Determination of long-term radiation ageing in polymers - Part 4: Effects of different temperatures and dose rates under radiation conditions

IEC TR 61244-4:2019(E) provides general guidance for the evaluation/verification of electrical insulation materials (EIM) and electrical insulation systems (EIS) intended to be used in types of equipment exposed to ionizing radiation. Beside sensors, actuators/motors as well as plugs and terminals, cables are a well-known typical application of those EIM and EIS. Their type spectrum covers low voltage power cables, control cables and instrumentation cables. Because of their comparable simple design, cables are the ideal type of equipment to study EIM and EIS degradation processes. But the results of these studies can be easily transferred to the enumerated types of equipment.
Nonetheless, this document provides a state-of-the art report on qualification/verification procedures used to simulate simultaneous effects of temperature and radiation at varying intensities rather than give detailed test programmes valid for specific test methods.
NOTE 1 Use of this document with specific products can require specification of additional product related procedures.
NOTE 2 Some of the procedures described in this document are emerging technologies. Therefore, specified prerequisites, former experiences as well as boundary conditions can be additionally taken into account.

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Status
Published
Publication Date
24-Sep-2019
Current Stage
PPUB - Publication issued
Completion Date
25-Sep-2019
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IEC TR 61244-4
Edition 1.0 2019-09
TECHNICAL
REPORT
Determination of long-term radiation ageing in polymers –
Part 4: Effects of different temperatures and dose rates under radiation
conditions
IEC TR 61244-4:2019-09(en)
---------------------- Page: 1 ----------------------
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---------------------- Page: 2 ----------------------
IEC TR 61244-4
Edition 1.0 2019-09
TECHNICAL
REPORT
Determination of long-term radiation ageing in polymers –
Part 4: Effects of different temperatures and dose rates under radiation
conditions
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 17.240; 29.035.01 ISBN 978-2-8322-7429-3

Warning! Make sure that you obtained this publication from an authorized distributor.

® Registered trademark of the International Electrotechnical Commission
---------------------- Page: 3 ----------------------
– 2 – IEC TR 61244-4:2019 © IEC 2019
CONTENTS

FOREWORD ........................................................................................................................... 3

INTRODUCTION ..................................................................................................................... 5

1 Scope .............................................................................................................................. 6

2 Normative references ...................................................................................................... 6

3 Terms, definitions and abbreviated terms ........................................................................ 6

3.1 Terms and definitions .............................................................................................. 6

3.2 Abbreviated terms ................................................................................................... 7

4 Radiation induced degradation mechanisms at standard ambient conditions .................... 8

4.1 General conventions ............................................................................................... 8

4.2 Effect of presence of oxygen ................................................................................... 9

4.3 Effect of dose-rate effect-1: Physical aspects .......................................................... 9

4.4 Effect of dose-rate-2: Chemical aspects ................................................................ 10

4.5 Research on degradation mechanism ................................................................... 10

4.6 Experiences and acceleration factors .................................................................... 11

4.7 Low dose-rate tests and LOCA survivability .......................................................... 11

4.8 Effect of LOCA test environment ........................................................................... 11

4.9 Order effects, synergistic effects, and antagonistic effects .................................... 12

4.10 Sequential test conditions equivalent to simultaneous methods............................. 12

4.11 Studies after TMI and later .................................................................................... 13

4.12 Arrhenius law and limitations ................................................................................ 13

4.13 Slow degradation behaviour observed in service-aged cables ............................... 16

4.14 Inverse temperature effects .................................................................................. 17

4.15 Role of antioxidants .............................................................................................. 17

4.16 Other thermal and radiation environment degradation characteristics .................... 18

5 Accelerated ageing test methods ................................................................................... 18

5.1 Sequential ageing test .......................................................................................... 18

5.1.1 General ......................................................................................................... 18

5.1.2 Sequence of testing and synergistic effects ................................................... 19

5.1.3 Guidance on test sequences .......................................................................... 20

5.1.4 Assessment of accelerated ageing................................................................. 20

5.2 Simultaneous ageing test ...................................................................................... 21

6 Conclusion .................................................................................................................... 22

Bibliography .......................................................................................................................... 23

Table 1 – Dose rate conditions which do not cause inhomogeneous degradation .................. 10

Table 2 – E value according to materials, manufacturers, and grades ................................. 14

Table 3 – Changes in various properties due to inverse temperature and annealing

recovery effects, investigated by several instrumental analyses [111]. .................................. 17

Table 4 – Typical standard acceleration ageing sequence for qualification ............................ 19

Table 5 – More recent standard acceleration ageing sequence for qualification .................... 19

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IEC TR 61244-4:2019 © IEC 2019 – 3 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
DETERMINATION OF LONG-TERM RADIATION AGEING IN POLYMERS –
Part 4: Effects of different temperatures
and dose rates under radiation conditions
FOREWORD

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The main task of IEC technical committees is to prepare International Standards. However, a

technical committee may propose the publication of a Technical Report when it has collected

data of a different kind from that which is normally published as an International Standard, for

example "state of the art".

IEC TR 61244-4, which is a Technical Report, has been prepared by IEC technical committee

112: Evaluation and qualification of electrical insulating materials and systems.

The text of this Technical Report is based on the following documents:
Draft TR Report on voting
112/442/DTR 112/446/RVDTR

Full information on the voting for the approval of this Technical Report can be found in the

report on voting indicated in the above table.
---------------------- Page: 5 ----------------------
– 4 – IEC TR 61244-4:2019 © IEC 2019

This document has been drafted in accordance with the ISO/IEC Directives, Part 2.

A list of all parts in the IEC 61244 series, published under the general title Determination of

long-term radiation ageing in polymers, can be found on the IEC website.

The committee has decided that the contents of this document will remain unchanged until the

stability date indicated on the IEC website under "http://webstore.iec.ch" in the data related to

the specific document. At this date, the document will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
A bilingual version of this publication may be issued at a later date.
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IEC TR 61244-4:2019 © IEC 2019 – 5 –
INTRODUCTION

IEC 60216 (all parts) and IEC 60544 (all parts) give reference and guidance for managing

accelerated thermal and radiological ageing steps for type testing procedures applicable to

electrical insulating materials. The actual application of electrical equipment usually requires

the consideration of effects which are a consequence of simultaneous occurrence of

temperature and radiation at varying intensities.

The CIGRE WG D1.42 study presents degradation data in particular with respect to cable and

wire insulation materials gathered from tests where thermal and radiation loads were applied

simultaneously. Even if there is a broad range of materials available from the industry, only

insulation materials commonly used were selected for this study. These materials are

crosslinked polyethylene (XLPE), ethylene-propylene-rubber (EPR), silicon-rubber (SIR) and

polyvinylchloride (PVC). Using these test data, power plant operators were in the position to

meet requirements defined by regulatory bodies in the frame of ‘long term operation

application’, showing that most insulation materials which have been in operation for 30 to 40

years were in good condition. Furthermore, material samples were collected from real

positions and test results were compared with reference samples, unaged as well as

artificially aged.

The main objective of the industry is to yield reliable values of the residual lifetime of the

insulation materials and linked pieces of equipment made up of these materials. However

more research is necessary as the in-service degradation of insulating materials appears to

be deviating from estimation based on accelerated ageing tests. For a better determination of

the degradation processes of insulation materials it is important to gain a wider knowledge on

material degradation and linked synergistic effects at low intensities of thermal and

radiological loads. Thus, this document aims to summarize the results, and in some areas

update the literature references, from CIGRE WG D1.42, to provide a state-of-the-art

document on qualification procedures capable to represent multifactor ageing (hereby thermal

and radiological ageing).
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– 6 – IEC TR 61244-4:2019 © IEC 2019
DETERMINATION OF LONG-TERM RADIATION AGEING IN POLYMERS –
Part 4: Effects of different temperatures
and dose rates under radiation conditions
1 Scope

This part of IEC 61244 provides general guidance for the evaluation/verification of electrical

insulation materials (EIM) and electrical insulation systems (EIS) intended to be used in types

of equipment exposed to ionizing radiation. Beside sensors, actuators/motors as well as plugs

and terminals, cables are a well-known typical application of those EIM and EIS. Their type

spectrum covers low voltage power cables, control cables and instrumentation cables.

Because of their comparable simple design, cables are the ideal type of equipment to study

EIM and EIS degradation processes. But the results of these studies can be easily transferred

to the enumerated types of equipment.

Nonetheless, this document provides a state-of-the art report on qualification/verification

procedures used to simulate simultaneous effects of temperature and radiation at varying

intensities rather than give detailed test programmes valid for specific test methods.

NOTE 1 Use of this document with specific products can require specification of additional product related

procedures.

NOTE 2 Some of the procedures described in this document are emerging technologies. Therefore, specified

prerequisites, former experiences as well as boundary conditions can be additionally taken into account.

2 Normative references

The following documents are referred to in the text in such a way that some or all of their

content constitutes requirements of this document. For dated references, only the edition

cited applies. For undated references, the latest edition of the referenced document (including

any amendments) applies.

IEC 60544 (all parts), Electrical insulating materials – Determination of the effects of ionizing

radiation

IEC TS 61244 (all parts), Determination of long-term radiation ageing in polymers

3 Terms, definitions and abbreviated terms
3.1 Terms and definitions

For the purposes of this document, the terms and definitions given in IEC TS 61244 (all parts)

and IEC 60544 (all parts) apply.

ISO and IEC maintain terminological databases for use in standardization at the following

addresses:
• IEC Electropedia: available at http://www.electropedia.org/
• ISO Online browsing platform: available at http://www.iso.org/obp
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IEC TR 61244-4:2019 © IEC 2019 – 7 –
3.2 Abbreviated terms
Abbreviated term Definition
ACA assessment of cable ageing for nuclear power plants
AO anti-oxidant
CEA Commissariat à l'Energie Atomique
CERN European Organization for Nuclear Research
CRIEPI Central Research Institute of Electric Power Industry
CSPE chrolo-sulphonated polyethylene
DED dose to equivalent damage
DLO diffusion-limited oxidation
DOR Department of Operation Reactor
DSC differential scanning calorimetry
EAB elongation at break
EdF Electricité de France
EIM electrical insulation materials
EIS electrical insulation systems
EPDM ethylene propylene diene rubber
EPR ethylene propylene rubber
EPRI Electric Power Research Institute
EQ environmental qualification
FR flame retardant
HDPE high density polyethylene
HELB high energy line break
IAEA International Atomic Energy Agency
IEEE Institute of Electrical and Electronics Engineers
IEEJ Institute of Electrical Engineers of Japan
IH inhibitor (antioxidant)
IR infrared
IRSN Institut de Radioprotection et de Sûreté Nucléaire
JAEA Japan Atomic Energy Agency
JAERI Japan Atomic Energy Research Institute
JAMPSS Japan Ageing Management Program on System Safety
JNES Japan Nuclear Energy Safety Organization
LDPE low density polyethylene
LET linear energy transfer
LOCA loss of coolant accident
MSLB main steam line break
NISA Nuclear and Industrial Safety Agency
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– 8 – IEC TR 61244-4:2019 © IEC 2019
Abbreviated term Definition
NPP nuclear power plant
NRC Nuclear Regulatory Commission
ORNL Oak Ridge National Laboratory
PE polyethylene
PVC polyvinyl chloride
National Institutes for Quantum and Radiological Science and
QST
Technology
SIR (SiR) silicone rubber
SNL Sandia National Laboratories
TED time to equivalent damage
TMI Three Mile Island
XLPE cross-linked polyethylene
XLPO cross-linked polyolefin
4 Radiation induced degradation mechanisms at standard ambient conditions
4.1 General conventions

The effects of radiation on polymer materials have been studied in both material reformation

and degradation since the 1950s [1-3] . Specific attention has been paid to polymer materials

used for electrical insulation and jackets of cables in the 1960s, where commercial nuclear

power plants were designed and built extensively [3]. The first results describing the

behaviour of insulation materials were reported from ORNL [4-6] and Harwell Atomic Energy

Research Establishment [7].

Secondary electrons can be generated in the bulk material due to the photoelectric effect, the

Compton effect, etc., when polymer materials are exposed to ionizing radiation. These

electrons can induce molecular ionization and electronic excitation [8]. Such physical

phenomena were believed to be the main cause of material degradation at that time, because

radiation penetrated well into the bulk material. It was also assumed that the contribution of

radiation to degradation was far greater than from heat for the same reason [9]. Accelerated

degradation through the presence of oxygen was also known, but this factor was

underestimated in fundamental research. Chain scission was considered to occur only in the

presence of oxygen, and cross-linking was assumed in non-oxygen atmospheres [3,10].

These early studies focused on the physical effects of radiation such as collision cross-

sections and linear energy transfers (LET) [3]. All studies of academic interest were thought to

be completed in 1967 [2]. However, the importance of oxidation had still not been identified

and most experiments were conducted in vacuum or under an inert atmosphere. As a result,

dose rate effects [9] were not considered and accelerated ageing experiments were performed

at high dose rates based on the “equal dose equal damage” concept. Permissible total

radiation dose [11] was adopted to define the radiation resistance of polymers. Such

databases can be found in various reports from ORNL [4-6], CERN [12-15], JAEA (current

QST) [16], and EPRI [17] and in many other literatures [18-25].
_____________
Numbers in square brackets refer to the bibliography.
---------------------- Page: 10 ----------------------
IEC TR 61244-4:2019 © IEC 2019 – 9 –

The ‘equal dose equal damage’ concept was also reflected in the IEEE type test standard [26]

for qualification of safety-related equipment in nuclear power plants, and accelerated ageing

with dose rates up to 10 kGy/h was accepted. The over dose approach, which uses a greater

amount of radiation than the in-service simulated total dose, was also used at this time to

improve maintainability. This approach was also used to compensate for uncertainties in the

knowledge at the time [27]. SNL strongly emphasized that these tests did not take into

account dose-rate effects [28].
4.2 Effect of presence of oxygen

Radiation is one source of radical generation and chemical bond breaking, and heat may

generate radical, while heat assists oxidation reactions. Material degradation occurs via

synergistic effects of radical generation and oxidation based on an auto-oxidation scheme.

Mechanical properties such as tensile strength do not exhibit significant degradation [29]

under irradiation in an inert gas environment. EAB, resistivity, and dielectric loss (tanδ) do not

, whereas significant adverse effects for these properties
change when irradiation occurs in N

are observed when irradiation occurs in air [30]. Furthermore, infrared (IR) absorption due to

carbonyl groups is small immediately after a material is irradiated, but the IR peak would be

increased by subsequent thermal ageing in a sequential accelerated ageing test. This

generation of carbonyl groups correlates with decrease in EAB [29]. Seguchi et al. have

reported that 80 % to 90 % of oxygen that contributes to oxygen reactions would be

accumulated in the bulk, and not less than 80 % of that forms carboxylic acid [10].

4.3 Effect of dose-rate effect-1: Physical aspects

It is widely known that the DED value decreases as the dose rate decreases, and this

phenomenon is called a dose-rate effect, irrespective of the causes. Put in other words, whilst

strong radiation induces severe material degradation, if total dose is used as the base, a

lower radiation environment can lead to more severe degradation [31] at a given total dose.

Such dose-rate effects were identified as early as 1956 at the basic research level

[6, 22, 32-34]. Wilski et al. have pointed out that inhomogeneous degradation in bulk was the

cause [35-37]. Gillen et al. experimentally clarified the relation between the inhomogeneous

degradation and decrease in EAB [38,39]. This inhomogeneous degradation is caused by

DLO, which occurs if the oxidation reaction rate is faster than the oxygen supply from the

outside air and its diffusion into the bulk of the material. Inhomogeneous degradation is also

observed in a high-temperature environment even if no radiation is present [40]. Gillen et al.

also quantitatively demonstrated that the rate of oxygen supply from air is decreased with

oxidation, which results in enhancing DLO in the subsequent degradation process [40].

Further advancements in the characterization of DLO and its effect on lifetime predictions

under accelerated thermal ageing was provided in a seminal paper by Gillen et al. [41].

Dose-rate effects in a fluoro-rubber has been investigated [42] and found that degradation

processes (embrittlement or flexibility) are dependent on radiation dose-rates. Therefore,

accelerated ageing tests that give rise to inhomogeneous degradation were suspected to

simulate the ageing process in service. It has been commonly recognized that accelerated

ageing tests should be carried out under conditions that do not generate DLO. Limits of test

conditions that give homogeneous degradation have been already summarized elsewhere;

one of the examples listed in IEEE Std 775 [43] is shown in Table 1. Conditions to prevent

DLO can also be investigated by analytical calculation as detailed by Gillen et al. [41].

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– 10 – IEC TR 61244-4:2019 © IEC 2019
Table 1 – Dose rate conditions which do not cause inhomogeneous degradation
Dose rate
Gy/h
Sheet thickness
Material
0,5 mm 1,0 mm 1,5 mm 2,0 mm
HDPE 50 13 5,8 3,2
LDPE 440 110 49 27
EPR 4 800 1 200 530 300
EPDM 2 100 520 230 130
Hypalon 1 200 300 130 75
Neoprene 520 130 58 32
SIR 5 100 1 300 580 320
PVC 440 110 49 27
NOTE Irradiation temperature is 25 °C [43].
4.4 Effect of dose-rate-2: Chemical aspects

It is clear that lifetime or so-called TED would show a constant value when the radiation dose-

rate is low enough that the thermal oxidation reaction is dominant for material degradation.

Since constant TED values induce a ‘thermal line’ in DED curves, these exhibit so-called

chemical dose-rate effects [39,44], to be distinguished from the DLO-derived physical dose-

rate effects.

Initially, this chemical dose-rate effect was not recognized. Gas analysis, frequently used at

the beginning of investigations, showed that oxygen absorption increased with a decrease in

dose rate [37]. The oxidation reaction was not accelerated when the oxygen concentration

was increased [10], although there is sparse supporting data for this observation at present. It

has also been reported that the chemical dose-rate effects were no longer observed if

polymeric materials are stabilized by inhibitors [45]. IEEJ technical report [11] recognized the

importance of dose-rate effects only after such phenomena were confirmed by long-term tests

performed at dose-rates as low as several tens of mGy/h in the US and France [46,47] or by

EQ tests on XLPO cables [48].
4.5 Research on degradation mechanism

Chemical reaction is considered as the mechanism of degradation in insulating polymeric

materials since the above-mentioned two types of dose rate effects have been established

and the importance of oxygen recognized. Further, irradiated materials are observed to

degrade without exhibiting an induction period if they are subsequently heated [28].

Degradation factors appear to accumulate in the bulk during irradiation. Peroxide species

(ROOH) are reported as such factors; for example, thermal decomposition of ROOH would

promote an auto-oxidation chain reaction, resulting in thermal degradation of PE [49-52].

Clough et al. have found that removing ROOH by phosphine gas suppresses degradation [53].

It is also reported that radiation ageing followed by thermal ageing for sequential accelerated

ageing generates rapid degradation, even when one to six months have passed between

radiation and thermal ageing. The accumulated degradation factor is considered to be stable

ROOH because radicals directly generated by radiation are generally difficult to sustain for

such a long period. Activation energy (E ) values obtained for EAB measurements [54,55]

coincide with that for ROOH decomposition [29,53]. This also suggests that ROOH is the rate-

controlling factor in material degradation, which also supports the use of an auto-oxidation

scheme that encompasses ROOH.
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IEC TR 61244-4:2019 © IEC 2019 – 11 –

Fuse et al. have recently proposed a new model with taking into account the influence of

antioxidants (see 4.15), of various temperature effects and divides the effects of thermal and

radiation on radical formation. This work highlighted the need for further work in these areas

to refine their auto-oxidation model [56].
4.6 Experiences and acceleration factors

Attention was drawn to the ability of safety-related equipment to continue to function during

LOCA conditions by surveys disclosed to the public by the mid-1980s [11]. Cables are

simultaneously exposed to high-temperature steam and strong radiation in the LOCA

environment, so attention was focused on synergis
...

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