IEC TR 61244-4:2019

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.

General Information

• Status: Published
• Publication Date: 24-Sep-2019
• ICS: 01 - GENERALITIES. TERMINOLOGY. STANDARDIZATION. DOCUMENTATION
• ICS: 17.240 - Radiation measurements
• ICS: 29.035.01 - Insulating materials in general
• Technical Committee: TC 112 - Evaluation and qualification of electrical insulating materials and systems
• Drafting Committee: WG 2 - TC 112/WG 2
• Current Stage: PPUB - Publication issued
• Start Date: 25-Sep-2019
• Completion Date: 21-Oct-2019

Standards Content (Sample)

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
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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
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 17.240; 29.035.01 ISBN 978-2-8322-7429-3

– 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
a
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

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.

– 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.

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).
– 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

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

– 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.

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].

– 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]
a
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.
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 synergistic effects in this complex environment [3].
The order effect is synergistic and was an issue related to the sequential type test, as
indicated in the phase of basic studies. Consideration of these effects lead to concerns that
the accelerated ageing test conditions selected would have resistance against LOCA
conditions. Acceleration factors, which are roughly the ratio of type test time to the assumed
actual in-service period, has been recognized to be too high since at least 1990. This concern
is addressed in 4.7 below.
4.7 Low dose-rate tests and LOCA survivability
The US NRC, SNL and CEA have studied the effects of accelerated test conditions on LOCA
durability, by exposing various cables to mild irradiation conditions from 5 Gy/h to 100 Gy/h
and temperatures from 40 °C to 70 °C for up to five years [46]. One of the results showed that
accelerated ageing tests give rise to material degradation, and that the subsequent LOCA test
further degrades functional characteristics. Most cables investigated maintained insulation
functions even after the LOCA test, but one of the XLPE cables degraded excessively and
failed.
Concerns about the resistance to LOCA conditions against material degradation due to low
dose rate irradiation were raised by several SNL studies [57-59]. The US NRC pointed out the
possibility that insulations of multi-conductor cables swelled from jackets during the LOCA
period, and the swelling point could be the failure point. The study further concluded that
cables with a jacket bonded to the insulation provide low durability against LOCA [60]. In the
US, Wyle Laboratories aged insulating materials with an equivalent of 60 years of in-service
exposure and found that increase in leakage current and dielectric breakdown strength
occurred in some cables after the LOCA test. Based on this finding, the US NRC reported that
extending cable life might increase degradation of the material [60].
Germany and France conducted long-term irradiation tests over ten years [61,62]. EPRI has
also conducted a long-term irradiation test and installed sample cables in nuclear plants that
were also evaluated as part of their studies [61], [63-65]. No property changes due to in
service ageing have been found for these samples, but the evaluation is still underway.
CERN’s long-term test results are summarized in the literature [66,67].
4.8 Effect of LOCA test environment
Equipment to perform simultaneous heat and radiation irradiation called SEAMATE-II was

to verify the equivalence of the
built in former JAERI Takasaki in 1979 [47], [68-71]
simultaneous and sequential methods. The US also conducted similar studies and reported no
differences between the two kinds of testing methods in the case of LOCA conditions [58].
This result was attributed to the environments in which the test objects were irradiated; cable
samples were irradiated under oxygen-free conditions in the simultaneous method, whereas
irradiation in the sequential method was performed in air. This difference in atmosphere
derives from the process that oxygen-decomposed species are expelled from autoclave
equipment together with compressed steam and oxygen [72]. Thus, it was reported that the
sequential method provided a more severe environment. The effect of oxygen concentration
during LOCA tests was also investigated in the above-mentioned NRC/SNL/CEA joint
research [46]. The results indicated that oxygen concentration did not affect the performance
decrease in XLPE, ethylene tetrafluoroethylene, SIR, EPR, and FR-XLPE, although
performance was affected in the case of chloroprene rubber. The research has suggested the

– 12 – IEC TR 61244-4:2019 © IEC 2019
necessity to evaluate the temporal change in the oxygen concentration in containment during
a LOCA period. It was also reported that material degradation tends to be alleviated under
high humidity [3]. Chemical spray used as a coolant in the pressurized water reactor LOCA
test was reported to have small effect on material degradation compared to γ-ray irradiation or
steam temperature [3,47,73,74].
4.9 Order effects, synergistic effects, and antagonistic effects
As has already been discussed above, radiation induces radical formation, which is the
ignition process of chemical reactions, whereas heat may generate radicals. It follows that
degradation is enhanced by the reverse (backward) sequential method in which radiation is
performed in advance, rather than by the normal sequential method, at least qualitatively. The
optical absorption of carbonyl groups that is frequently used as a degradation index [28]
remains small under irradiation at room temperature, but increases by thermal degradation
thereafter [29]. However, material degradation is also greatly affected by specimen shape,
manufacturing process, material, composition, humidity, oxygen, temperature, etc. As a result,
order effects in sequential tests and thermal and radiation synergistic effects were not
recognized for some time. Synergistic effects were confirmed with CSPE but not with EPR in
the literature [46]. As these two kinds of radiation effects have gradually been observed in
many other materials, both have been considered to be generated by the same mechanism
[38].
Order effects also affect the type test standardized by IEEE, since this standard does not
specify the test sequence [53]. Concerns about the reproducibility of in-service environment
by sequential methods would also arise in the case that degradation behaviour exhibits strong
synergistic effects [29]. The issue becomes complex in a high-temperature ageing
environment, because heating would also take part in radical generation. Clarifying the
reaction rate and its activation energy (E ) against relevant elementary chemical reactions is
a
necessary to quantitatively evaluate the contribution of heat and radiation but would be
difficult.
The technical reports published by IEEJ [11,47] have recognized from studies at SNL [75] that
degradation in many polymeric materials becomes more severe in the reverse (backward)
sequential method than in the forward method. The report [11] has also mentioned that
irradiation should be performed first in order to follow the NRC DOR guidelines [76]. It is also
worth noting that stress factors under in-service environment are not yet completely
understood, and that difficulties still need to be overcome for the present ageing test
procedures to provide lifetime estimation [11]. An accelerated ageing test is considered as a
qualitative screening technique [11].
In a series of experiments evaluating the lifetime evaluation of NPP cables a reverse
(backward) sequential (radiation then thermal) ageing combination was found to be more
severe than the more common sequential order (thermal then radiation). The simultaneous
ageing was found to be in between these two approaches in terms of severity. This work
highlighted the importance of oxidation and the influence of antioxidants in these results [77].
4.10 Sequential test conditions equivalent to simultaneous methods
The biggest issue, yet to be resolved, in synergistic and order effects is to identify test
conditions for the reverse (backward) sequential ageing test equivalent to those in
simultaneous methods. Literature [78,79], has recommended carrying out heat ageing after
irradiating materials at 70 °C to simulate degradation by the simultaneous method. It is also
effective to irradiate with a dose rate of 650 Gy/h and then perform heat ageing at 120 °C, or
to irradiate materials with 1,2 kGy/h and heat at 140 °C. The maximum acceptable dose rate
to induce homogeneous oxidation within EPR and XLPE materials was found to depend on
irradiation temperature. The temperature influences the rate of diffusion of oxygen into the
material as noted in 4.3. Hence, a dose rate of 50 Gy/h was found to be acceptable for 1 mm
thick XLPE at room temperature. This maximum dose rate increased to 800 Gy/h if the
irradiation took place at 100 °C. Much higher values were obtained for EPR material which
exhibits a higher oxygen diffusion rate. However, the higher irradiation temperature can also

induce additional chemical reactions which can impact the ageing mechanisms. Based on
these findings the irradiation temperature should not exceed 100 °C [80].
4.11 Studies after TMI and later
There have been many long-term studies investigating low dose-rate radiation since the early
1990s. This research was prompted by the standardization of several time-temperature-dose
rate superposition methodologies which include the one named ‘superposition of DED data’ by
IEC and IAEA. These two organizations have also standardized evaluation and monitoring
methods for material degradation, due to the concern over nuclear accidents (e.g. Chernobyl).
As long-term irradiation tests require years for evaluation, the amount of research has
gradually decreased. In the case of Japan, the Nuclear Technology Assessment Committee of
the Ministry of Economy, Trade and Industry designated evaluation technology for cable
degradation as one of the urgent implementation items in ageing management in 1999. The
project called ‘Assessment of Cable Ageing for Nuclear Power Plants (referred to as ACA)’
was performed by JNES [81].
The objective of the ACA research was to validate the Japanese national standard for type
testing, in response to international trends in low dose-rate accelerated ageing procedures.
Therefore, low dose-rate irradiation was performed on safety-related cables used in Japanese
NPPs. As a result, it was found necessary to bend the Arrhenius line in order to correlate in-
service cable conditions to test results obtained between 100 °C and 130 °C. Studies up to
then had focused on radiation effects and temperature characteristics which assumed the
normal Arrhenius law. This bending called ‘non-Arrhenius behaviour’ was also approved by
SNL [82]. However, SNL also suggested that the ACA research reconsider the following two
points in their evaluation. First, the use of tentative activation energy value (E ) with respect
a
to polymeric materials in which the in-service data did not indicate significant degradation.
Second, no inverse temperature effect was taken into account with respect to crystalline
polymers like XLPE and EPR.
4.12 Arrhenius law and limitations
Non-Arrhenius behaviour has also been recognized on the basic research level, similar to the
case for other irradiation effects described above. IEEJ technical reports [11,47] stated, with
respect to Arrhenius behaviour, that XLPO has a linear characteristic against thermal life
whereas EPR exhibits ‘poor’ linearity. JAEA also presented related studies in 1984 [9,83]. The
Arrhenius law is based on the assumption that the activated molecule concentration and the
molecular collision probability increase at a constant rate with increase in temperature
resulting in promotion of chemical reactions. Consequently, the ease of reaction changes, and
nonlinearity occurs, if ageing tests exceed any temperatures such as glass transition
temperature and/or melting point, no matter if they are of the base material or of additives
related to degradation.
The E values are also important together with other governing factors with respect to non-
a
Arrhenius behaviour. IRSN reported that in-service cable degradation takes place more
quickly than predicted by a conventional linear Arrhenius plot. This finding agrees with that
reported in the ACA project [81]. IRSN expressed concern that the uniform use of the E value
a
had a limited applicable range [84]. Table 2 summarizes the E values reported. Both the
a
E values and the temperature that exhibits the nonlinearity vary in materials, manufacturers,
a
and grades.
– 14 – IEC TR 61244-4:2019 © IEC 2019
Table 2 – E value according to materials, manufacturers, and grades
a
Low temperature High temperature
a
Grade Organization Reference
Manufacturer
E E
Temperature Temperature
a a
°C °C
kJ/mol kJ/mol
XLPE
Rockbestos 2/C #14 AWG   127 180 to 200 NRC 63
Brandrex CLPO 72 50 to 144 72 50 to 144 SNL 82
Surprenant
ITT   110 90 to 130 SNL 82
Exane II
54 to
Unknown Unknown 60 to 78 110 90 to 170 SNL 35, 85
Unknown Unknown   128 Unknown EPRI 86
(61 to
Company A CV  110 100 to 120 JNES 84
78)
Company B CV   100 100 to 120 JNES 81
Company C CV   95 100 to 120 JNES 81
Company A FR-CV   66 100 to 120 JNES 81
Company B FR-CV   84 100 to 120 JNES 81
EPR
American
3/C #16 AWG   119 141 to 160 NRC 9
Insulated Wire
Flameguard
Anaconda   106 100 to 170 SNL 87
14AWG
Durasheeth
Anaconda   100 99 to 139 SNL 87
12AWG
Elastoset
Eaton Dekoron   106 110 to 160 SNL 82
16AWG
Okonite 12AWG   89 100 to 140 SNL 82
Unknown Unknown 75 < 110 115 110< SNL 88
Unknown Unknown   116 Unknown EPRI 86
Unknown Unknown 58 < 120 126 120 < JAEA 89
Company C EP rubber   94.5 100 to 120 JNES 81
FR-EPR,
Company A   110 100 to 120 JNES 81
black
FR-EPR,
Company A   102 100 to 120 JNES 81
white
Company A FR-EPR, red   96 100 to 120 JNES 81
FR-EPR,
Company B   84 100 to 120 JNES 81
black
FR-EPR,
Company B   85 100 to 120 JNES 81
white
Company B FR-EPR, red   87 100 to 120 JNES 81

Low temperature High temperature
a
Grade Organization Reference
Manufacturer
E E
Temperature Temperature
a a
°C °C
kJ/mol kJ/mol
FR-EPR,
Company C   87 100 to 120 JNES 81
black
FR-EPR,
Company C   84 100 to 120 JNES 81
white
Company C FR-EPR, red   85 100 to 120 JNES 81
SIR
Company A KGB   47 135 to 175 JNES 81
Company B KGB   42 135 to 175 JNES 81

Company C KK   50 135 to 175 JNES 81
Chlorosulfonated
polyethylene
(Hypalon)
Flameguard
Anaconda   107 < 100 SNL 87
inner
Flameguard
Anaconda   107 < 100 SNL 87
outer
Kerite FR   107 < 100 SNL 87
Rockbestos Firewall III 88 40 to 100 107 < 100 SNL 87
Eaton Dekoron Elastoset   107 < 100 SNL 87
Brandrex Unknown   107 < 100 SNL 87
BIW Bostard 7E 88 82 to 100 107 < 100 SNL 87
Samuel Moore Dekoron 88 40 to 100 102 100 to 125 SNL 87
80 to 80 to
Unknown Unknown 22 to 125 22 to 125 SNL 90
105 105
Unknown Unknown 22 > 120 57 < 120 JAEA 91
Mitsubishi
Mitsubishi Cable Unknown 55 > 120 97 < 120 61
Cable
80 –
Unknown Unknown   unknown NRC 9
Unknown Unknown  > 100  < 120 EdF 92
Chloroprene
(Neoprene)
Okonite Unknown 89 24 to 70 71 70 to 131 SNL 82
Unknown 86 to
Burke Rubber   65
...