Fig. 7-3 Thermal arrest (peak) indicating the melting point of an alumina specimen The peak was generated by the release of latent heat in the liquid to solid transition during the cooling process.

Fig. 7-4 Calibration curve of temperature obtained using three standard ceramic specimens A more accurate melting point for fuel pellets can be estimated using this calibration curve.

In Japan, the reliability of current Light Water Reactor (LWR) fuel is well recognized, and the fuel is safely utilized in the present nuclear power plants. Today main focus of our efforts is concentrated on the development of higher burnup fuel to improve fuel economy. To serve this purpose, we are carrying out post-irradiation examinations of newly-designed fuel after irradiation in LWR. We have developed and constructed various test apparatuses and have been collecting valuable experimental data. Some new results obtained with one of the apparatuses are described on the melting point drop of the fuel during irradiation. The pellet specimen is enclosed in a tungsten capsule and heated to about 3,000 degrees cent. by a high-frequency furnace. Temperatures at the bottom face of the capsule are continuously measured by a radiation thermometer. The melting point of the specimen can be estimated by the thermal arrest which occurs due to the latent heat of melting. An example of thermal arrest in an alumina specimen is shown in Fig. 7-3, where the thermal arrest peak due to the release of latent heat can be seen in the cooling stage. In addition, we have obtained a calibration curve for temperature, as shown in Fig. 7-4, using three-standard ceramic materials: HfO2, Y2O3, and Al2O3. Little data exists on the drop in the melting point of LWR fuel during irradiation. So data acquired by the use of this test apparatus is extremely valuable for evaluating the reliability of high-burnup LWR fuel.

Reference T. Yamahara et al., Development of Post-irradiation Examination at the Reactor Fuel Examination Facility, Proc. Tech. Comm. Meet., Oct. 17, 1994, Cadarache, France, IAEA-TECDOC-822 (1995).

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