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Fig. 11-1 Instrumentation procedures to install thermocouple into irradiated fuel rod After cutting the cladding of an irradiated fuel rod, the fuel pellets are fixed by solid CO2. The drilling work and the removing work are repeated until the depth of the center hole has the prescribed value (50 mm).

Fig. 11-2 Drilling machine installed in hot cell

Fig. 11-3 Photograph of butt welding work of instrumentation device to irradiated fuel rod

The extension of burnup of LWR fuel is currently an important issue from an economical point of view. Information on fission product (FP) gas release and the center line temperature of fuel pellets during power transients is important for the study of the pellet clad interaction (PCI) mechanism of LWR fuel rods. To study the irradiation behavior of high-burnup LWR fuel, an FP gas pressure gage and a thermocouple for centerline temperature measurement of a fuel rod being irradiated in an LWR have been developed in the Department of Japan Materials Testing Reactor (JMTR). The procedures to install a thermocouple in an irradiated fuel rod are shown in Fig. 11-1. The most difficult technique was drilling a hole along the center line of cracked pellets to insert a thermocouple. This was accomplished by immobilizing the pellets with solid carbon dioxide (CO2) during the drilling work, which was done with diamond drills. Figure 11-2 shows a photograph of the drilling machine installed in a hot cell. Holes 2.5 mm in diameter and 50 mm deep can easily be drilled on the center line of irradiated fuel pellets in this machine. Figure 11-3 shows a photograph of the butt welding of the instrumentation device to the irradiated fuel rod. This technology will allow comprehensive experiments concerning the irradiation behavior of high-burnup LWR fuel to be carried out.

Reference M. Shimizu et al., Development of Thermocouple Re-instrumentation Technique for Irradiated Fuel Rod, JAERI-Tech 95-037 (1995).

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