Fig. 5-1 Failure probability dependence on burnup The probability of through-coating failure of fuel particles with defects in the SiC layer caused in fabrication, as well as those without defects, is shown as a function of burnup. In the case of the intact fuel particles, no failure occurs up to 33 GWd/t of burnup, which is maximum value for the HTTR fuel. By monitoring radioactivity in the cooling gas of the reactor and comparison it with the concentration of fission-product gas as determined from the failure probability, an abnormal rate of fuel particles during operation.

Fig. 5-2 Development of the advanced high-burnup fuel High-burnup fuel is designed with a thickened porous buffer layer and the SiC layer, thus the lifetime of fuel due to through-coating failure is increased about 1.8 times that of first-loading-fuel. This results from strengthening of the SiC layer and restraint of internal pressure owing to an increase in the volume accommodating fission product gasses in the buffer layer.

The High Temperature Test Research Reactor (HTTR) is being constructed by the Japan Atomic Energy Research Institute (JAERI), with its first criticality scheduled for June, 1998. A variety of research and development activities at JAERI have focused on the fuel. Tri-isotropic (TRISO) coatings on the HTTR fuel have function as cladding on light-water reactor fuel. TRISO coatings consist of a low-density carbon buffer adjacent to the spherical fuel kernel, an isotropic PyC layer (inner PyC), a silicon carbide (SiC) layer and a final PyC (outer PyC) layer, as shown in the photograph. The HTTR is charged with 900 million coated-fuel particles, the estimating the probability of coating failure under high-temperature irradiation is a matter of primary concern for fuel safety. Conventional models have been developed to calculate stress in the SiC layer based upon classic elasticity theory. In these models the fission product gas is released from the fuel kernel when the SiC coating layer has failed, even if either the inner or outer PyC layers are intact. The results obtained by these models, however, do not adequately represent the results obtained from irradiation tests. Therefore, a new failure model has been developed based upon the fact that the failure fraction of the coated particle depends not only on the failure of the SiC layer, but also on the failures of both the PyC layers, as illustrated in the Fig. 5-2. In this newly developed model the fission product gas is released from the particle when all layers failed, but no release occurs when only the SiC layer failed. By using this new model, the failure probabilities and the lifetime of the HTTR's first-loading fuel and advanced high-burnup fuel can be more precisely and practically estimated. Furthermore, useful information for the design of high burnup fuel may also be obtained.

Reference K. Sawa et al., Development of a Coated Fuel Particle under High Burnup Irradiation, J. Nucl. Sci. Technol., 33 (9), 712 (1996).

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