Study on coated layer material performance of coated particle fuel FBR (1)

Design studies of 'Helium Gas Cooled Coated Particle Fuel FBR' are being conducted since it has the following superior features, and it is considered to be one of the most promising concepts in Feasibility Study on Commercialized Fast Reactor Cycle System in Japan (F/S). 1. Helium gas as coolant is applicable to a high temperature direct gas turbine cycle with high thermal efficiency, 2. Large heat capacity and large Doppler coefficient induced by coated layer material may achieve the attractive core safety characteristic such as so-called 'melt down proof' at the sever accident ('Depressurization accident + Without scram + Natural circulation'). As for the conventional design of coated particle fuel, TRISO fuel with SiC as main layer and PyC as buffer and surface protect layers has expressed a lot of excellent achievements in High Temperature Gas Reactor studies. However, it is impossible to directly use this fuel design in FBR, because high burn-up (∼150GWd/t) aimed for economics demands higher strength, and high fast neutron fluence makes it difficult to adopt PyC as surface protect layer due to its dimensional instability in fast neutron spectrum. Therefore, materials with higher strength for main layer and alternative material for surface protect layer are necessary. Nevertheless, at present situation usable data of strength of coat ceramics besides SiC is scarce, and refractory metals as possible candidates of surface protect layer have little experience in fabrication. Based on these backgrounds, we have investigated characteristics of potential candidate coat materials (for main layer and surface protect layer) to understand the possibility of coating applications to the above described design concept in F/S, have selected some potential candidates, and have conducted tests of strength properties for main layer materials and tests of vapor deposition properties for surface protect layer materials. The results of these examinations come to the following conclusions. (1) As for TiN coat by the current coating technology, the Weibull modulus are gained to be 17.5 ∼ 22.3 (in conditions of thin layer and room temperature), and the strengths with 10-4 of coating layer failure reliability are evaluated to be 35.6 ∼ 100.1 kgf/mm2. These results reveal TiN has superior property in strength, and TiN is considered to be promising as main layer material. As for Si3N4 coat also, experimental results show capability to have superior property in strength. (2) Vapor deposition properties between W and main layer materials (TiN, SiC and Si3N4) are experimentally evaluated. These results reveal that the property by the conventional method is inferior, therefore, W is considered to require some additional studies to form a suitable coating layer. Contrary, Mo has superior vapor deposition properties on TiN, moreover the residual stress has tendency to be large in compression side. Therefore, Mo is considered to be promising as the surface protect layer material. (3) In the reference fuel design of F/S Phase I, if the maximum value of TiN coat strength evaluated in this study (100kgf/mm2) is applied for fuel integrity and optimization of thickness of buffer and main layer is conducted, the fuel volume fraction is evaluated to be increased up to 1.21 times of reference value. This reveals that the strength of TiN coat largely contribute to the core performance. (author)