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Ageing of polymers in electrical equipment used in nuclear power plants R. Clavreul Electricit e de France, Research and Development Division, Ecuelles, 77818 Moret sur Loing Cedex, France Abstract Ageing of polymers in electrical equipment used in nuclear power plants has been studied in (Electricit e de France) EDF for several years. The objective of such studies is to predict the polymers lifetime in normal and accidental conditions. The prediction of polymers behaviour in normal conditions requires accelerated tests in order to get rapidly experimental results. Experimental conditions must carefully be chosen and representative of real ageing. Accelerated ageing is usually done by applying higher temperature, (dose) or dose rate. When such experiments are done, the eects of temperature, (dose) or dose rate are first determined. In a second step, experimental results are extrapolated to real conditions. To predict lifetime of polymers, the following recommendations have to be checked: in order to assume that accelerated tests are representative of normal ageing, the observed mechanisms in experiments must be the same as those in real conditions. For accidental conditions, the same tests as those described in standards can be applied to polymers. The simulation of any accident occurring just after the installation of electrical equipment in nuclear power plants is easy to manage: only the accidental test can be carried out on the electrical equipment. To determine whether polymers in electrical equipment would have a good behaviour or not when an accident would occur after a period of several years or decades in normal conditions in a nuclear power plant, the accidental test must be done on aged materials; their physical, mechanical and electrical characteristics must be relevant to aged polymers in normal con- ditions. In order to detect any evolution of properties during ageing, the electrical, mechanical or chemical tests have to be proceeded on polymers samples. The characterisation tests which are applied on non-aged and aged samples depend on the nature of the polymers, their application in electrical equipment and their environment. The IEC 544.2 standard (Guide for Determining the Eects of Ionising Radiation on Insulating Materials: Part 2: Procedures for Irradiation and Test, 2nd ed., 1991-08) indicates a list of tests in which it is possible to select the methods which are sensitive to ageing conditions and representative of the given application. Ó 1999 Elsevier Science B.V. All rights reserved. Keywords: Polymers; Electrical equipment; Nuclear power plants; Lifetime prediction 1. Introduction To predict the lifetime of electrical equipment in nuclear power plants, experiments can be done either on the electrical equipment, or on materials. Whatever the sampling, the environment of the material during ageing must be equivalent. The choice of the sampling depends on the following parameters: the price of the electrical equipment, the facility or diculty for sampling, the sample Nuclear Instruments and Methods in Physics Research B 151 (1999) 449–452 www.elsevier.nl/locate/nimb 0168-583X/99/$ – see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 9 ) 0 0 1 0 0 - 7

Ageing of polymers in electrical equipment used in nuclear power plants

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Ageing of polymers in electrical equipment used in nuclear powerplants

R. Clavreul

Electricit�e de France, Research and Development Division, Ecuelles, 77818 Moret sur Loing Cedex, France

Abstract

Ageing of polymers in electrical equipment used in nuclear power plants has been studied in (Electricit�e de France)

EDF for several years. The objective of such studies is to predict the polymers lifetime in normal and accidental

conditions. The prediction of polymers behaviour in normal conditions requires accelerated tests in order to get rapidly

experimental results. Experimental conditions must carefully be chosen and representative of real ageing. Accelerated

ageing is usually done by applying higher temperature, (dose) or dose rate. When such experiments are done, the e�ects

of temperature, (dose) or dose rate are ®rst determined. In a second step, experimental results are extrapolated to real

conditions. To predict lifetime of polymers, the following recommendations have to be checked: in order to assume that

accelerated tests are representative of normal ageing, the observed mechanisms in experiments must be the same as

those in real conditions. For accidental conditions, the same tests as those described in standards can be applied to

polymers. The simulation of any accident occurring just after the installation of electrical equipment in nuclear power

plants is easy to manage: only the accidental test can be carried out on the electrical equipment. To determine whether

polymers in electrical equipment would have a good behaviour or not when an accident would occur after a period of

several years or decades in normal conditions in a nuclear power plant, the accidental test must be done on aged

materials; their physical, mechanical and electrical characteristics must be relevant to aged polymers in normal con-

ditions. In order to detect any evolution of properties during ageing, the electrical, mechanical or chemical tests have to

be proceeded on polymers samples. The characterisation tests which are applied on non-aged and aged samples depend

on the nature of the polymers, their application in electrical equipment and their environment. The IEC 544.2 standard

(Guide for Determining the E�ects of Ionising Radiation on Insulating Materials: Part 2: Procedures for Irradiation

and Test, 2nd ed., 1991-08) indicates a list of tests in which it is possible to select the methods which are sensitive to

ageing conditions and representative of the given application. Ó 1999 Elsevier Science B.V. All rights reserved.

Keywords: Polymers; Electrical equipment; Nuclear power plants; Lifetime prediction

1. Introduction

To predict the lifetime of electrical equipmentin nuclear power plants, experiments can be doneeither on the electrical equipment, or on materials.

Whatever the sampling, the environment of thematerial during ageing must be equivalent. Thechoice of the sampling depends on the followingparameters: the price of the electrical equipment,the facility or di�culty for sampling, the sample

Nuclear Instruments and Methods in Physics Research B 151 (1999) 449±452

www.elsevier.nl/locate/nimb

0168-583X/99/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved.

PII: S 0 1 6 8 - 5 8 3 X ( 9 9 ) 0 0 1 0 0 - 7

Page 2: Ageing of polymers in electrical equipment used in nuclear power plants

dimensions required for characterisation afterageing. In EDF laboratories, cables and pieces ofelectrical equipment have been aged under normaland accidental conditions: joints for pressurecaptors, reinforcing pieces for electrical equip-ment. To simulate normal ageing under gammaradiation, quali®cation standards of electricalequipment recommend the application of only onedose rate between 100 and 10000 Gy/h. If suchexperiments are available to determine the abilityof electrical equipment in nuclear environment,they are not su�cient to predict their lifetime. InEDF laboratories, ageing accelerated tests includ-ing a minimum of three dose rates are usuallycarried out in order to be able to have the lifetimeprediction in normal conditions between 0.001 and0.1 Gy/h. For accidental radiation conditionswhere the dose rate is su�ciently high to do theexperiment in a short time, the most representativetest is the real radiation one.

2. Usual methods for lifetime prediction

Two usual methods are recommended for thelifetime prediction in gamma radiation environ-ment:

2.1. Graphic method according to IEC 1244-2standard

This method is based on the extrapolation oftest data obtained under isothermal conditions inair. The test data obtained at di�erent dose ratesare used to determine the endpoint doses which areextrapolated graphically to the service dose rate.For example, we can use elongation at break as thecritical property as recommended in IEC 544.2 [1].If we assume the homogeneity of oxidationthrough the specimen thickness, we can plot therelative elongation at break e/e0 (e for the studiedmaterial aged during time t; e0 for non-aged ma-terial) versus absorbed dose. The end point criteriamight for example reach e/e0� 0.5. Then the doseat which the end point criteria is reached or thedose to equivalent damage DED is plotted versusthe dose rate in a log/log plot [2]. This plot is found

to be linear in some materials, enabling extrapo-lation to lower dose rates (see Fig. 1).

2.2. Kinetic method developed in EDF

The EDF model has been developed for longterm prediction in combined radiation and thermalenvironment. It is a kinetic model where theproperty P depends on the temperature T, the doserate I and the time t [3]:

P �t�=P0 � �1� �bÿ 1�Kt�T ; I�t�1=1ÿb:

P(t) is the property value at time t, P0 its initialvalue, and b the overall order of the degradationprocess.

The rate constant Kt(T,I) should be expressedas the sum of two terms, the one depending onlyon T, the second one depending on T and I:

Kt�T ; I� � Kth�T � � Kr�T ; I�;

Kt�T ; I� � k0t exp �ÿEa=RT � � k0rIa exp �ÿE0a=RT �;

where k0t and k0r are constant; Ea and E0a are ac-tivation energy for thermal and radiation ageing; ais a characteristic constant.

3. Application of the methods to silicone rubber in

electrical equipment

Silicone is a rubber used in electrical equipmentfor several applications: electrical insulation, wa-terproofness... Tensile test pieces have been aged inair at 70°C according to the following conditions[6]: 100, 1000 and 7000 Gy/h up to 250 kGy. Afterageing, elongation at break has been measured.Then, we have determined the end point dose ateach dose rate for the relative elongation e/e0� 0.5.

3.1. Lifetime prediction by the graphic methodaccording to IEC 1244.2 standard

First, the experimental Dose to EquivalentDamage or DED has been determined at 100, 1000and 7000 Gy/h (see Table 1).

450 R. Clavreul / Nucl. Instr. and Meth. in Phys. Res. B 151 (1999) 449±452

Page 3: Ageing of polymers in electrical equipment used in nuclear power plants

In a second step, the DED values have beenobtained from 0.001 to 0.1 Gy/h by the graphicmethod based on the extrapolation of the test dataobtained under isothermal conditions at 70°C (seeFig. 1).

The Time to Equivalent Damage or TED (seeTable 2) can be estimated from DED values: thelifetime prediction of silicone rubber estimated bythe graphic method in normal conditions in nu-clear plants environment is longer than 45±50years for each dose rate below 0.1 Gy/h.

3.2. Lifetime prediction by the kinetic method

The DED has been determined for e/e0� 0.5reached at experimental dose rates (see Table 1).From the experimental data, the kinetic model hasbeen obtained; it is described below:

e=e0 � �1� �bÿ 1�Kt�T ; I�t�1=1ÿb;

Table 2

Dose and time to equivalent damage (DED±TED) and lifetime

prediction (LP) of silicone

Dose rate DEDa TEDa LPb

(Gy/h) (kGy) (years) (years)

0.1 40 �45 >50

0.01 34 388 >50

0.001 28 3000 40 < LP <50

a Graphic method.b Kinetic method.

Fig. 1. Dose to equivalent damage (DED) of silicone versus the dose rate.

Table 1

Experimental dose to equivalent damage (DED) of silicone

Dose rate (Gy/h) DED (kGy)

7000 100

1000 85

100 70

R. Clavreul / Nucl. Instr. and Meth. in Phys. Res. B 151 (1999) 449±452 451

Page 4: Ageing of polymers in electrical equipment used in nuclear power plants

e=e0 � �1� �bÿ 1�3:23

� 10ÿ4I0:9 exp�247=T �t�1=1ÿb

with 2 < b < 3:It has then been possible to estimate the lifetime

of silicone rubber for each combined conditions oftemperature and dose rate (see Table 2): the life-time prediction of silicone estimated by the kineticmethod in nuclear environment at 70°C in therange of dose rates of 0.001±0.1 Gy/h is againlonger than 40±50 years.

4. Limits of the methods

Both methods give similar results for the life-time prediction of silicone rubber in normal con-ditions in nuclear plants environment. The graphicmethod is rapid and simple and is recommendedfor the study of radiation e�ect under isothermalconditions. The kinetic method allows to predictthe lifetime of polymers in combined radiation andthermal ageing. But these two methods have thefollowing limits:· a minimum of three to four experimental DED

obtained at three-four dose rates is necessary tohave a good lifetime prediction of any polymerin nuclear environment; to study the e�ect oftemperature, three to four experimental condi-tions with di�erent temperatures has to be used;

· extrapolation low dose rates by graphic methodor kinetic method is possible only if the radia-tion ageing is dominant compared to the ther-mal ageing. It is the case for silicone rubberwhich is stable at temperatures higher than130°C. Extrapolation to very dose rates (<0.05Gy/h) is of course not representative of thepolymer degradation; lifetime of several centu-ries are not realistic;

· the experimental radiation conditions must besu�ciently low in order to get an homogeneousoxidation of the polymer during ageing and toassume that the mechanisms with acceleratedtests are similar to those in normal conditions.Complementary tests such as infrared spectros-

copy, dynamic mechanical analysis or hardnessmeasurement are recommended to check thehomogeneity of the material after acceleratedageing;

· it has been shown by other authors that homo-geneous oxidation is not always easy to achieve;in this case the graphic method is not usable be-cause the experimental points do not ®t astraight line [4]. Moreover, other thermal e�ectsappear sometimes, and the kinetic method musttake into account several activation energies [5].

5. Conclusion

Two di�erent methods were used to extrapolateexperimental data to operating conditions, and topredict the lifetime of a silicone rubber in electricalequipment of nuclear power plants: the graphicmethod recommended by IEC 1244.2 standard,and a kinetic model developed in EDF. The twomethods agree with each other and give similarresults; they may be recommended for lifetimeprediction of silicone rubber in radiation envi-ronment. Further investigation will try to check ifthe methods are applicable to other commonlyused rubbers and plastics.

References

[1] Guide for Determining the E�ects of Ionising Radiation on

Insulating Materials: Part 2: Procedures for Irradiation and

Test, 2nd ed., 1991±08.

[2] Determination of Long-Term Radiation Ageing in Poly-

mers: Part 2: Procedures for Predicting Ageing at Low Dose

Rates, 1st ed., 1006±02.

[3] B. Pinel, F. Boutaud, A practical model for the lifetime

prediction of LOCA (EPDM-CSPE) and (PVC) cables in

nuclear power stations, JICABLE, Versailles, France, 1995.

[4] Ch. Chevalier, K. Coste, A. Fontaine, M. Tavlet, these

Proceedings (IRaP'98), Nucl. Instr. and Meth. 151 (1999)

438.

[5] M. Celina, K.T. Gillen, J. Wise, R.L. Clough, Anomalous

aging phenomena in a crosslinked polyole®n cable insula-

tion, Radiat. Phys. and Chem. (1996).

[6] R. Clavreul, Nucl. Instr. and Meth. B 131 (1997) 192.

452 R. Clavreul / Nucl. Instr. and Meth. in Phys. Res. B 151 (1999) 449±452