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Polymeric materials in nuclear power plants: Lifetime prediction, condition monitoring and simulation of ageing
KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Polymeric Materials.
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Nuclear power plants generate a significant part of the world’s electrical power consumption. However, many plants are nearing the end of their designed lifetime, and to extend the lifetime it is important to verify that every component can withstand the added service time. This includes polymeric materials, which become brittle with time. By predicting their lifetime and monitoring their condition, unnecessary downtime of the plant can be avoided, and secure operation can be ensured. The lifetime can be predicted by extrapolating results from accelerated ageing to service conditions, or by simulation of the degradation process.

In this study, lifetime predictions through extrapolation were performed on samples of a polyvinyl chloride (PVC) core insulation and an acrylonitrile butadiene rubber (NBR) membrane, which were thermally aged in air. The lifetime of the PVC cable was predicted using Arrhenius extrapolation, and using a method based on Langmuir, Clausius-Clapeyron, and Kirchhoff’s equations.

The lifetime of the NBR membrane was predicted using extrapolation in the temperature domain using an Arrhenius approach coupled with an extrapolation in pressure-domain, yielding realistic lifetimes.

Two cable insulations, one made from crosslinked polyethylene (XLPE) and the other from ethylene propylene rubber (EPR) were aged under the simultaneous effect of elevated temperature and γ-radiation investigated using several condition monitoring techniques. In particular, two non-destructive techniques, dielectric spectroscopy and nuclear magnetic resonance, showed promising results be developed and used in situ.

Finally, a computer model simulating the diffusion and consumption of oxygen in XLPE was developed, based on assumptions that diffusion, consumption and solubility were dependent on the total degree of oxidation. The model showed promise for further development.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2015. , 75 p.
Series
TRITA-CHE-Report, ISSN 1654-1081 ; 2015:29
National Category
Textile, Rubber and Polymeric Materials
Identifiers
URN: urn:nbn:se:kth:diva-168193ISBN: 978-91-7595-587-2 (print)OAI: oai:DiVA.org:kth-168193DiVA: diva2:814648
Public defence
2015-06-11, K2, Teknikringen 28, KTH, Stockholm, 13:00 (English)
Opponent
Supervisors
Note

QC 20150529

Available from: 2015-05-29 Created: 2015-05-27 Last updated: 2015-05-29Bibliographically approved
List of papers
1. Plasticizer migration from PVC cable insulation - The challenges of extrapolation methods
Open this publication in new window or tab >>Plasticizer migration from PVC cable insulation - The challenges of extrapolation methods
2014 (English)In: Polymer degradation and stability, ISSN 0141-3910, E-ISSN 1873-2321, Vol. 101, no 1, 24-31 p.Article in journal (Refereed) Published
Abstract [en]

A single strand PVC-P insulation including an internal metal conductor removed from the jacketed assemblies of a signal cable showed brittleness after 30 years service at 25 +/- 3 degrees C in air. The PVC compound contained diisodecyl phthalate (DIDP), di(2-ethylhexyl) phthalate (DEHP) and a sizeable fraction of filler. Single strand insulation samples with internal metal conductor were aged in air at elevated temperatures for different periods of time after which the strain at break, the Young's modulus and the plasticizer content were assessed by tensile testing and liquid chromatography. Isothermal evaporation rates from pristine DIDP and DEHP and solutions of the two plasticizers were obtained by thermogravimetry. Data for Young's modulus, strain at break and plasticizer contents were extrapolated to service temperature using two different extrapolation methods, Arrhenius extrapolation (constant activation energy) and a method based on models by Langmuir, Clausius-Clapeyron and Kirchhoff. These methods assume that evaporation of plasticizers to the surrounding gas phase is the dominant deterioration mechanism. Both methods predicted only a minor decrease in plasticizer content after 30 years of ageing at 28 degrees C and thus a material with adequate mechanical properties. Liquid chromatography showed that the single strand cable samples contained a very low DIDP content (4 wt.%) and an anomalously high DEHP content; a finding that cannot be explained by the expected evaporative loss mechanism. It is suggested that DIDP was efficiently extracted by contact with a DEHP-rich interface at the insulation surface, a process which is active during plant operation, but could not be simulated by controlled laboratory accelerated ageing studies.

Keyword
Poly(vinyl chloride), Di(2-ethylhexyl)phthalate, Diisodecyl phthalate, Ageing
National Category
Polymer Technologies
Identifiers
urn:nbn:se:kth:diva-145081 (URN)10.1016/j.polymdegradstab.2014.01.021 (DOI)000334000400004 ()2-s2.0-84896317790 (Scopus ID)
Funder
Swedish Radiation Safety Authority
Note

QC 20140509

Available from: 2014-05-09 Created: 2014-05-08 Last updated: 2017-12-05Bibliographically approved
2. Long-term performance of a DEHP-containing carbon-black-filled NBR membrane
Open this publication in new window or tab >>Long-term performance of a DEHP-containing carbon-black-filled NBR membrane
2014 (English)In: Polymer testing, ISSN 0142-9418, E-ISSN 1873-2348, Vol. 34, no 1, 25-33 p.Article in journal (Refereed) Published
Abstract [en]

A NBR membrane containing carbon black (36 wt.%) and di(2-ethylhexyl) phthalate (DEHP; 11 wt.%) that had been used at temperatures up to 45 C in pressurised air showed cracking after 2 years in service. Samples were aged in air at elevated temperatures and their mechanical properties were assessed by tensile testing, the glass transition temperature was obtained by DSC, and the DEHP content was determined by liquid chromatography. The loss of DEHP was controlled by the boundary conditions at low temperatures and the loss rate was constant at 90 C within a certain DEHP concentration range (8 to 11 wt.%). The presence of carbon black and DEHP made it impossible to determine oxidation products by infrared spectroscopy. Strain-at-break data were analysed in a way that enabled the effect of DEHP migration to be separated from the effect of thermal oxidation. This allowed extrapolation in both temperature and oxygen pressure domains of high temperature/low oxygen pressure data to the service conditions. The analysis showed that both DEHP evaporation and thermal oxidation had a significant impact on the strain-at-break, but that the latter was the more important. Data for the mechanical properties and the glass transition temperature indicated that oxidation was non-uniform with increasing depth in the specimens. This condition of the 4.5 mm thick samples meant that it was inappropriate to use the specimen Young's modulus for extrapolation purposes.

Keyword
Acrylonitrile butadiene rubber, Ageing, Di(2-ethylhexyl)phthalate, Extrapolation, Oxidation
National Category
Polymer Technologies
Identifiers
urn:nbn:se:kth:diva-136765 (URN)10.1016/j.polymertesting.2013.12.011 (DOI)000335426000005 ()2-s2.0-84892692690 (Scopus ID)
Note

QC 20140305. Updated from manuscript to article in journal.

Available from: 2013-12-09 Created: 2013-12-09 Last updated: 2017-12-06Bibliographically approved
3. Dielectric spectroscopy as a condition monitoring technique for cable insulation based on crosslinked polyethylene
Open this publication in new window or tab >>Dielectric spectroscopy as a condition monitoring technique for cable insulation based on crosslinked polyethylene
2015 (English)In: Polymer testing, ISSN 0142-9418, E-ISSN 1873-2348, Vol. 44, 135-142 p.Article in journal (Refereed) Published
Abstract [en]

Dielectric spectroscopy was evaluated as a condition monitoring technique for aged polyethylene electrical insulation in nuclear power plants. Bare core insulations of crosslinked polyethylene were aged at 55 and 85 °C under exposure to 60Co γ-radiation at different dose rates (0.42, 0.76 and 1.06 kGy h−1) to different total doses. The samples were studied by dielectric spectroscopy and tensile testing, and the crystallinity, mass fraction of soluble component and density were determined. The oxidation profiles along the depth of the insulations were assessed by infrared microscopy. The aged samples showed an increase in both the real and imaginary parts of the dielectric permittivity over the whole frequency range studied, an increase in the mass fraction of soluble component and in the material density, and a decrease in the strain-at-break. The imaginary part of the dielectric permittivity at 100 kHz increased in a linear fashion with increasing material density, the latter being strictly related to the extent of oxidation of the material according to infrared spectroscopy and differential scanning calorimetry. The generic relationship between the imaginary part of the permittivity and the density included all the data obtained under different ageing conditions. The results suggest that dielectric spectroscopy can be used for in-situ measurements of the degree of oxidation of polyethylene cables, in order to obtain information about the condition of the cable insulation to enable the remaining lifetime to be predicted.

Keyword
Polyethylene, Radiation ageing, Thermal ageing, Dielectric response, Density, Condition monitoring
National Category
Polymer Technologies
Identifiers
urn:nbn:se:kth:diva-168185 (URN)10.1016/j.polymertesting.2015.04.004 (DOI)2-s2.0-84928666071 (Scopus ID)
Funder
EU, FP7, Seventh Framework Programme
Note

QC 20150602

Available from: 2015-05-27 Created: 2015-05-27 Last updated: 2017-12-04Bibliographically approved
4. Non-destructive condition monitoring of aged ethylene-propylenecopolymer cable insulation samples using dielectric spectroscopy and NMR spectroscopy
Open this publication in new window or tab >>Non-destructive condition monitoring of aged ethylene-propylenecopolymer cable insulation samples using dielectric spectroscopy and NMR spectroscopy
Show others...
(English)Manuscript (preprint) (Other academic)
Abstract [en]

The causes of changes in dielectric response as a result of thermal and irradiative ageingof a cable insulation of ethylene propylene copolymer rubber containing 38 wt.% filler wereinvestigated. Samples were aged in three different combinations of irradiation dose rate andtemperature, 0.42 kGy h–1 at 85 °C, and 1.58 kGy h–1 at 55 and 85 °C, and subsequentlystudied by dielectric spectroscopy, NMR spectroscopy using a portable spectrometer andtensile testing. The extractable mass fraction and density were determined, and related to theimaginary part of the dielectric permittivity at 100 kHz. The ageing led to an increase in thedielectric permittivity, stiffness, density and degree of oxidation, together with a decrease inboth strain-at-break and relaxation time, as revealed by NMR spectroscopy. Except for thestrain-at-break, the properties changed in a linear fashion with increasing imaginary part ofthe dielectric permittivity at 100 kHz, with a particularly good agreement with respect to thedensity. As these properties are affected by the degree of oxidation, the results show that bothNMR using a portable spectrometer and dielectric spectroscopy can be used as conditionmonitoring techniques to detect the degree of oxidation in complex systems such as filledcopolymers.

Keyword
Ethylene propylene rubber, Radiation ageing, Thermal ageing, Dielectric response, Portable NMR spectrometer, Condition monitoring
National Category
Textile, Rubber and Polymeric Materials
Identifiers
urn:nbn:se:kth:diva-168192 (URN)
Funder
EU, FP7, Seventh Framework Programme
Note

QS 2015

Available from: 2015-05-27 Created: 2015-05-27 Last updated: 2015-05-29Bibliographically approved
5. Modelling of oxygen diffusion and consumption in an XLPE cableinsulation for nuclear power applications using an oxidationdependentdiffusion-reaction approach
Open this publication in new window or tab >>Modelling of oxygen diffusion and consumption in an XLPE cableinsulation for nuclear power applications using an oxidationdependentdiffusion-reaction approach
(English)Manuscript (preprint) (Other academic)
Abstract [en]

The oxidation profile of an XLPE conductor insulation after accelerated ageing atdifferent temperature/dose-rate combinations was assessed through infrared spectroscopy, tostudy heterogeneous oxidation in the samples. These profiles showed clear drops in oxidationdegree towards the centre of the cable, to such an extent that more than half of the thicknesswas essentially not oxidized. The oxygen diffusion and consumption was then modelledthrough computer simulations to obtain the diffusion coefficient and reaction rate, includingtheir possible dependencies on the oxidation degree. The model was based on the assumptionthat diffusion and consumption rates, and solubility of oxygen in polyethylene weredependent on the degree of oxidation relative to a maximum degree of oxidation. Good fitswith experimental data were obtained, and the model could be applied to data from literaturewith promising results.

Keyword
Polyethylene, Radiation ageing, Thermal ageing, Oxygen diffusion, Computer modelling, Lifetime prediction
National Category
Textile, Rubber and Polymeric Materials
Identifiers
urn:nbn:se:kth:diva-168189 (URN)
Funder
EU, FP7, Seventh Framework Programme
Note

QS 2015

Available from: 2015-05-27 Created: 2015-05-27 Last updated: 2015-05-29Bibliographically approved

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