M. Haverty , W. Bodel , B.J. Marsden Nuclear Graphite Research Group

1. The effect of compressive pre-stress on the thermal expansion behaviour of anisotropic nuclear grade graphite M. Haverty, W. Bodel, B.J. Marsden Nuclear Graphite Research Group The University of Manchester maureen.haverty@postgrad.manchester.ac.uk

2. Contents • General Methodology used • Preliminary Study • Main Study • Comparison between two studies • High Resolution images

3. Motivation • Previous studies have shown a change in CTE with stress • Gilsocarbon: Applied uniaxial compressive and tensile stress 1 • Steels: Applied uniaxial tensile stress and pre-stress beyond elastic limit2 • Are these changes observed in PGA? • What causes these changes? Preston, S.D. & Marsden, B.J., 2006. Changes in the coefficient of thermal expansion in stressed Gilsocarbon graphite. Carbon, 44(7), pp.1250–1257 Rosenfield, A.R. & Averbach, B.L., 1956. Effect of Stress on the Expansion Coefficient. Journal of Applied Physics, 27(2), pp.154–156. .

4. CTE Measurement

5. Standards and Methodologies Used • CN821-1 “Advanced technical ceramics-Monolithic ceramics-Thermo-physical properties-Part 1: Determination of thermal expansion” • Provides test method including, sample size; reference standards and heating rates • Accuracy if followed: • 5K/min  0.5 x 10-6 K-1 • 2K/min  0.1 x 10-6 K-1

6. CTE Measurement equipment • Netzsch Proteus DIL • Pushrod dilatometer • Nitrogen atmosphere • Al2O3Reference standard • Glow runs • Filler piece for short samples (Al2O3)

7. Thermal Expansion Measurements Al2O3 Pushrod Al2O3 Sample Holder Al2O3 Filler Piece Graphite sample

8. Preliminary Test

9. Method • Samples were cut in the AG and WG directions • Uniaxial compressive stress applied • Compressive strength: 27 MPa • Max temp: 250 °C

10. Applying Stress • Universal Load Testing Machine • Compressive stress • Sample subdivided into two sister samples • Excess used to face off to correct tolerance Φ =12 mm Φ =12 mm H =6 mm H =18 mm

11. Comparison of unstressed samples with the literature • Comparing unstressed sample values with Sutton and Howard 3 • Two blocks of PGA, measured in WG and AG directions • Average CTE for 50°C increments, e.g. 100-150°C • CTE plotted at midpoint of increment e.g. 125 °C 3. Sutton, A.L. & Howard, V.C., 1962. The role of porosity in the accommodation of thermal expansion in graphite. Journal of Nuclear Materials, 7(1), pp.58–71.

13. CTE Preliminary Results • Average CTE plotted • Reference temperature of 50 °C used, e.g. 50-250 °C • CTE plotted at midpoint of temperature increment

26. Main Study

27. Samples • CTE measured on 10 samples to ascertain sample variability • Two samples in each direction selected randomly for pre-stressing Φ =6 mm H =6 mm

28. Method • CTE measured • Pre-stress applied (fraction of compressive strength) • Properties re-measured • Max temp: 450 °C • AG: 0%; 20%; 40%; 90% • WG: 0%; 20%; 40%

29. Comparison of unstressed samples with the literature • Comparing unstressed sample values with Sutton and Howard 3 • Two blocks of PGA, measured in WG and AG directions • Average CTE for 50°C increments, e.g. 100-150°C • CTE plotted at midpoint of increment e.g. 125 °C 3. Sutton, A.L. & Howard, V.C., 1962. The role of porosity in the accommodation of thermal expansion in graphite. Journal of Nuclear Materials, 7(1), pp.58–71.

32. Sample Variability

50. Comparison between preliminary and main study