Hiroya Homma

Mechanical aging trend in ethylene propylene rubber-insulated safety cables sampled from bwr nuclear power containment

Statistical analysis has been performed on variously aged ethylene propylene rubber insulation on cables removed from boiling water reactor-type containment. Temperature and dose rate conditions of such cables are not identical. Therefore, elongation at break data observed for such insulation are converted to each equivalent period under 60 °C, 10 mGy/h condition. The aging trend is found to be approximately slower by half than that expected from accelerated aging test results. Furthermore, this aging trend is used as a reference to fit to each elongation value, and the fitted curve is extrapolated to find the end of service life, which is defined as 70 % of absolute elongation. Lifetimes estimated from this method and from predictions by the accelerated aging test are compared on an Arrhenius plot. It is suggested that the latter lifetime estimation is shorter than actual. The temperature dependence also suggests a difference in a basic chemical reaction under the two aging conditions, which results in apparent differences in activation energies and pre-exponential factors.

Study on surface degradation of polymer insulating materials caused by leakage current

Some artificial aging tests were performed using test rods made of Silicone Rubber (SR) and Ethylene Propylene Rubber (EPDM), based on the Rotating Wheel Dip Test (RWDT) method and the Salt Fog Chamber Test (SFCT) method, and aspects of the surface degradation were evaluated. Following the tests, it was concluded that the surface degradation of polymer insulating materials is directly related to the partial discharge and that measurement of the cumulative charge of the leakage current is useful in evaluating the degradation. Moreover, an energized exposure test using prototype composite insulators was carried out for five years. The accelerating factor from the artificial aging test for actual use was estimated by comparing the amounts of cumulative charge in the artificial aging test and in the exposure test.

A fundamental study on the surface degradation of polymer insulation materials in DC voltage application

Polymer insulator has the advantage of light weight, high strength and high performance on antipollution. At the present, polymer insulator is being tested on AC transmission lines and a few DC transmission lines in Japan. This paper reports on the surface degradation and characteristics of leakage current on silicone rubber (SR) under DC(+) and AC artificial aging test (the Rotating Wheel Dip Test; RWDT), and leakage current characteristics of polymer phase-to-phase spacers subjected to DC(+) and AC coastal exposure tests.

Evaluation on flashover voltage property of snow accreted insulators for overhead transmission lines, part I - field observations and laboratory tests to evaluate snow accretion properties

This paper presents evaluation of snow accretion properties of long-rod and cap & pin insulators focusing on the phenomena found in the snowstorm in 2005, which related to wet snow accretion packed with sea-salt. Field observations at severe natural environment and laboratory tests under defined conditions were parallelly carried out in order to clarify wet snow accretion process under various conditions. In field observations, it was confirmed that snow easily accreted on insulators at temperature range especially between 0 and +1.0°C. The long-rod insulators sheds were bridged more easily than that of cap & pin type because of its narrower shed spacing. In laboratory tests, cylindrical snow accretion, which was found in the snowstorm in 2005, was achieved. The mechanisms on explaining snow accretion on long-rod insulators were found to be similar to those confirmed for wires. The laboratory simulations agree well with the field observations. In order to estimate the amount of snow flux on insulators caused by a snow event for given climatic conditions, an index called the snow accretion potential ability (SAP) was proposed. SAP is defined by using the snow flux and associated with the weight term in the index for the snow stress product (SSP). SAP was adopted to evaluate snow accretion characteristics by field observations and laboratory tests, and the snow accretion phenomenon during a snowstorm in 2005 was discussed.

Evaluation of flashover voltage property of snow accreted insulators for overhead transmission lines, part III - 154 kv full-scale flashover voltage test of snow accreted insulators -

In December 2005, Japan experienced a major outage in Niigata Kaetsu area due to a large amount of wet snow mixed with sea-salt accreted on several transmission line insulators. To clarify the causes of the snow-induced outage and increase reliability of the networks, a 154 kV class full-scale snow test procedure to evaluate various insulator designs was developed, and artificial flashover voltage tests of snow accreted insulators were carried out. High voltage flashover tests showed that the flashover voltage of both long-rod and cap & pin insulators was decreased with the increase of snow conductivity. Also, cap & pin insulators showed significantly higher flashover voltage than long-rod insulators. Thus substitution of long-rod insulators with cap & pin insulators appears to be reasonable as a countermeasure against snow induced flashovers.

Relationship between remaining antioxidant content and radiation-thermal degradation in crosslinked polyethylene

To clarify the deterioration mechanism of cables installed in nuclear power plants, three kinds of XLPE sheets with various antioxidant contents were aged by 100 Gy/h γ-ray irradiation at room temperature, 60, and 100°C. The remaining antioxidant content estimated from an oxidation induction time of a differential calorimetric thermogram as well as the elongation at break decreases faster as the initial antioxidant content decreases. On the other hand, an evaluation parameter called oxidation degree, which is the absorbance ratio of the carbonyl group to the methylene group obtained by infrared spectroscopy, increases with aging time. The oxidation degree increases with an increase in the elongation at break, and the relationship of these two parameters can be plotted around a sigmoid curve. Furthermore, the elongation at break and oxidation degree show reasonably stable transitions until the antioxidant content decreases to a certain value. After such a period, the elongation at break starts to change drastically and becomes sufficiently low value. It is clarified from these experimental results that there exists a certain antioxidant content that effectively inhibits oxidation.

Mechanism elucidation of degradation dynamics of ethylene propylene rubber cable insulations sampled from nuclear power containment using additional aging

Ethylene propylene rubber insulation under operation for 16 years in boiling water reactor containments was subjected to additional thermal aging at 110°C. After 70 days of aging, the mechanical property of the insulation was decreased such that the limit guaranteeing durability against design-basis events was breached. This degradation rate is three times slower than that predicted on the basis of an accelerated aging test with control samples. Infrared spectroscopic analysis revealed suppression in the formation of carbonyl products in the service-used cables. According to the gel fraction measurement, cross-linking is the dominant reaction in such service cables. The unique slow degradation dynamics observed in service cables could be attributed to such oxidation control and the cross-linking reaction. The present study also conducts nuclear magnetic resonance analysis to elucidate the cross-linking point in the main chain.

Slow degradation dynamics under additional aging in EPR insulations sampled from BWR containment

Ethylene propylene rubber insulations under operation for 16 years in boiling water reactor containments were subjected to additional thermal aging at 110°C. 70 days were needed to decrease their mechanical property to the limit that they cannot secure the durability against design-basis events. This degradation rate is found to be three times slower than that predicted on the basis of an accelerated aging test in control samples. Infrared spectroscopic analysis reveals suppression in the formation of carbonyl products in the service-used cables. According to the gel fraction measurement, cross-linking is the prominent reaction in such service cables. The unique slow degradation dynamics observed in service cables could be attributed to such oxidation control and cross-linking reaction.

Statistical analysis of aging trends in EPR-insulated safety cables sampled from BWR containments

Statistical analysis was performed on aging trends of ethylene-propylene rubbers used for safety cables within boiling water reactor (BWR) containments of nuclear power plants. In order to obtain a regressed aging trend curve, observed mechanical elongation at break data was converted for equivalent period in an environment of 60°C and 10 mGy/h. Dropping speed is found to be approximately two times slower in service-used cables than that predicted by an accelerated aging test. This newly found aging trend was used as a reference to fit to each elongation value, and the fitted curve is extrapolated to find available service durations. An Arrhenius plot shows that the duration predicted from the acceleration test is shorter than the actual duration. Activation energy is lower and the pre-exponential factor in the Arrhenius plot is higher in newly estimated service durations. Low-oxygen-concentration environment in BWR containments could be a contributor to such behavior.

Applicability of positron annihilation spectroscopy to evaluation of polymer materials

Free volumes of six types of polyethylene with different densities and one type of cross-linked polyethylene were evaluated by the positron annihilation technique, and relationships between free volumes and material characteristics such as material density and mechanical characteristics were investigated. As a result, it was found that the estimated free volume radius and its number density decreased with an increase in the material density, and the free volume radius was estimated to be about 0.3nm. We proposed a parameter related to both the volume and number density of free volumes and referred to it as the relative free volume. The relative free volume decreased with an increase in the material density and increased with a decrease in the yield and tensile strength.