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

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 7 th 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

Work of fracture for linear materials – Size effects

Work of fracture for irradiated Gilsocarbon

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

R 20x4.5 20x5.5 50x6 50x11 50x12 50x15 50x7 50x6 50x550x4 20x6 7x6 50x50 A 50x19 50x29 B C D E FG HI J KL 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.

Ultrasonics – shear wave measurements results GraphitePolyethylene Relatively constant for graphite samples with lengths > ~7 mm. <7 mm, PE and graphite samples generally show increased mean shear wave velocity.

Ultrasonics – longitudinal wave measurements results Polyethylene Mk2 Mk3

Ultrasonics – longitudinal wave measurements results Graphite

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.