Development of New Techniques for AGR Graphite

Presented by: Nassia Tzelepi

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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)

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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

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Developing New Techniques

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•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

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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.

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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.

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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

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DYM(2p/2t) <GPa>

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Hunterston B R4 2010 Hunterston B R3 2012 Dungeness B R21 2009 Hinkley Point B R4 2008 Hinkley Point B R4 2012

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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.

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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

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Square Root(Ligament Area) {mm}

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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

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Work of fracture for linear materials – Size effects

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Vertical bars indicate 95% confidence band

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PC45 Filter Carbon

Macor Machinable Ceramic

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Fast Fracture occurred

Work of fracture for irradiated Gilsocarbon

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Previous 42% 6x6 Virgin95% Conf on 30 samples

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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

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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

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Ultrasonics – shear wave measurements results

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Graphite Polyethylene

• Relatively constant for graphite samples with lengths > ~7 mm. • <7 mm, PE and graphite samples generally show increased mean shear

wave velocity.

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Mk3 0.5MHz delay lines

Mk3 1MHz delay lines

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Ultrasonics – longitudinal wavemeasurements results

Polyethylene Mk2

Mk3

Ultrasonics – longitudinal wave measurements results

Graphite

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Mk3 1MHz Delay lines

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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.