Comparative study on degradation of ethylene-propylene rubber for nuclear cables from gamma and beta irradiation

Polymer Testing 60 (2017) 102e109

Contents lists available at ScienceDirect

Polymer Testing journal homepage: www.elsevier.com/locate/polytest

Material Properties

Comparative study on degradation of ethylene-propylene rubber for nuclear cables from gamma and beta irradiation Yi Gong a, Shi-Meng Hu a, Xiao-Lei Yang a, Jing-Lu Fei a, Zhen-Guo Yang a, *, Xiu-Qiang Shi b, Yong-Cheng Xie b, Ai-Hua Guo c, Jian-Feng Xu c a b c

Department of Materials Science, Fudan University, Shanghai 200433, PR China Shanghai Nuclear Engineering Research & Design Institute (SNERDI), Shanghai 200233, PR China Shanghai Institute of Process Automation Instrumentation (SIPAI), Shanghai 200233, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 6 January 2017 Received in revised form 14 March 2017 Accepted 15 March 2017 Available online 16 March 2017

Considering safety is the priority concern of nuclear power plants, equipment qualification testing of the nuclear components manufactured should be paid special attention to. Thereinto, equivalent conversion (1:1) from the absorbed beta doses to gamma doses is a rule of thumb for irradiation qualification testing of the polymers used as nuclear cables, however whether it is reasonable and applicable to Chinese domestic polymers still requires investigation. In this paper, both gamma and beta irradiation testing with the actual dose rates and total absorbed doses in China Advanced Passive (CAP) series nuclear power plant was performed upon one domestically manufactured ethylene-propylene rubber intended for nuclear cable insulation in China. The mechanical and the electrical properties before and after irradiation were measured to compare the extent and the trend of degradation between the two irradiation types, and related oxidation degradation mechanism especially its attenuation along the thickness was quantitatively addressed. © 2017 Published by Elsevier Ltd.

Keywords: Gamma irradiation Beta irradiation Ethylene-propylene rubber Cable insulation Nuclear power plants

1. Introduction Safety, the priority concern of nuclear power plants, is the capability of structures, systems and components (SSCs) to perform safety-related functions under both normal and accidental conditions against radiation risks [1], and is commonly credited by equipment qualification (EQ) and maintained by ageing management (AM) [2], thereby premature failure incidents could be prevented and conceived economic benefits could be ensured. Fundamentally, the precondition of safety relies on the reliability of materials. In practice, besides failure analysis of nuclear components with finished materials during operation [3e6], accelerated ageing testing [7e10] of raw materials under simulated service conditions before manufacturing is the other effective approach for improvement of materials reliability and longevity, among which irradiation is the characteristic factor to be taken into account. As a matter of fact, ionizing irradiation degradation of polymers used in nuclear power plants (predominantly applied to the nuclear

* Corresponding author. E-mail address: [email protected] (Z.-G. Yang). http://dx.doi.org/10.1016/j.polymertesting.2017.03.017 0142-9418/© 2017 Published by Elsevier Ltd.

cables) has been attracting expansive researches since several decades ago [11e14], and effects including irradiation types [15,16], absorbed doses [17,18], dose rates [19], temperatures [20e22], and their synergisms [23e26] have been investigated. Thereinto, beta (b) irradiation testing is used to represent the post-accident environment in nuclear power plants, and is usually realized by electron beam accelerator. However, the dose rates of traditional electron beam accelerators are more than 10,000 kGy/h [27e29], at least three orders of magnitude higher than the actual b irradiation dose rates after design basis events (DBE) in containment [30]. Thus, the results of erstwhile b irradiation testing seem not be conservative enough if considering the dose rate effect in air, i.e. a lower dose rate with longer exposure time exerts severer degradation effect on polymers than a higher dose rate with shorter exposure time does when the total absorbed doses are same [31,32]. In this situation, the reasonability of equivalent conversion (1:1) from beta doses to gamma doses as a rule of thumb in EQ of nuclear components [33,34] deserves to be revalidated [35]. To this end, in addition to gamma (g) irradiation by common 60 Co radioactive source, beta irradiation by a modified electron beam accelerator capable of generating dose rates from 10 kGy/h to 70 kGy/h (with patents) was conducted on one domestically

Y. Gong et al. / Polymer Testing 60 (2017) 102e109

manufactured ethylene-propylene rubber (EPR) product used for nuclear cable insulation of China Advanced Passive (CAP) series nuclear power plants in China. After irradiation testing, the mechanical and the electrical properties were measured to compare the different degradation effects from gamma and beta irradiation, and relevant degradation mechanisms were investigated by means of scanning electron microscope (SEM), attenuated total reflection Fourier transform infrared spectroscopy (ATR FT-IR) and FT-IR microscope. The results indicated that the degradation effects from both gamma and beta irradiation with dose rates and total absorbed doses in same order of magnitude (~10 kGy/h, ~103 kGy) were close on this EPR product, verifying the classic ‘equal dose e equal damage’ approximation in theory and the equivalent conversion from beta doses to gamma dose in practice for EQ testing of nuclear components. 2. Experimental 2.1. Material The ethylene-propylene rubber is the domestically manufactured commercial product intended for application as nuclear cable insulation of CAP series nuclear power plants, and was provided as 2 mm-thick sheet from Shanghai Nuclear Engineering Research & Design Institute (SNERDI) for investigation. Samples of type 2 dumb-bell in ISO 37-2011 standard [36] and round plate with diameter of 80 mm were cut from the EPR sheet for irradiation, and subsequently subjected to the mechanical and the electrical properties measurement respectively. 2.2. Irradiation testing The 60Co radioactive source and the patented electron beam accelerator (0.7e1.5 MeV) was utilized to perform gamma and beta irradiation testing respectively on the EPR samples in air in Shanghai Institute of Process Automation Instrumentation (SIPAI). The irradiation conditions, as listed in Table 1, were specified according to the gamma and the beta irradiation dose rates and total doses after DBE in containment of Advanced Passive 1000 (AP1000) nuclear power plants [37]. Samples exposed to gamma irradiation were vertically hung around the 60Co source, while those exposed to beta irradiation were horizontally and directly placed on a stainless steel plate with a 4 420 mm effective irradiation area under the moving electron beam, and flipped over at the middle of the total exposure time for better homogeneousness.

103

accordance with the ISO 37-2011 standard via WDS-W-5kN electronic universal testing machine (Chengde Precision Testing Machine Co., Ltd.), and the number of test pieces was 7 under each irradiation condition; the round plate samples were measured for volume resistivity according to the IEC 62631-3-1 standard [38] by ZC36 megger (Shanghai No.6 Electric Meter Works Co., Ltd.), and the number of test pieces was 3 under each irradiation condition. 2.4. Characterization In order to identify the degradation mechanisms, Nicolet Nexus 470 ATR FT-IR (Thermo Fisher Scientific) was utilized to detect the functional groups changes of the samples before and after irradiation. Also, the cross sections of the irradiated dumb-bell samples were observed under S-520 SEM (Hitachi), and were even scanned by Nicolet iN10 FT-IR microscope (Thermo Fisher Scientific) after being cut into 15-mm-thick slices via Shandon Finesse 325 manual microtome (Thermo Fisher Scientific) to investigate the oxidation extent gradient along the depth. 3. Results and discussion 3.1. Mechanical properties It is widely accepted that elongation at break is the most appropriate and sensitive index for evaluating irradiation degradation of elastomers [39]. As is displayed in Fig. 1(a), Eb of EPR from both gamma and beta irradiation decrease with absorbed doses in a roughly exponential decay pattern, indicating similar degradation extents on EPR from these two irradiation types. In more detail, the 50% relative drops of the initial Eb values (commonly regarded as the end-point for cable insulation elastomers) locate in around 500 kGy, and the ultimate Eb values are less than 10% of the initial. However, with respect to the tensile strength at break, although in general they increase with absorbed doses, one exception is at the 1000 kGy absorbed gamma doses, seen in Fig. 1(b), which decreases nearly 20% compared with the value at previous dose (500 kGy), and consequently the situation that the TSb increase rate from gamma irradiation is higher than that from beta irradiation before this dose (0e500 kGy) inverts after it (1000e5000 kGy). Besides, it can be learned that the statistical dispersion of TSb is not as good as that of Eb, verifying the appropriateness of the latter for degradation evaluation of elastomers even though they are obtained simultaneously by one tensile test. 3.2. Electrical properties

2.3. Measurement Before and after irradiation, the dumb-bell samples were tested for tensile strength at break (TSb) and elongation at break (Eb) in Table 1 Conditions of the gamma and the beta irradiation. Irradiation type

Dose rate (kGy/h)

Exposure time (h)

Absorbed dose (kGy)

gamma (g)

10

beta (b)

20

10 50 100 500 1000 25 50 100 200 250

100 500 1000 5000 10000 500 1000 2000 4000 5000

As shown in Fig. 2, volume resistivity from both irradiation types increase with absorbed doses in low dose ranges until reaching peaks (gamma at 100 kGy, beta at 500 kGy), and then decrease in a sharp manner (gamma irradiation) and a gradual manner (beta irradiation) respectively. In detail, after the peaks, the volume resistivity from gamma irradiation slightly change in the range of ~1012 U$m (one order of magnitude lower than the initial values) with absorbed doses, while those from beta irradiation are all in the same order of magnitude of the initial values. Considering the 10% end-point criteria (90% drop) for volume resistivity in the standard [31], the critical absorbed dose of gamma irradiation for EPR can thereby be determined as 500 kGy, conforming with the Eb results mentioned above; on the contrary, EPR begins ‘failed’ after 500 kGy absorbed dose of beta irradiation based on the Eb results, while it seems still qualified even after enduring 5000 kGy beta irradiation from the electrical point of view, coinciding with the conclusion in literature [40,41] that cable materials could retain normal electrical functions even if only ~5% of the initial Eb values was left.

104

Y. Gong et al. / Polymer Testing 60 (2017) 102e109

Fig. 1. Mechanical properties versus absorbed doses (a) elongation at break (b) tensile strength at break.

bending vibration of methyl eCH3), 1165 and 970 cm
1 (reveal the existence of the head-to-tail structure

CH3

CH3

CH

CH2 CH CH2

in

polypropylene monomer), and 727 cm
1 (imply the number of methylene in e(CH2)ne is more than 4) demonstrates the backbones of the EPR samples are basically not destroyed by irradiation [43,44]. 3.4. SEM and FT-IR microscope

Fig. 2. Dependence of logarithmic volume resistivity on absorbed doses.

3.3. ATR FT-IR Since the EPR samples are not transparent, FT-IR with ATR accessories was utilized to identify the chemical structure changes after irradiation. Fig. 3 presents the FT-IR spectra of the samples unirradiated, irradiated with lowest (500 kGy) and highest (5000 kGy) absorbed beta doses, and irradiated with same absorbed gamma doses for comparison. It is explicit that the newly generated functional groups from irradiation mainly accumulate in the wavenumber range of 1200e1800 cm
1 despite of irradiation types and absorbed doses, among which the absorption peaks at around 1730 cm
1 stand for the stretching vibration of carbonyl C¼O; at around 1600 cm
1 and 1390, 1320 cm
1 represent the asymmetric and symmetric stretching vibration of carboxylate eCOOe respectively; and at around 1225 cm
1 are the stretching vibration of ether CeOeC. All of them indicate a strong interaction with the oxygen in air due to irradiation [42]. However, the presence of the absorption peaks at around 2920 cm
1 (assigned to the asymmetric stretching vibration of methyl eCH3), 2850 cm
1 (assigned to the symmetric stretching vibration of methylene eCH2e), 1460 cm
1 (assigned to the in-plane bending vibration of methylene eCH2e), 1376 cm
1 (assigned to the in-plane symmetric

Fig. 4 displays the SEM micrographs of the cross sections of the dumb-bell samples after tensile test. It's obvious that compared with the unsmooth surface with dimples of the unirradiated samples (Fig. 4(a)), the surfaces of the irradiated samples with highest absorbed gamma doses (10000 kGy) and beta doses (5000 kGy) are plain and even, seen in Fig. 4(b) and (c). This phenomenon indicates a ductile-brittle transition of the fracture mode of the EPR samples due to ductility decrease after irradiation [45], and is in agreement with the mechanical properties changes presented in Fig. 1. Then, the cross sections of such two irradiated dumb-bell samples were cut into 15-mm-thick slices by microtome, and scanned under FT-IR microscope. It is semi-quantitative demonstrated in Fig. 5(a) and (b) that carbonyl (~1710 cm
1), the typical evidence of oxidation (degradation) primarily concentrates on the edges, and its absorption intensity gradually decreases from the surface to the interior, as marked with red arrows in Fig. 6(a) and (b). If take this carbonyl absorption intensity as the relative oxidation index, it can be quantitatively learned from Fig. 7 that the oxidation extent from both irradiation types decreases in nearly an exponential decay manner along the depth, and the seriously affected distance is less than 150 mm. 3.5. Comprehensive discussion Based on the results of the mechanical and the electrical properties measurement presented above, it could be concluded that the degradation extent and trend of this domestically manufactured EPR product from both gamma and beta irradiation with equivalent absorbed doses in their respective actual dose rates are generally the same. Furthermore, if only take the most-commonlyused elongation at break as index, the degradation patterns from both irradiation types are nearly identical (Fig. 1(a)). This result phenomenologically testifies the reasonability of the equivalent

Y. Gong et al. / Polymer Testing 60 (2017) 102e109

105

Fig. 3. FT-IR spectra of the blank and the irradiated EPR samples.

Fig. 4. SEM micrographs of the cross sections of the dumb-bell samples after tensile test. (a) unirradiated (b) with 10000 kGy absorbed gamma doses (c) with 5000 kGy absorbed beta doses.

conversion (1:1) from absorbed beta doses to gamma doses in EQ testing of nuclear components. From the theoretical point of view, it is also addressed that the degradation extents from both gamma and beta irradiation are approximately equal upon polymers since their linear energy transfer (LET) values (L, MeV/mm) are close, which is termed as the deposited energy (E) per unit distance (l) when the charged particles of ionizing irradiation traverse a material, seen in equation (1) [46]. For example, the gamma and the beta irradiation with energy around 1 MeV will cause nearly equal damage upon polymers with thicknesses less than 3 mm [13],

above which the beta particles are incapable of penetrating the material. In our case, considering the fact that the energies of both the 60Co radioactive source and the electron beam accelerator were about 1.25 MeV, and the thickness of the EPR samples was 2 mm, equal damage/degradation from the gamma and the beta irradiation is possible, i.e. equivalent conversion of the absorbed doses between the two irradiation types is practicable and reasonable.

L¼

dE dl

(1)

106

Y. Gong et al. / Polymer Testing 60 (2017) 102e109

Fig. 5. Carbonyl profile on the cross sections of the irradiated dumb-bell samples (X-axis is along the 2-mm thickness). (a) with 10000 kGy absorbed gamma doses (b) with 5000 kGy absorbed beta doses.

Y. Gong et al. / Polymer Testing 60 (2017) 102e109

107

Fig. 6. FT-IR spectra along the depth away from the surfaces of the irradiated dumb-bell samples. (a) with 10000 kGy absorbed gamma doses (b) with 5000 kGy absorbed beta doses.

Fig. 7. Relative oxidation as determined from the carbonyl absorption intensity versus depth away from the surface.

CH3 CH2

CH3

CH CH2

CH2

CH2

CH2

+ CH2

CH2

C CH2

CH2

CH2

CH3

C

(2)

C

CH2

irradiation

CH3 CH2

In terms of the degradation mechanisms from irradiation in vacuum, EPR is always classified into the ‘crosslinking group’, i.e. the polymer chains are predominantly cross linked from mutual combination of the free radicals that are produced by irradiation, and a three dimensional network structure will be finally formed. When oxygen is present, such combination reaction will be competed with the oxidation reaction of the free radicals, and the polymer chains will be destroyed with the formation of ketone, ether, ester, carboxylate and so on [47e49]. In conclusion, such degradation mechanisms of EPR in air can be divided into three main steps: 1) Generation of the free radicals, which preferentially occurs in the tertiary hydrogen of the polymer chains, seen in the reaction shown in equation (2). 2) Combination and oxidation of the free radicals, from which crosslinking is induced and intermediate products peroxides are generated, seen in equations (3) and (4). 3) Degradation of the polymer chains, seen in equations (5)e(7) respectively, and as a result, fresh functional groups including carbonyl C¼O, carboxylate eCOOe and ether CeOeC are formed, which are regarded as the solid evidence of irradiationinduced degradation upon polymers in air, and are indeed detected in our case according to Fig. 3.

CH2

CH3

CH2

C

CH2

CH2

C

CH2

CH2

CH3

(3)

108

Y. Gong et al. / Polymer Testing 60 (2017) 102e109

CH3 CH2

CH3 + O2

C CH2

CH2

CH2

CH2

CH3 CH2

CH2

CH2

C CH2

CH3 CH2

CH3

+

·CH2

(5)

O

O

C(O2·) CH2

CH2

O

C(O2·) CH2

(4)

C(O2·)

CH2

CH2

CH2

(6)

+ ·OCH2

C CH3

Acknowledgements

·CH2

+ ·OCH2

O CH2

CH2

(7)

Certainly, it's not difficult to infer that the oxidation reaction is controlled by the oxygen diffusion, hence it begins from the samples surfaces where air is accessible while the combination reaction (crosslinking) primarily occurs in the interior. This assumption has been verified by means of FT-IR microscope, and the distribution of carbonyl C¼O, the most distinct symbol of oxidation, was revealed to obey an exponential decay manner along the depth (Fig. 7). If take the similar degradation pattern of elongation at break (Fig. 1(a)) into account, a correlation among the thickness, the dose rate, and the service time might be deduced when sufficient data are available in the future, which will contribute to both the prediction of the service life of this EPR product, and the optimization of the nuclear cables design. 4. Conclusions Gamma (60Co) and beta (electron beam) irradiation testing with the actual dose rates and total absorbed doses was performed on one domestically manufactured EPR product intended for nuclear cables insulation of CAP series nuclear power plants in China. The results indicated that the degradation extent was close and the degradation trend was similar from both irradiation types, testifying the reasonability of equivalent conversion (1:1) from absorbed beta doses to gamma doses in equipment qualification testing of nuclear components in industry. Crosslinking and oxidation were identified as the two predominant mechanisms of degradation, in which the latter initiated from the surfaces and attenuated in an exponential decay manner along the depth.

This work was supported by the National Science and Technology Major Project of the Ministry of Science and Technology of China entitled ‘Large-Scale Advanced Pressurized Water Reactor Nuclear Power Plants’ (No. ZB01K01), and the start-up fund by Fudan University (No. JIH2021003). Gratitude should also be given to Shanghai Institute of Applied Physics (SINAP) affiliated to Chinese Academy of Science, and Thermo Fisher Scientific.

References [1] Safety Fundamentals No. SF-1, Fundamental Safety Principles, International Atomic Energy Agency, Vienna, Austria, 2006. [2] Specific Safety Requirements No. SSR-2/1, Safety of Nuclear Power Plants, Design, International Atomic Energy Agency, Vienna, Austria, 2012. [3] Z.G. Yang, Y. Gong, J.Z. Yuan, Failure analysis of leakage on titanium tubes within heat exchangers in a nuclear power plant. Part I: electrochemical corrosion, Mater. Corros. 63 (2012) 7e17. [4] Y. Gong, Z.G. Yang, J.Z. Yuan, Failure analysis of leakage on titanium tubes within heat exchangers in a nuclear power plant. Part II: mechanical degradation, Mater. Corros. 63 (2012) 18e28. [5] F.J. Chen, C. Yao, Z.G. Yang, Failure analysis on abnormal wall thinning of heattransfer titanium tubes of condensers in nuclear power plant Part I: corrosion and wear, Eng. Fail. Anal. 37 (2014) 29e41. [6] F.J. Chen, C. Yao, Z.G. Yang, Failure analysis on abnormal wall thinning of heattransfer titanium tubes of condensers in nuclear power plant Part II: erosion and cavitation corrosion, Eng. Fail. Anal. 37 (2014) 42e52. [7] A.K. Behera, C.E. Beck, A. Alsammarae, Cable aging phenomena under accelerated aging conditions, IEEE Trans. Nucl. Sci. 43 (1996) 1889e1893. [8] A. Shimada, M. Sugimoto, H. Kudoh, K. Tamura, T. Seguchi, Degradation mechanisms of silicone rubber (SiR) by accelerated ageing for cables of nuclear power plant, IEEE Trans. Dielectr. Electr. Insulation 21 (2014) 16e23. [9] A. Shimada, M. Sugimoto, H. Kudoh, K. Tamura, T. Seguchi, Radiation ageing technique for cable life evaluation of nuclear power plant, IEEE Trans. Dielectr. Electr. Insulation 19 (2012) 1768e1773. [10] J. Cassidy, S. Nesaei, R. McTaggart, F. Delfanian, Mechanical response of high density polyethylene to gamma radiation from a Cobalt-60 irradiator, Polym. Test. 52 (2016) 111e116. [11] I. Kuriyama, N. Hayakawa, Y. Nakase, S. Kawawata, J. Ogura, K. Kasai, T. Onishi,

Y. Gong et al. / Polymer Testing 60 (2017) 102e109

[12]

[13] [14] [15]

[16]

[17]

[18]

[19]

[20]

[21]

[22]

[23]

[24]

[25] [26] [27] [28]

[29]

[30]

Radiation-resistance of cable insulating materials for nuclear-power generating stations, IEEE Trans. Electr. Insulation 13 (1978) 164e171. CERN 79-04, Compilation of Radiation Damage Test Data, Part I: Cable Insulating Materials, European Organization for Nuclear Research, Geneva, Switzerland, 1979. NP-2129, Radiation Effects on Organic Materials in Nuclear Plants, Electric Power Research Institute, Palo Alto, CA, USA, 1981. N. Hildreth, An update on irradiation of wire and cable, IEEE Trans. Nucl. Sci. 28 (1981) 1794e1796. € nbacher, Effects of radiation types and F. Hanisch, P. Maier, S. Okada, H. Scho dose rates on selected cable-insulating materials, International J. Radiat. Applications Instrum. Part C. Radiat. Phys. Chem. 30 (1987) 1e9. t, Comparison of degradation effects induced B. Bartoní cek, V. Pla cek, V. Hna by gamma radiation and electron beam radiation in two cable jacketing materials, Radiat. Phys. Chem. 76 (2007) 857e863. lez, P. Silva, M. Ichazo, The effect of gamma raR. Perera, C. Albano, J. Gonza diation on the properties of polypropylene blends with styreneebutadieneestyrene copolymers, Polym. Degrad. Stab. 85 (2004) 741e750. N.Z. Noriman, H. Ismail, The effects of electron beam irradiation on the thermal properties, fatigue life and natural weathering of styrene butadiene rubber/recycled acrylonitrileebutadiene rubber blends, Mater. Des. 32 (2011) 3336e3346. hal, Dose rate effects in radiation V. Pla cek, B. Bartonı cek, V. Hn at, B. Ota degradation of polymer-based cable materials, Nucl. Instrum. Methods Phys. Res. Sect. B Beam Interact. Mater. Atoms 208 (2003) 448e453.  vy, A. Gusarov, M.J. Konstantinovi T. Sarac, N. Quie c, The study of temperature and radiation induced degradation of cable polymers: a comparison between the mechanical properties of industrial and neat EPDM, Procedia Struct. Integr. 2 (2016) 2405e2414. A. Shimada, M. Sugimoto, H. Kudoh, K. Tamura, T. Seguchi, Degradation distribution in insulation materials of cables by accelerated thermal and radiation ageing, IEEE Trans. Dielectr. Electr. Insulation 20 (2013) 2107e2116. T. Seguchi, K. Tamura, H. Kudoh, A. Shimada, M. Sugimoto, Degradation of cable insulation material by accelerated thermal radiation combined ageing, IEEE Trans. Dielectr. Electr. Insulation 22 (2015) 3197e3206. K.T. Gillen, R.L. Clough, Time teperature dose-rate superposition - a methodology for extrapolating accelerated radiation aging data to low-dose rate conditions, Polym. Degrad. Stab. 24 (1989) 137e168. J.V. Gasa, Z. Liu, M.T. Shaw, Relationship between density and elongation-atbreak of naturally and artificially aged cable materials used in nuclear power plants, Polym. Degrad. Stab. 87 (2005) 77e85. S.P. Cygan, J.R. Laghari, Effects of multistress aging (radiation, thermal, electrical) on polypropylene, IEEE Trans. Nucl. Sci. 38 (1991) 906e912. J.R. Laghari, Complex electrical thermal and radiation aging of dielectric films, IEEE Trans. Electr. Insulation 28 (1993) 777e788. L. Costa, I. Carpentieri, P. Bracco, Post electron-beam irradiation oxidation of orthopaedic UHMWPE, Polym. Degrad. Stab. 93 (2008) 1695e1703. R. Rosario-Melendez, L. Lavelle, S. Bodnar, F. Halperin, I. Harper, J. Griffin, K.E. Uhrich, Stabilitý of a salicylate-based poly(anhydride-ester) to electron beam and gamma radiation, Polym. Degrad. Stab. 96 (2011) 1625e1630. I. Zembouai, M. Kaci, S. Bruzaud, I. Pillin, J.-L. Audic, S. Shayanfar, S.D. Pillai, Electron beam radiation effects on properties and ecotoxicity of PHBV/PLA blends in presence of organo-modified montmorillonite, Polym. Degrad. Stab. 132 (2016) 117e126. NUREG/CR-5175, Beta and Gamma Dose Calculations for PWR and BWR Containments, U.S. Nuclear Regulatory Commission, Washington D.C., U.S.A, 1989.

109

[31] IEC 60544-2, Electrical Insulating Materials e Determination of the Effects of Ionizing Radiation on Insulating Materials e Part 2: Procedures for Irradiation and Test, International Electrotechnical Commission, Geneva, Switzerland, 2012. [32] J. Boguski, G. Przybytniak, Benefits and drawbacks of selected condition monitoring methods applied to accelerated radiation aged cable, Polym. Test. 53 (2016) 197e203. [33] IEEE Std 383, IEEE Standard for Qualifying Electric Cables and Splices for Nuclear Facilities, The Institute of Electrical and Electronics Engineers, Inc., New York, U.S.A, 2015. [34] IEC/IEEE 60780-323, Nuclear Facilities e Electrical Equipment Important to Safety e Qualification, International Electrotechnical Commission, Geneva, Switzerland, 2016. [35] N. Fuse, H. Homma, T. Okamoto, Remaining issues of the degradation models of polymeric insulation used in nuclear power plant safety cables, IEEE Trans. Dielectr. Electr. Insulation 21 (2014) 571e581. [36] ISO 37, Rubber, Vulcanized or Thermoplastic - Determination of Tensile Stress-strain Properties, International Organization for Standardization, Geneva, Switzerland, 2011. [37] X.X. Wang, P.Z. Zhang, X.J. Liu, Calculation and analysis for the radiation condition in the containment of PWR after severe accident, Nucl. Sci. Eng. 33 (2013) 163e167 (in Chinese but with an English abstract). [38] IEC 62631-3-1, Dielectric and Resistive Properties of Solid Insulating Materials e Part 3-1: Determination of Resistive Properties (DC Methods) e Volume Resistance and Volume Resistivity e General Method, International Electrotechnical Commission, Geneva, Switzerland, 2016. [39] M. Madani, R.A. Aly, Monitoring of the physical aging of radiation cross-linked conductive rubber blends containing clay nanofiller, Mater. Des. 31 (2010) 1444e1449. [40] K. Anandakumaran, W. Seidl, P.V. Castaldo, Condition assessment of cable insulation systems in operating nuclear power plants, IEEE Trans. Dielectr. Electr. Insulation 6 (1999) 376e384. [41] Y.T. Hsu, K.S. Chang-Liao, T.K. Wang, C.T. Kuo, Correlation between mechanical and electrical properties for assessing the degradation of ethylene propylene rubber cables used in nuclear power plants, Polym. Degrad. Stab. 92 (2007) 1297e1303. [42] T. Kurihara, T. Takahashi, H. Homma, T. Okamoto, Oxidation of cross-linked polyethylene due to radiation-thermal deterioration, IEEE Trans. Dielectr. Electr. Insulation 18 (2011) 878e887. [43] A. Rjeb, L. Tajounte, M.C. El Idrissi, S. Letarte, A. Adnot, D. Roy, Y. Claire, A. Perichaud, J. Kaloustian, IR spectroscopy study of polypropylene natural aging, J. Appl. Polym. Sci. 77 (2000) 1742e1748. [44] Y. Gong, Z.G. Yang, Fracture failure analysis of automotive accelerator pedal arms with polymer matrix composite material, Compos. Part B Eng. 53 (2013) 103e111. [45] K.G. Schmitt-Thomas, Z.G. Yang, T. Hiermer, Development of torsion loading facility used for evaluating shear performance of polymeric composite implant rods, Polym. Test. 17 (1998) 117e130. [46] S.M. Seltzer, D.T. Bartlett, D.T. Burns, G. Dietze, H.G. Menzel, H.G. Paretzke, A. Wambersie, Fundamental quantities and units for ionizing radiation, J. ICRU 11 (2011) 1e41. [47] K.V.A. Kumar, H.B. Ravikumar, S. Ganesh, C. Ranganathaiah, Electron beam induced microstructural changes and electrical conductivity in bakelite RPC detector material, IEEE Trans. Nucl. Sci. 62 (2015) 306e313. [48] T. Zaharescu, S. Jipa, Evaluation of radiochemical effects in ethylene-propylene elastomers, Polym. Test. 16 (1997) 107e113. [49] T. Zaharescu, C. Podinǎ, Radiochemical stability of EPDM, Polym. Test. 20 (2001) 141e149.