To evaluate the performance of nuclear fuel, it is necessary to measure the fission gas release, fuel temperature, fuel stack elongation, cladding elongation, fuel rod inner pressure, coolant temperature, and neutron flux during irradiation tests. Linear variable differential transformer (LVDT) is applied to the in-pile instruments for measuring the fuel rod inner pressure. In this study, design modification of LVDT was carried out to measure the fuel elongation and cladding elongation, without changing the structural soundness of the conventional nuclear fuel test rig used in HANARO. The magnetic core was directly connected to the fuel stack or cladding using a supporter, and it moved through the guide rail of LVDT sensor according to the deformation of fuel pellets or cladding. The performance verification and data calibration of the modified LVDT were verified by the deformation simulation system, where displacement was induced using a micrometer.

When a new nuclear fuel is developed, an irradiation test needs to be carried out in the research reactor to analyze the performance of the new nuclear fuel. In order to check the performance of a nuclear fuel during the irradiation test in the test loop of a research reactor, sensors need to be attached in and out of the fuel rod and connect them with instrumentation cables to the measuring device located outside of the reactor pool. In particular, to check the temporary temperature change at the center of a nuclear fuel during the irradiation test, a thermocouple should be instrumented at the center of the fuel rod. Therefore, a hole needs to be made at the center of fuel pellet to put in the thermocouple. However, because the hardness and the density of a sintered UO<SUB>2</SUB> pellet are very high, it is difficult to make a small fine hole on a sintered UO2 pellet using a simple drilling machine even though we use a diamond drill bit made by electro deposition. In this study, an automated drilling mach ne using a CVD diamond drill has been developed to make a fine hole in a fuel pellet without changing tools or breakage of workpiece. A sintered alumina (Al<SUB>2</SUB>O<SUB>3</SUB>) block which has a higher hardness than a sintered UO<SUB>2</SUB> pellet is used as a test specimen. Then, it is verified that a precise hole can be drilled off without breakage of the drill bit in a short time.

To develop a new nuclear fuel, it needs to make a test fuel rod and carry out burn-up test in the test loop of a research reactor to check the irradiation characteristics of the nuclear fuel. At that time, several sensors such as thermocouples, LVDTs and SPNDs are needed to be attached in and out of the fuel rod and connect them with instrumentation cables. Then, the instrumentation cables deliver the signals measured by the sensors to the measuring device located outside of the reactor pool. In particular, to install a thermocouple in a fuel rod, it needs to drill off holes on the alumina blocks and sintered UO2 pellets. However, because the hardness of a sintered UO2 pellet is 700 Hv (or HRC 61) and that of an alumina block is 1480 Hv, a special drilling machine which adapts a diamond coated drill bit had developed. In this study, several case experiments have been carried out to find an optimal drilling condition of the drilling machine. And, using the optimal drilling condition, minimum numbers of the holes that a drill bit can drill off are verified.