Yoshiaki Nakase

Effect of Dose Rate on Degradation Behavior of Insulating Polymer Miaterials

The dose rate effect on the mechanical and electrical properties of representative insulating and jacket materials has been studied. Crosslinked polyethylene (XLPE),thylene ethylene-propylene terpolymer (EPDM), and some chlorine containing polymeric materials are irradiated in air at several dose rates. XLPE degrades significantly at lower dose rates, but EPDM shows opposite dose rate dependency to XLPE. Dose rate effect is interpreted as a competition between the dissolution and diffusion of oxygen into the sample and the oxidation reaction by irradiation. Ingredient compounding in chlorine containing polymeric materials may depress the dose rate effect.

Radiation Resistance of Cable Insulating Materials for Nuclear Power Generating Stations

The radiation resistance of sixteen types of representative rubber and plastic materials for cable insulation was studied. The endurance of some crosslinked polymers under the combined environment of thermal aging-radiation-steam (high temperature) was also examined in relation to the chlorine contents of the base polymer and additives (antioxidant, vulcanizers, etc.). Crosslinked polyethylene insulation and polyvinylchloride jacket cables were tested by the IEEE Std. -383 method. The results showed that those cables were not satisfactory under loss of coolant accident (LOCA) simulating conditions. The cables utilizing EP rubber insulation and special chloroprene jacket have been found to pass the IEEE-383 test.

Formation of layer-like voids in irradiated polytrioxane

The formation of voids and defects is investigated by X-ray diffraction methods, by weight loss measurement and by electron microscopy, for an irradiated polytrioxane (PTOX) obtained by theγ-ray-induced polymerization of trioxane in the solid state. The main reasons for the formation of the layer-like voids are transverse decomposition at a selective point accessible to the radiation in the micro-fibrils, and the subsequent extensible depolymerization which may occur from the unstable chain end with increase of the radiation dose. The analysis of the SAXS curve for the irradiated PTOX, according to Tsvankins method gives reasonable results for the void structure, where the reduction of the sub-crystal fraction and also the weight loss of the sample by irradiation are taken into account.

Radiation-Induced Solid-State Polymerization in Binary Systems. VI. Polymerization of Vinyl Compounds in the Supercooled Liquid Phase

Abstract The radiation-induced polymerization of glass-forming systems containing vinyl monomers was investigated. Irradiation below the secondorder transition temperature (Tg) of the systems causes no in-source polymerization but does cause a very rapid postpolymerization in the course of heating above Tg. Differential thermal analysis was carried out to estimate Tg and to follow the postpolymerization.

Effect of γ-irradiation on the melting behaviour of poly(trioxane)

Poly(trioxane) obtained by radiation-induced polymerization in the solid state, was subjected to 1 Mrad γ-rays and the change in melting behaviour and the thermogravimetric curve examined. Little structural change in the X-ray scattering patterns was observed. The heating curve of as-polymerized poly(trioxane) exhibited a double endothermic peak under a high polymer yield, but the endotherm on the higher temperature side disappeared at a heating rate lower than 8° C min−1. On irradiating over 25 krad, or rolling, the endotherm was again absent even at heating rates above 16° C min−1. The endotherm at higher temperatures is interpreted as being due to superheating caused by entropy restriction. The irradiation or rolling causes chain scission at a selective point in the crystallites. These scissions reduce the superheatability of a poly(trioxane) with a high polymer yield. The lamellar-type poly(oxymethylene) crystal was also studied for comparison.

Radiation Resistance of Ethylene-Propylene Terpolymer Vulcanized by Sulfur I. Basic Ingredient Compounding Material

The radiation resistance of ethylene-propyleneethylidene norbornene terpolymer (EPDM) vulcanized with sulfur was studied. Several mechanical properties of original EPDM demonstrate maxima with irradiation. The property values at the maxima are observed to be a function of the gel fraction which was varied over a wide range by choice of the amounts of sulfur curing agent and accelerator. The deterioration of the materials by irradiation was evaluated by observing the change from these maximum values. EPDM with higher gel fractions showed less deterioration than that with lower gel fractions. The dose rate affects the mechanical properties. This is interpreted as evidence of a diffusion limited oxidation reaction during irradiation. At dose rates less than 1 x 105 rad/h, the effect is insignificant and the data are realistically considered to reflect changes which may occur in the containment area of a nuclear reactor with a nominal dose rate of 100 rads/h. Hardness can be taken as a parameter to evaluate the deterioration.

Radiation Resistance of Ethylene-Propylene Rubbers Cured by Peroxide

The radiation resistance of ethylene-propylene-ethylidene norbornene terpolymer (EPDM) cured by peroxide was studied. Some properties of EPDM showed maxima with irradiation. The gel fraction dropped rapidly following the maximum, at a rate which increased with decreasing dose rate. The dose rate affects the rate of deterioration of mechanical properties; the lower the dose rate, the more the deterioration per unit of absorbed dose. No effect of original gel fraction on the deterioration rate was observed in this study. This is in contrast with the results of a previous study in which EPDM vulcanized by sulfur showed less deterioration in the samples with higher gel fractions than those with lower gel fractions [1]. It is suggested that the high radiation resistance of EPDM vulcanized by sulfur is attributed to the presence of sulfur atoms in the specimen, but not to the existence of crosslinks themselves, since radiation resistance increases in proportion to the concentration of sulfur in the rubber.

Radiation-Induced Conductivity in Polymeric Insulating Materials Degraded under Specified Conditions

Various polymeric insulating materials for cables were degraded by simulated irradiation and environmental conditions for normal operation and under accident at a nuclear power reactor. Thermally stimulated currents were observed only in the crystalline samples, and the higher thecrystallinity, the larger the amounts of detrapped carriers. The change of fine structure of the degraded sample was investigated by the change of X-ray crystallinity, melting behavior, and glass transition temperature. The radiation-induced conductivity was studied during irradiationand a decay curve was measured after the irradiation. Analysis of the conductivity decay curve enabled us to detect at most four kinds of carriers with different time constants. Long-lived carriers were hardly observed in the non-crystalline samples, while many were seen in the crystalline samples. With the decrease of crystallinity by degradation, only short-lived carriers were observed, indicating the existence of trapping sites for the long-lived carriers in or aroundthe polymer crystallites. Treatment of samples with high temperature steam and chemicals showedno special effect on the samples except for polyimide which dissolved in alkaline solution.

Monte Carlo calculations of the behavior of 300 keV electrons from accelerators

Abstract A Monte Carlo method for the passage of electrons based on a single scatterring model has been developed. A code constructed is operable on personal computers, and has been applied to analyze electron behavior in a layered system consisting of Ti (an accelerator window), air, CTA and backing material irradiated by 300 keV electrons, in the static and dynamic irradiation. The energy spectra and the angular distributions of electrons at the CTA surface as well as depth distributions of energy deposition in the CTA layer of 114 μm thickness placed on various backing materials have been obtained. The backscattering coefficients for various backing materials have been calculated. These results indicate that the characteristics of the electrons at the forward surface of CTA in the dynamic irradiation are similar to those of the electrons diffusely incident on the backing material. Some of these results are in good agreement with our experiments.

Electron beam dosimetry for a thin-layer absorber irradiated by 300-keV electrons

Abstract Depth-dose distributions in thin-layer absorbers were measured for 300-keV electrons from a scanning-type irradiation system, the electrons having penetrated through a Ti-window and an air gap. Irradiations of stacks of cellulose triacetate(CTA) film were carried out using either a conveyer (i.e. dynamic irradiation) or fixed (i.e. static) irradiation. The sample was irradiated using various angles of incidence of electrons, in order to examine the effect of obliqueness of electron incidence at low-energy representative of routine radiation curing of thin polymeric or resin layers. Dynamic irradiation gives broader and shallower depth-dose distributions than static irradiation. Greater obliqueness of incident electrons gives results that can be explained in terms of broader and shallower depth—dose distributions. The back-scattering of incident electrons by a metal(Sn) backing material enhances the absorbed dose in a polymeric layer and changes the overall distribution. It is suggested that any theoretical estimations of the absorbed dose in thin layers irradiated in electron beam curing must be accomplished and supported by experimental data such as that provided by this investigation.