Elsevier

Applied Surface Science

Volume 484, 1 August 2019, Pages 845-852
Applied Surface Science

Full length article
Effect of irradiation dose rates on ethylene-propylene rubber for nuclear cables

https://doi.org/10.1016/j.apsusc.2019.04.152Get rights and content

Highlights

  • Irradiation testing with multiple dose rates was performed on one ethylene-propylene rubber to study the dose rate effect.

  • By infrared microscope, the dose rate effect based on the oxidation profile as extent and penetration depth was demonstrated.

  • Degradation of the measured mechanical, electrical and chemical properties was found influenced by the dose rate effect.

  • This paper will facilitate deeper understanding of the dose rate effect and the theories of diffusion-limited oxidation.

Abstract

The effect of dose rates is the critical concern of irradiation testing in equipment qualification for polymeric materials used in nuclear power plants, and the rule ‘the lower dose rates, the severer degradation’ is always regarded as the truth due to the limited diffusion of oxidation. But it's obviously not economic and practical from the engineering point of view. In this paper, aiming at investigating such dose rate effects, gamma irradiation testing with multiple dose rates was performed on one domestically manufactured ethylene-propylene rubber that was intended for the nuclear cable insulations of the advanced pressurized water reactors in China. In detail, before and after testing, the mechanical, electrical and chemical properties were measured from the specimens, and their fractographs after tensile test were observed, and particularly the oxidation profiles on the whole cross sections of them were detected by the infrared microscope to quantitatively compare the degradation extent under different dose rates. Based on the results, the correlation among the dose rates, the measured properties, and the penetration depth of oxidation was clarified, the relevant degradation mechanisms were discussed, and a suitable dose rate for engineering practice was recommended.

Graphical abstract

The effect of dose rate is the critical concern of irradiation testing in equipment qualification for polymeric materials used in nuclear power plants, and the rule ‘the lower dose rate, the severer degradation’ is always regarded as the truth due to the limited diffusion of oxidation. But it's obviously not economic and practical from the engineering point of view. In this paper, for purpose of investigating such dose rate effect, gamma irradiation testing with multiple dose rates was performed on one domestically manufactured ethylene-propylene rubber that was intended for use as the nuclear cable insulations in China. Before and after testing, upon the specimens, the mechanical, electrical and chemical properties were measured, the fractographs were observed, and particularly the oxidation profiles on the whole cross sections were detected to compare the degradation extent under different dose rates. The results indicated that the severest degradation was found occur at the intermediate dose rate instead of the lowest one due to the competing mechanisms between plasticizers volatilization and chain scission, seen in Fig. 1. But the oxidation penetration depth discovered indeed increases with the decrease of the dose rates, and even obeys roughly an arithmetic progression relation, as revealed in Fig. 2, from which the conventional concept that the degradation extent is inversely proportional to the dose rates is testified. Based on the results, the correlation among the measured properties, the penetration depth and extent of oxidation, and the dose rates were clarified, the relevant degradation mechanisms were discussed, and a suitable dose rate for engineering practice was recommended.

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Introduction

It has to be admitted that nuclear power is yet a dilemma to the governments and a controversy to the public though stringent regulations have been issued by the administrators, stricter operations have been implemented by the operators, and substantial investigations have been conducted by the researchers after the Fukushima Daiichi nuclear disaster for the sake of safety - the priority concern of nuclear power plants. In essence, safety relies on the capability of nuclear equipment to perform designated functions under postulated service conditions, and is generally predicted by equipment qualification [1], protected by ageing management [2], and prevented by failure analysis [[3], [4], [5], [6], [7], [8], [9], [10]] against the degradation factors as pressure, temperature, humidity, chemicals, vibration, and particularly the characteristic one, irradiation. Thus, full understanding of the behaviors of nuclear equipment under irradiation, especially the nuclear cables that are mainly composed of polymeric materials and therefore are sensitive to irradiation, will not only provide the technical basis for safe and economic operation of nuclear power plants, but also help to boost the global confidence in nuclear power.

Actually, this issue has been attracting abundant researches in the past several decades, and a series of standardized methods have been formed to specify the requirements and to recommend the conditions of irradiation testing on nuclear cables [11,12]. Thereinto, the effect of dose rates has been paid special attention to on account that the degradation of the materials' properties doesn't increase with the dose rates due to the diffusion-limited oxidation [[13], [14], [15]], which is opposite to what conventional concept believes. This phenomenon has been extensively demonstrated on the mechanical properties as tensile stress [16] and elongation at break (Eb) [17], on the physical properties as density [18], degree of crystallinity [19] and surface structure [20], and on the chemical properties as hydroperoxides and carbonyls yield [21], oxidation induction time [18] and gel fraction [22]. For this reason, it seems that the dose rates in the irradiation testing should be as low as possible for the sake of conservativeness, but undoubtedly it is impossible and uneconomic. Hence, suitable ranges that are feasible in technology and practical in engineering deserve to be determined for qualification of the nuclear cables and related polymeric materials, but the precondition is to ensure they are homogeneously oxidized.

To this end, focuses have been mainly put on identification of the degradation trends along the thicknesses of the polymeric materials by means of properties detection of density, hardness, and carbonyl etc. on the stacked specimens [23,24] after irradiation under different dose rates. Thanks to the invention of the more efficient instrument - the infrared microscope, the functional groups of interest on the whole cross sections of polymeric materials, e.g. carbonyl, carboxyl, hydroxyl etc. can be directly detected in precision of micrometer, which then makes quantitative study of the penetration depth of oxidation possible. However, it still has not been widely used for dose rate effects, and also the specimens studied were the products with patented formulas [25,26], whose universal applicability yet remains in doubt. That is, although the dose rate effects is a general problem and the materials studied were common types, e.g. EPR (ethylene-propylene rubber), EVA (ethylene-vinyl acetate) etc., the testing results were always product-specific, making it obliged to evaluate the irradiation resistance of every ‘new’ polymeric material before its adoption for the nuclear cables.

In this context, in order to meet the demand for domesticalization of the polymeric materials for nuclear cables of the advanced PWR (pressurized water reactor) nuclear power plants in China, and to determine suitable dose rate ranges for subsequent equipment qualification of them, the effect of irradiation dose rates was investigated in detail on one domestic EPR product as representative in this paper. Besides the conventional mechanical, electrical and chemical properties measurement of the specimens after irradiation, their whole cross sections were particularly scanned by the infrared microscope for detection of the oxidation profile. The results revealed that the extent of homogeneity on the cross sections did decrease with the dose rates, but unexpectedly the relation between the macroscopic properties and the dose rates was relatively ‘abnormal’, i.e. the ‘the lower dose rates, the severer degradation’ rule failed. Indeed, this might be partly ascribed to the deviation of the measurements themselves, but it really challenged the current knowledge of and facilitated a more comprehensive understanding of the dose rate effects, which should be the achievement of this paper.

Section snippets

Material

The ethylene-propylene rubber was the commercial product domestically developed and produced for application as the nuclear cable insulations of the advanced PWR nuclear power plants in China, and was provided as 2 mm-thick sheet from Shanghai Nuclear Engineering Research & Design Institute Co., Ltd. (SNERDI) for investigation. The type 2 dumb-bell specimen with gauge length of 20 mm and total length of 75 mm in the ISO 37 standard [27], as well as the round-disc specimen with diameter of 80 mm

Mechanical properties

As displayed in Fig. 1, the Eb and TSb values of the dumb-bell specimen after irradiation accumulated around 0.5 and 1.0 (normalized) respectively, despite of the difference in dose rates, which should be attributed to the effect of the absorbed dose 500 kGy. However, the minimum values of these two properties both located at the dose rate of 5 kGy/h instead of 0.5 kGy/h, i.e. not conforming to the conventional concept that the irradiation degradation extent is inversely proportional to the

Conclusions

Gamma irradiation testing with multiple dose rates was carried out on one domestic EPR product intended for nuclear cable insulations of the advanced PWR nuclear power plants in China to investigate the dose rate effects. With the assistance of the FT-IR microscope, the dose rate effects grounded on the correlation among the dose rates and the penetration depth and extent of oxidation was indeed demonstrated. However, degradation of the macroscopic properties including mechanical, electrical

Acknowledgements

This work was supported by the National Science and Technology Major Project of the Ministry of Science and Technology of the People's Republic of China entitled ‘Large-Scale Advanced Pressurized Water Reactor Nuclear Power Plants’ (No. ZB01K01), the National Natural Science Foundation of China (No. 51803033), and the start-up fund by Fudan University (No. JIH2021003). Gratitude should also be given to Prof. Jing-Ye Li and Prof. Lin-Fan Li from Shanghai Institute of Applied Physics (SINAP)

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