3.3 Microscopic Observation of Defective Particles Is a Key to Success
-Improvements in the Quality of Coated Fuel Particles for High-Temperature Gas-Cooled Reactors-

Fig. 3-5 SiC-defective fuel particle (ceramograph of TRISO-coated particle)
The appearance of a SiC layer failed due to a partially carbonized fuel kernel is shown. CO gas synthesized by the reaction of a carbothermic reduction on the surface of the fuel kernel is released through a crack of the inner PyC (pyrolytic carbon) layer. The resulting CO gas obstructs the formation of the SiC layer in the coating process and produces a SiC-defective fuel particle.

Fig. 3-6 Improvements in the quality of fuel compacts by the modification of the manufacturing processes
The result of improvements in the quality of fuels is clarified by the relation between the SiC-defective fraction and the number (N) of the SiC-defective particles in the fuel compact such as N=1, 2, 5 and 10, at the improved steps. For example, one particle is defective in one fuel compact containing 13,500 particles, when N=1. This means that the defective fraction is 7.4 *10-5. Although the number of SiC-defective particles is about ten on the average using ordinary technology, the number decreases to two or three with the improved coating process. Finally, the average number greatly decreases to less than unity using both improved coating and compaction processes.

In the high-temperature engineering test reactor (HTTR, a 30-MW (thermal) high-temperature gas cooled reactor which has been constructed by JAERI, and which went critical on November 10, 1998), about 109 coated fuel particles (about 1 mm in diameter) are loaded. It is essential that the coated layers of these fuel particles are sound in order to confine fission products. To ensure the safety of the reactor and to reduce the cost of fuel manufacturing, the improvement in quality and the decrease of the defective fraction of fuel particles are indispensable.
Aiming at high-quality products, defective particles in about a million manufactured fuel particles have been selected and thoroughly examined to determine the causes of the coating failures. The selected particles have been classified according to the morphology of defective coated layers and studied with respect to the mechanisms of the coating failure. The appearance of a SiC-defective layer which is one of six typical failures, is shown in Fig. 3-5. A study to search for the optimum conditions in both the coating and compaction processes of the fabrication was performed. Thus, it has been found that the defective fraction of the SiC coated layer is decreased greatly by the following improvements; (1) optimizing the mode of the particles fluidization, (2) developing the process without unloading and loading the particles at intermediate coating steps, and (3) optimizing the combinations of pressing temperature and speed of the over-coated particles. These results are summarized in Fig. 3-6.
The quality of Japanese coated fuel particles is greatly improved and is nowadays superior to that of those of Germany and the United States, countries which have taken a leading part in coated fuel particles manufacturing. At present only Japan possesses this technology, as both the other countries have stopped manufacturing coated fuel particles.

Reference
K. Minato et al., Improvements in Quality of As-Manufactured Fuels for High-Temperature Gas-Cooled Reactors, J. Nucl. Sci. Technol., 34 (3), 325 (1997).

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Persistent Quest-Research Activities 1998
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