Development of New Techniques for AGR Graphite Presented by: Nassia Tzelepi.
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Transcript of Development of New Techniques for AGR Graphite Presented by: Nassia Tzelepi.
Development of New Techniques for AGR Graphite
Presented by: Nassia Tzelepi
2
Overview
•Background
•Measurement techniques developed for AGR graphite
•Electronic Speckle Pattern Interferometry (ESPI)
•ESPI – CTE
•Work of fracture
•Ultrasonic Poisson’s ratio
•Thermal conductivity
•Electrical resistivity
•Resonant Ultrasound
Spectroscopy (RUS)
3
Monitoring Graphite Behaviour
NNL Graphite PIE
• NNL carries out all the testing and characterisation of monitoring samples that are taken from the Magnox and AGR cores in unique world-class facilities based at Sellafield.
• Measurements have been carried out by NNL and its predecessor companies for over 40 years.
NNL Graphite PIE
•Density by mensuration
•Density by immersion
•Laser mensuration
•Open pore volume
•Gas diffusivity
•Gas permeability
•Thermal conductivity (diffusivity)
•Rate of release of stored energy
•Total stored energy
•Dynamic Young’s modulus
•Static Young’s Modulus and Poisson’s ratio using DIC
•Coefficient of thermal expansion
•Compressive strength
•Ultimate tensile strength
•3-point and 4-point bend strength
•Graphite-air reactivity and activation energy
•Deposit concentration and reactivity
6
Developing New Techniques
7
•Is the measurement still valid at these sample sizes?
•Is the measurement representative of the bulk material?
•Will the measurement be still accurate and reproducible on highly oxidised graphite?
Validation can only be done with unirradiated graphite which has very different properties
Challenges in measuring irradiated graphite
8
Developing new techniques
• Large development programme is usually required for each technique.
•Proof of concept (e.g. modelling)
•Proving trials on unirradiated graphite, reference materials and oxidised graphite simulants
• Prove accuracy and reproducibility
• Investigate size effects
• Prove reproducibility on oxidised graphite
•Measurements on irradiated samples
• Prove reproducibility on irradiated graphite
• Prove consistency with previous method
•Participate in inter-laboratory studies, e.g. through ASTM.
9
Electronic Speckle Pattern Interferometry (ESPI)
•Optical technique that measures the displacement field through change in speckle patterns.
•Like a fingerprint, these speckles are inherent to the investigated surface. •Under load, the object is deformed and hence, the speckle interferogram also changes. •The displacements can be calculated Young’s modulus.
10
Electronic Speckle Pattern Interferometry (ESPI)
•Existing technique measures Dynamic Young’s Modulus (DYM) using the ultrasonic Time-of-Flight technique
•The calculation requires an assumed value of Poisson’s ratio.
•Static values are used in the safety case assessments.
•Experimental challenges:
•Vibrations
•Sample alignment
•Using ESPI, we can:
•measure Poisson’s ratio
•compare Static with Dynamic YM
•validate the DYM technique.
•The ESPI test is carried out during the routine 3-point bend fracture test.
ESPI - Young’s Modulus vs. Stress
ESPI - Validation of DYM
5
7
9
11
13
15
17
19
21
23
25
5 7 9 11 13 15 17 19 21 23 25
DYM(2p/2t) <GPa>
ES
PI (A
rea
) Yo
un
g's
mo
du
lus
<G
Pa>
Hunterston B R4 2010 Hunterston B R3 2012 Dungeness B R21 2009 Hinkley Point B R4 2008 Hinkley Point B R4 2012
13
ESPI-CTE
•Same principles but the samples are now under thermal strain in order to measure the Coefficient of Thermal Expansion (CTE).
•Experimental challenges:
•Vibrations
•Sample alignment
•Expansion in the z-direction
•Hot air turbulence above the samples.
ESPI-CTE
• Advantages:• faster throughput of
measurements• suitable for high weight loss, no
mechanical contact• validation of existing method
using dilatometers.
ESPI
• 7th EU Framework project VANESSA, led by Liverpool University:• VAlidating Numerical Engineering Simulations: Standardisation
Actions (VANESSA)
• Two Inter-Laboratory Studies (ILS) • Calibration of optical systems for strain field measurement • Validation protocol - for computational solid mechanics models.
• CEN Workshop Agreement on the validation of computational solid mechanics models based on comparisons to strain fields from optical measurement systems.
16
Work of fracture
•There is currently no valid technique to measure the fracture properties of irradiated graphite
•What happens after crack initiation
•Measurement of the energy released per unit area of fracture surface
•Deep chevron notched samples for slow crack growth.
Work of fracture for virgin Gilsocarbon – Size effects
100
150
200
250
300
1 3 5 7 9 11 13 15
Square Root(Ligament Area) {mm}
Fra
ctu
re E
ner
gy
{J/m
2}
Vertical bars indicate 95% confidence band
20x20x10042%
10x10x5042%
6x6x20/Wings25% 42% 58%
Virgin Graphite Samples from brick Heysham 2, 72/4/C
Virgin Graphite Samples from brick Heysham 2, 72/1/D
15x15x10042%
Work of fracture for linear materials – Size effects
0
10
20
30
40
50
60
1 3 5 7 9 11 13 15
Square Root(Ligament Area) {mm}
Fra
ctu
re E
ner
gy
{J/m
2}
Vertical bars indicate 95% confidence band
20x20x10042%
10x10x5042%
6x6x20/Wings25% 42% 58%
PC45 Filter Carbon
Macor Machinable Ceramic
15x15x10042%
Fast Fracture occurred
Work of fracture for irradiated Gilsocarbon
0
50
100
150
200
250
1200 1300 1400 1500 1600 1700 1800 1900 2000
Density {kg/m3}
Fra
ctu
re E
ner
gy
{J/m
2}
Virgin
C99
C09
B08
Previous 42% 6x6 Virgin95% Conf on 30 samples
20
Work of fracture
•Advantages
•Provides information on the fracture properties of irradiated graphite.
•Uses existing mechanical testing equipment.
•Uses pieces of irradiated graphite that cannot be used for any other tests.
•Validations of experimental
Results with FE analysis.
•Next step
•Routine use on irradiated
graphite.
•Produce ASTM standard?
Ultrasonics – DYM and Poisson’s ratio
• The main objectives of this development programme:
• A Poisson’s ratio value of 0.21 is used in the calculation of the DYM• This value is assumed to be constant for virgin and irradiated graphite• This value is based on a small number of historic measurements.
• Investigate the size effects related to Time-of-Flight (ToF) technique• Shear and longitudinal.
• Develop the ToF technique so that it remains accurate and reproducible for highly oxidised graphite.
Ultrasonics – main uncertainties
Dispersion
The spatial resolution of the wave signal into components of different frequencies due to, in the case of graphite, sample geometry
- related to the ratio of the sample diameter to the transmitted signal wavelength (D/λ) and therefore, for a graphite sample with infinitely large lateral dimensions, there should be no dispersion.
Attenuation
The gradual loss of intensity as the signal passes through a medium
-the reduction in the main frequency of the received signal.
Major causes of uncertainty and cannot properly quantify the effect
R20x4.5
20x5.5
50x650x1150x1250x15
50x7 50x6 50x550x4 20x6
7x6
50x50 A
50x19 50x29 B C
D E F G
H I J K L M
N
S
Ultrasonics - size effects
The lateral dimension of the sample should be much larger than the wavelength of the transmitted pulse (D>>λ). This is so that the sample can be approximated as an infinite medium.
2VCE
1211
C
Ultrasonics – shear wave measurements results
810
820
830
840
850
860
870
880
2 7 12 17Length (mm)
Ve
loc
ity
(m
/s)
50mm diameter samples
20mm diameter samples
1500
1550
1600
1650
1700
1750
1800
0 10 20 30 40 50
Length (mm)
Vel
oci
ty (
m/s
)
50mm diameter samples
20mm diameter samples
7mm diameter samples
Graphite Polyethylene
• Relatively constant for graphite samples with lengths > ~7 mm. • <7 mm, PE and graphite samples generally show increased mean shear
wave velocity.
2100
2120
2140
2160
2180
2200
2220
2240
0 5 10 15 20 25 30 35 40 45 50 55
Length (mm)
Velo
cit
y (
m/s
)
Mk2 1.25MHz
Mk3 0.5MHz delay lines
Mk3 1MHz delay lines
Mk2 0.5MHz
Ultrasonics – longitudinal wavemeasurements results
Polyethylene Mk2
Mk3
Ultrasonics – longitudinal wave measurements results
Graphite
2480
2500
2520
2540
2560
2580
2600
2620
2640
2660
2680
0 5 10 15 20 25 30 35 40 45 50 55Length (mm)
Velo
cit
y (
m/s
)
Mk2 1.25MHz
MK3 0.5MHz Delay Lines
Mk3 1MHz Delay lines
Mk2 0.5MHz
Conclusions
• We continuously strive to improve the accuracy and reproducibility of the measurements employed in the core monitoring programme.
• New techniques are providing new insight into irradiated graphite behaviour.
• As the graphite cores age, there is a strong requirement for accurate and reproducible measurements.• Large programmes are required to validate each new
technique and overcome the engineering challenges of installing and using the equipment remotely.
The presenter would like to thank EDF Energy Nuclear Generation for funding and technical contribution.