Study at TCU

While the permittivity and conductivity of poly-L-lactic acid (PLLA) and polyethylene terephthalate succinate (PETS) are comparable to low-density polyethylene (LDPE), those of polycaprolactone butylene succinate (PCL-BS) and polybutylene succinate (PBS) are much higher. This is because PLLA and PETS are in the glass state at room temperature, while PCL-BS, PBS, and polybutylene succinate adipate (PBSA) are in the rubber state. All the polymers tested have almost similar impulse breakdown strength at room temperature. As for dc or ac breakdown strength, PLLA and PETS show a relatively higher strength than PCL-BS and PBS.

With an aim to develop a good coil winding insulation film, fillers of boehmite alumina in the shape of a roughly rectangular plate were added with ratios of 10 and 20 wt% to polyimide. It was found that samples with fillers treated using the methacrylic silane coupling agent have the strongest adhesion at the filler/polyimide interfaces and the lowest dielectric loss factor at high temperatures. A positive relationship between the filler/polyimide adhesion and the thermal conductivity is also indicated.

The spatial resolution of a cable fault location method by frequency domain reflectometry was examined experimentally by giving two abnormalities intentionally along a 31.5-m coaxial cable. As a result, two clear signal peaks appear, which can locate the presence of two hot spots perfectly if they are separated by more than 30 cm at a position about 15 m from the cable end with a measuring device. In addition, a clear signal peak with a small shoulder is recognized if the two hot spots are 1 cm away.

Various kinds of polymers are used for electrical insulation in nuclear power plants. They are subjected to thermal and radioactive stresses, depending on their locations. They are also subjected to high-temperature steam in the event of an accident. Regarding this, sheets of flame-retardant crosslinked polyethylene (FR-XLPE) were subjected to heat, gamma-rays, and steam exposure, and their infrared absorption spectra, stress-strain relations, indenter moduli, and permittivity spectra were measured. The low dose rate at the high temperature enhances the diffusion of oxygen into the sample. Oxidation and cross-linking proceed at the high dose rate. Such oxidation and cross-linking make FR-XLPE hardened by the formation of three-dimensional structures.

Some safety-related low-voltage cables used in nuclear power plants are required to maintain the insulation performance even during severe accidents. It is not clear whether oxygen affects the insulation performance of the cable in the steam exposure during a severe accident. Flame-retardant ethylene propylene diene rubber insulation cables were used as safety-related cables. When oxygen is present in water vapor, the oxygen concentration decreases during the exposure since oxygen is consumed by the oxidation of insulators and other materials. Whether or not the cable was subjected to aging does not alter the insulation resistance of the cable significantly under the assumed severe accident condition.