seminar on “degradation of polymers in npps” boras, 21 sep 2016 · 2016-09-27 · 1 seminar on...
TRANSCRIPT
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Seminar on “Degradation of polymers in NPPs”Boras, 21 Sep 2016
Dr Sue BurnayJohn Knott Associates Ltd., UK
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Contents of the seminar
Why is ageing of polymeric components of concern?
Basics of polymers & their degradation mechanismsAreas of concern for components in NPPs
Assessment of aged polymeric componentsCondition monitoring methods Fingerprinting of polymeric materialsFailure analysis of seals & special problem areas
Lifetime prediction methods
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The need for ageing management - why is ageing of polymeric components of concern?
Dr Sue BurnayJohn Knott Associates Ltd., UK
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Why should we be interested in ageing of polymers in nuclear plant?
SealsMajor equipment (e.g. valves, actuators) contain seals or gasketsIn AGRs, seals form part of the pressure boundary
CablesTypical NPP contains >1000 km of electrical cablesCables provide the vital link for power, control and instrumentation for safety systems in a NPP. If the cable fails, the safety system cannot function
Polymeric coatings used in many areasSeals, cable insulation and jacket materials, and many coatings are all polymeric.
Seals can often be replaced during routine maintenance of equipment, but cables need to operate for lifetime of plant Cables are passive components, so maintenance is not an option. Replacement is expensive, and removal may be impractical
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Polymers – what are they?Macroscopic properties determined by long chain structureMade up of long molecular chains, usually C, H and O basedOther elements present can include S, Cl, F, N, SiSome crystallisation can occur in the simpler polymers (eg. PE) – this has implications for degradation
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Polymers commonly in use in NPPsCable insulation
XLPEEPR/EPDMSiRPVCPPO (Noryl)ETFE (Tefzel)PEEK
Cable jacketsEPR/EPDMCSPE (Hypalon) EVASiR
SealsEPR/EPDMNitrile rubbersFluoropolymers (eg. Viton)SiR
CoatingsEpoxy resinsRubbers
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Basic structure of some polymer chainsPolyethylene (simplest polymer)Polypropylene
Nitrile rubber
Silicone rubber
Fluoroelastomer
- CH2 - CH2 - CH2 - CH2 –
- CH2 – CH - CH2 –|CH3
- CH2 – CH = CH - CH2 – CH - CH2 -|
CNCH3 CH3
| |- Si – O – Si – O -
| |CH3 CH3
- CF – CF2 – CH2 – CF2 -|
CF3
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What is in a polymeric component?In a commercial polymer formulation the polymer content can be relatively small, with fillers, additives, extenders, plasticisersPolymer properties can be varied by changing proportions of fillers, plasticisers etc.Formulations are specific to individual manufacturers and can vary over time A limited number of polymer types are in regular use in NPPs, but the properties and degradation behaviour are strongly dependent on the formulation used (particularly for radiation degradation)
e.g. Typical elastomer formulationBase polymer 100 partsSulphur 2Accelerator 1.5Zinc oxide 5Stearic acid 50Carbon black 50Plasticiser 10Antioxidant 1
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Main degradation mechanisms
Oxidative degradation, both scission and crosslinking (in most polymers)Radiation cross-linkingPlasticiser loss (particularly in thermal ageing of PVC)Dehydrochlorination and defluorination (in halogenated polymers)
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Oxidative degradation – main mechanism for most polymers
Initiation RH → R•
Propagation R• + O2RO2• + RH
RO2HRO•
→→→→
RO2•RO2H + R•RO• + OH•R• + -CO-
Termination 2R•2RO2•
→→or
RRRO2R + O2ROH + -CO- + O2
The kinetics of oxidative degradation are complex, with multistage reactions that can end in either scission of the long chains or cross-linking between chains
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Scission and crosslinking - schematic
crosslinking
scission
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Effects of degradation on bulk properties of polymers
Tensile elongation decreasesTensile strength decreases (may show initial increase)Hardness increases (usually)Density increasesCompression set of seals increasesElectrical properties show little change until material is significantly degraded (usually)Dielectric properties can change significantlyChanges in colour and surface texture often visible
Tensile tests - schematic
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Degradation examples
Loss of plasticiser after thermal ageing (example is for a PVC cable after 20 years at 60C)Mechanical stress can accelerate degradation –jacket failure on the outside of a tight bend in cables
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Degradation stressors
Polymers are sensitive to environmental conditions within a NPPMain stressors –
TemperatureRadiation dose rate and total dose (unlike metals, polymers are susceptible to gamma irradiation)Presence of oxygen
Secondary stressors-MoistureMechanical stressOzoneChemical contamination
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Ageing of polymeric componentsThermal ageing is dominant for most polymers in a NPPRadiation ageing is only significant for a small proportion of theseEnvironmental qualification aims to demonstrate that cables can continue to fulfil their safety function throughout the lifetime of the plantSeal replacement intervals need to be knownRequirement to simulate the ageing that would occur in the plant, using worst case environmental conditionsThermal and radiation ageing needs to be simulated in an accelerated time frameIs this accelerated ageing an accurate simulation?
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Accelerated thermal ageing
Usually based on Arrhenius relationship, using following equation
t1 = t2 exp [E/R (1/T1 - 1/T2)]Where t1 is ageing time required at a temperature T1 to simulate a service life of t2 at a temperature T2. E is the activation energy for thermal ageing and R is the gas constant
Assumes that A single degradation mechanism is in operationDegradation mechanisms are the same at the higher temperature used for accelerated ageing Activation energy E is constant and value is known for specific polymer formulation
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0.1
1
10
100
1000
10000
100000
0.002 0.0022 0.0024 0.0026 0.0028 0.003 0.0032 0.0034
1/T (K)
Agei
ng ti
me
(day
s)
E = 23 kcal/mole (1 eV)
E = 18.4 kcal/mole (0.8 eV)
Activation energy - is the value appropriate?
40C, 40 years
140C
100CTesting at 140 C would be in error by factor of 6, whereas testing at 100 C, error would be a factor of 3
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Variation of Ea with temperature
2.4 2.5 2.6 2.7 2.8 2.9 3.0 3.1 3.2 3.31000/T, K-1
10-3
10-2
10-1
100
101
Empi
rical
a T
SMhy-aT7
102 kJ/mol
88 kJ/mol
elongation
O2 consumption
Example for a CSPE cable jacketOxygen consumption measurements used to access low temperature regionSimilar variations seen in many cable materials
40C140C
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Variation of activation energy with temperature –example for EPR cable
Ageing temperature
△E: 130 kJ/mol
△E: 63 kJ/mol
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Are the degradation mechanisms the same?
PVC cables – thermal ageing at < 90C is dominated by loss of plasticiser but > 90C mainly loss of HClEPR and PE-based cables – can be semi-crystalline materials. Thermal ageing at T > crystalline melting point may not be representative of ageing in plantImportant to use the lowest practicable temperature for the accelerated thermal ageing
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Factors affecting lifetime of polymeric components under irradiation
Diffusion-limited oxidationmust be taken into account in any simulation of service conditions (both for thermal and radiation degradation)
Radiation dose rate effectsparticularly in accelerated testing
Synergy between radiation and temperatureReverse temperature effectPolymer formulations
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Diffusion-limited oxidation
Oxidative degradation is limited by diffusion of oxygen into sampleUnder service conditions (low temperature/dose rate, long times) oxidation is usually homogeneousIn accelerated testing (high temperature/dose rate, short times) oxidation often heterogeneousBulk measurements (eg. tensile tests, compression set) on heterogeneous samples likely to be non-representative
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Diffusion – limited oxidationA visual example of heterogeneous oxidation (top 2 images) and homogenous oxidation (bottom image)The properties of the oxidised region (yellow) and unoxidised regions (clear) will be different
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Example of heterogeneous oxidation –oxidative cross-linking
Diffusion-limited oxidation can occur during both thermal and radiation ageingEdges of sample are more degraded than bulk of materialEffects become more severe as ageing proceeds
CSPE jacket material
Relative distance through sample
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25
VITON - 20C
-350
-300
-250
-200
-150
-100
-50
0
0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.6 0.65 0.7 0.75 0.8 0.85 0.9 0.95 1
Relative distance through seal
Pene
tratio
n (m
icro
ns)
As-received
940 hrs
1550 hrs
3046 hrs
4506 hrs
Example of heterogeneous oxidation –oxidative scission + radiation cross-linking (Viton seal)
Radiation cross-linking in central regions –material hardens with increasing dose
Oxidative scission at edges – material softens with increasing dose
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Dose rate effects
Assumption often made of equal dose = equal damage but many polymers are dose ratedependent Dose to equivalent damage (DED) decreases with decreasing dose rateNot seen in all polymers but can be significant in manyNeeds to be considered when simulating service conditions
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Dose rate dependence of dose to 100% elongation for XLPE insulation at 20C
100
1000
10000
1 10 100 1000 10000
log Dose rate (Gy/hr)
log
Do
se to
100
% e
lon
gat
ion
(kG
y)
Dose rate effect in a cable insulation material (XLPE)Dose to reach 100% elongation for XLPE cable insulation, radiation aged at 25 C
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Dose rate effect in EPR insulation, radiation aged at elevated temperature
FR-EPR radiation aged at 80C
0
50
100
150
200
250
300
350
400
450
500
10 100 1000
Radiation dose (kGy)
Elon
gatio
n at
bre
ak (%
)
3 Gy/hr18 Gy/hr100Gy/hr
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Synergy between radiation and temperature
At low dose rates (typically seen in NPPs)Degradation is dominated by thermal ageingDegradation is determined only by time at temperature
At high dose rates (typically seen in accelerated testing)
Degradation is dominated by radiation ageingDegradation tends to show only a small effect of temperature
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Effect of combined radiation/thermal ageing on DED – general trends
1.00E-01
1.00E+00
1.00E+01
1.00E+02
1.00E+03
1.00E-05 1.00E-04 1.00E-03 1.00E-02 1.00E-01 1.00E+00
Dose rate (Gy/s)
DED
(kG
y)
20 C
40 C
60 C
80 C
Dominated by radiation ageing -temperature has little effect in this region
Dominated by thermal ageing -tending towards a constant time to failure in this region
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Reverse temperature effect in semi-crystalline polymers in radiation ageing
Semi-crystalline polymers –Tend to show reverse temperature effect, with degradation being greater at ambient temperature than elevated temperature in the presence of radiationArises from recrystallisation and recombination of radicals at higher temperaturesOnly of concern where service and accelerated ageing temperatures span the crystalline melting point, e.g. in XLPE
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X L P E c a b le in s u la tio n
1 0 0
1 0 0 0
1 0 0 0 0
1 0 0 0 0 0
1 0 0 0 0 0 0
10 0 0 0 0 0 0
0 2 0 4 0 6 0 8 0 100 12 0 1 40 16 0
T e m p .(C )
Pre
dic
ted
time
to 1
00%
elo
ng
atio
n a
t 0.0
03 G
y/s
(hrs
)
P re d ic te d
A c tua l
Example of reverse temperature effect in XLPE cable insulation
Experimental data from long-term ageing programme at 10 Gy/hr
Note: time axis is logarithmic
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DSC traces for same XLPE material
Melting of crystallites within polymer
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Effect of formulation on degradation
Differences in formulation can significantly affect the rate of degradation of a cable materialParticularly important when radiation is presentChanges in pigments, anti-oxidants, fillers or stabilisers, as well as the base polymer, can all affect component lifetime.
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Pigment effects in an EPR insulation
FR-EPR insulation - radiation aged at 90 C, 3 Gy/hr
0
50
100
150
200
250
300
350
400
450
500
1,000 10,000 100,000
Ageing time (hr)
Elon
gatio
n at
bre
ak (%
)
blackwhitered
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Effect of pigment on degradation
Differences in formulation (e.g. pigments) can significantly affect the rate of degradation of a cable materialThe example above is for PE insulation from a USA reactor for a cable that ran through containment past several coolant lines for heat exchangers
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Factors to consider in accelerated ageing
Summary of factors that may need to be considered -Activation energy for thermal ageingDegradation mechanismsDiffusion-limited oxidation
for both thermal and radiation degradation
Radiation dose rate effectsSynergy between radiation and temperatureReverse temperature effectPolymer formulations
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Radiation ageing of polymers – pitfalls!Be very wary of using tables such as thisThe data have been generated from very high dose rate irradiation testsFactors such as diffusion-limited oxidation and dose rate effects have not been consideredOnly indicate the relative radiation resistance of different polymers at high dose rates
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Special sealing situations
Some seals may also need to cope with special situations, such as
High temperature transientsLow temperature transientsImpact or large vibrations
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High temperature transients
Despite obvious damage, these EPDM seals continued to seal during a transient to 150 COnly the surface was damaged, interior of seal still flexible
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Low temperature transients - material selection is critical
Sealing Flange Force vs Flange Temperature EPDM (125 kGy)
0
10
20
30
40
50
60
70
80
90
100
-40 -30 -20 -10 0 10 20 30
Temp (Deg C)
% S
ealin
g Fo
rce
Decreasing T
Increasing T
Sealing Flange Force vs Flange Temperature Viton E60C (150 Deg C)
0
10
20
30
40
50
60
70
80
90
100
-40 -30 -20 -10 0 10 20
Temp (Deg C)
% S
ealin
g Fo
rce
Decreasing T
Inreasing T
EPDM seal continued to seal below -35 C, but Viton seal failed at -20 C because it has reached its glass transition temperature
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Sealing after impact damage or with large vibrations
Need to consider residual sealing force, allowing for operational degradationNeed to assess whether seal is capable of bridging impact/vibrational gaps – from FE calculations on transport container design
Relative changes in sealing properties of elastomers
0
10
20
30
40
50
60
70
80
90
100
0 1 2 3 4 5 6 7 8 9 10
Degree of degradation
Rel
ativ
e pr
oper
ty v
alue
Sealing forceCompression setLeakage rate
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Conclusions
You should now have a better understanding of why polymer ageing is considered to be importantGeneric degradation processes and failure mechanisms are reasonably well understood Detailed behaviour of a polymer component is specific to its formulation
Thank you for listening