Reactor Fuel Examination Facility (RFEF)

Fig. 11-4 Measuring unit of ultramicro hardness tester in lead cell

Fig. 11-5 Indentations in zirconium liner by UHT (10 micrometer intervals)

Fig. 11-6 Measuring principle of thermal diffusivity by laser flash method Thermal diffusivity is evaluated from the change of temperature at the back surface of specimen after a laser beam exposure on the front surface.

This picture, 43KB

Fig. 11-7 Thermal diffusivity of Halden fuel pellet (63 GWd/t) The thermal diffusivity of irradiated pellets is lower than unirradiated ones below 1260K.

This picture, 69KB

Fig. 11-8 Fuel removal equipment Initially a 6 mm diameter hole was drilled at the center of the pellet-stack. This was followed by drilling a hole 7 mm diameter. A thin annulus of the fuel pellet remained inside the fuel cladding. The annulus was removed by a pellet removal die.

Japanese high-reliability fuel used in light water reactor (LWR) nuclear power plants has been developed by the efforts of the parties concerned. Fuel development for LWRs is directed toward higher burnup to effectively utilize resources, to reduce the cost of the nuclear fuel cycle, and to reduce the quantity of spent fuel generated. The significant factors that affect the reliability of high-burnup fuels are the degradation of thermal conductivity of the uranium dioxide pellets, the fission product (FP) gas release from fuel pellets, and the waterside corrosion of fuel cladding by the cooling water. The FP gas pressure in the fuel rod increases with the generation of FPs caused by the nuclear fission of the uranium. In addition, the release of the FP gas is promoted by the rim effect. High-reliability high-burnup fuels are produced on the basis of advanced techniques that address problems. Post irradiation examinations (PIEs) are carried out to confirm the performance expected from the design and to obtain a knowledge for the development of fuels with higher performance. The Reactor Fuel Examination Facility (RFEF) in the Japan Atomic Energy Research Institute (JAERI), has developed PIE equipment such as an ultramicro hardness tester (UHT) and a pellet thermal diffusivity measuring apparatus (PTD). The Science and Technology Agency (STA) has funded the development of equipment for PIEs of high-burnup LWR fuels since 1990. A puncture test apparatus and fuel removal equipment were also developed. Valuable data on irradiation behavior of the high-burnup fuel are provided by this PIE equipment. The UHT was developed to examine the change of mechanical properties in irradiated fuel cladding as a function of neutron fluence. The measuring unit of the UHT is shown in Fig. 11-4. The indenter of the UHT is pressed into the cross sections of fuel cladding (the thicknesses of fuel cladding and zirconium liner are about 0.6 and 0.07 mm respectively) at 10 micrometer intervals. This measurement can be performed in load ranges of 0.1 to 200 g. The hardness change in the zirconium liner was examined as shown in Fig. 11-5. This confirmed that the zirconium liner was effective for relaxation of Pellet-Cladding Interaction (PCI). The operating principle of the PTD is shown in Fig. 11-6. This apparatus can measure the thermal diffusivity of irradiated fuel pellets to high temperature (1800 degrees cent.). The thermal diffusivity data in fuel pellets irradiated to 63 GWd/t in the Halden boiling water reactor are depicted in Fig. 11-7. These data have attracted the worldwide attention. The puncture test apparatus is used to measure FP gas pressure, etc. The void volume in this system was reduced to about 30% of conventional systems, the measurement accuracy was improved, and the measurement time was decreased to less than 1/3. The fuel removal equipment, shown in Fig. 11-8, is capable of removing pellets, that have adhered to fuel cladding by PCI. This is done without any alteration of the cladding. A test specimen of the high-burnup fuel cladding is obtained by this method and material strength tests are successfully carried out. This equipment was selected as a noteworthy invention by the STA. It also has received an international patent.

Reference T. Kodaira et al., Present Status of PIE Techniques in Tokai Hot Cell Facilities, Proceedings, The 5th Asian Symposium on Research Reactors, May 29-31, 1996, Taejon, Korea (1996).

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