiency of fuel was established through a lot of R&D activities. In the fabrication of the HTTR fuel at Nuclear Fuel Industries, Ltd, the continuous four-layer coating technology was developed and applied in the coating process to produce high quality CFPs (Photo. 2). The fuel compacts are produced by warm-pressing of the CFPs with graphite powder. A fuel compact contains about 13,000 CFPs. The compaction process was also improved by optimizing the combination of the pressing temperature and the pressing speed to avoid the direct contact with neighboring particles in the fuel compact. Fourteen fuel compacts were encased in a long graphite sleeve, making up a fuel rod. A fuel element consists of fuel rods and a prismatic graphite block. The HTTR core is composed of 150 of fuel elements (Fig. 1). The release rate-to-birth rate ratio (R/B) is an important measure of the performance of the CFPs. A sample of the primary coolant gas is obtained in a bottle using the grab sample apparatus shown in Photo. 1. The sample is obtained by first evacuating the bottle and grab sample manifold, and then backfilling the system with the primary coolant gas. The type and activity of radioactive fission gas in the grab sample was measured using γ-spectroscopy. In the HTTR, Krypton-88 (half-life is approx. 2.8 hours) is selected as reference nuclide to evaluate fuel performance. Figure 2 shows measured results of (R/B)s of Kr-88 during 30 days continuous operation of the HTTR with 850 °C of reactor outlet coolant temperature. The (R/B)s in the HTTR are 10 to 1,000 orders lower than those of the Fort St. Vrain reactor (FSV) in the US (842MW of thermal power with about 780°C of outlet coolant temperature) and the AVR reactor in Germany (46MW of thermal power with about 950 °C of reactor coolant temperature). These results mean that the fuel quality in the HTTR is the highest. The (R/B)s of the FSV were about 100 times higher than the HTTR, although its outlet coolant temperature was lower. The fuel temperature of the HTTR and the AVR is evaluated to be almost the same although the coolant temperature of the AVR was about 950 °C. The (R/B)s of the AVR were over 10 times higher than the HTTR reflecting fission product retention performance. - BACK - |
