Evaluation and Comparison of Fracture Behavior of Selected Nuclear Graphite by Small Size SENB Specimens Se-Hwan Chi. Ph. D. 15 th International Nuclear Graphite Specialist Meeting (INGSM-15), September 15-18, 2014, Hangzhou, China NHDD Project (Nuclear Graphite Study), KAERI

Content 1.Background 2.Materials, Specimen, Test Jig 3.Fracture Toughness Testing and K IC, G-Δa Determination 4. Results 4.1 Specimen size effects on K IC and G-Δa of selected nuclear graphite grades 4.2 Comparison of Fracture Behavior of selected nuclear graphite grades. 5. Conclusion.

1.Background o Graphite core components in HTGR experience dimensional, mechanical, physical property changes owing to neutron irradiation and oxidation during operation resulting in an increase in fracture probability of the components.

For safety, monitoring and reflecting the mechanical property change of the core components on reactor operating condition are important as seen in the PWR RPV surveillance program. Surveillance Program of Core Materials PWR Limited capsule space for surveillance specimens requires small volume specimen and small specimen test techniques

Small specimen test techniques Various kinds of small specimens and small specimen test techniques are well developed in PWR technology

In HTGR, while limited space for surveillance specimens also needs graphite small specimen and small specimen test techniques as in PWR, related study and developments are progressing and further studies are required * *ASTM Symposium on Graphite Testing for Nuclear Applications: the Significance of Test Specimen Volume and Geometry and the Statistical Significance of Test Specime n Population (Sept , 2013 Hilton Seatle, Seatle, Washington, USA) HTTR (Japan) Surveillance Program of Core Materials No guide on strength measurements yet ! HTGR

Background / Purpose of Study In this study, the fracture toughness (K IC ) and G-Δa behavior of selected nuclear graphite grades were determined by small size fracture toughness specimens in size of 50.0 x 10.0 x 4.0 (4T) mm and 52 x 12 x 6.5 mm (6.5T)* based on a new ASTM fracture toughness testing procedure, ASTM D **. Obtained results ((K IC ) and G-Δa) were discussed in view from specimen size and microstructure of the grades. * The 4T and 6.5T specimen correspond to 1/15 (6.5T) and 1/30 (4T) of the ASTM D recommendation ** ASTM D The Standard Test Method for the Determination of Fracture Toughness of Graphite at Ambient Temperature

2. Materials, Specimen and Testing Fixture IG-110IG-430PCEANBG-17NBG-18NBG-25 Grain Size ( ㎛ ) Max. 800 Max: 1, Forming MethodIso-static Extrusion Vibration molding Iso-static Number of Specimen * 4T1310 acacacac T

2. Materials, Specimen and Testing Fixture Notch depth: 1.6 mm, Angle: 30° (EDM) Loading Rate: 0.1 mm/min

3. Fracture Toughness Testing and K IC, G-Δa Determination - g (a/W) for S/W = 8 Load-Displacement curve (IG-430) (NBG-18)

G-Δa Curve C n = D n /P n C n : Compliance for the point n (m/N) D n : displacement for the point n (m) P n : loading force for the point n (N). Initial Crack length, a o = notch depth a n = a n-1 + [(W-a n-1 )/2*((C n –C n-1 )/C n-1 )] G(a n ) = P 2 /2B*δC/δa [J/m 2 ], δC= C n – C n-1, δa= a n - a n-1 G-Δa, where Δa = a n – a 0

4. Results/Discussion 4.1 K IC MPa(m) 1/2 (1) M. Eto, et al, Int. Sym. On Carbon (1990), 8 type of specimens. (2) S. Fazluddin and B. Rand (2002, Univ. of Leeds), TB: 100 x 10 x 12, CT: 50 x 48 x 10 (3) Haiyan Li, CARBON (2013) 46, TB: 45 x 10 x 5 mm. (4) T. D Burchell, Proc. HTR2012 (2012), TB: 50 x 6 x 6 mm IG-110IG-430 NBG- 2 5-a NBG- 25-c NBG- 17-a NBG- 17-c NBG- 18-a NBG- 18-c PCEA-aPCEA-c K IC [MPa (m) 1/2 ] 4T 0.76 ± ± ± ± ± ± ± ± ± ± T 0.78 ± ± ± ± ± ± ± ± ±0.04 Ref (1) (2) 1.27 (3) ± ± (4) 0.941± (4)

4. Results/Discussion Observation 1.K IC tends to increases with specimen size 2.Observed anisotropy in K IC except IG-110 and Observed smaller value for IG-110 and larger value for NBG-18 and PCEA 4.1 K IC

4. Results/Discussion 4.2 G-Δa (4T )

4. Results/Discussion 4.2 G-Δa (6.5T)

4. Results/Discussion 4.2 G-Δa (Grade) 6.5T

4.3 Analysis of G-Δa Curve Based on the corresponding G and Δa of the intersection point A and B, the G IC, Δa, stable crack growth length to initial ligament (SCL)(%), an increase in ΔG during stable crack growth against G IC, i.e.,(G B -G A ) 100/ G IC, and stable strain energy release rate (ΔG B-A / Δa B-A ) were determined and compared. G IC A B 4. Results/Discussion

G IC A B 750 J/m J/m mm 2.06 mm ΔG = ( )/750 J.m 2 = 237 % SCL=100*( )/3.9 = 46 % Determination of SCL and Δ G

4. Results/Discussion 4.4 G-Δa Analysis Results IG-110IG-430 NBG- 2 5-a NBG- 25-c NBG- 17-a NBG- 17-c NBG- 18-a NBG- 18-c PCEA - a PCEA - c G IC [J/m 2 ], Δa (mm) 4T ΔaΔa T ΔaΔa SCL (%) 4T T ΔG (%) 4T 90 (133) 150 (120) 175 (175) 99 (85) 175 (100) 292 (167) 400 (172) 375 (225) 404 (248) 225 (100) 6.5T 225 (112) 575 (192) 600 (200) 1435 (208) 1310 (190) 1400 (175) 878 (157) 1775 (237) 1455 (217) (ΔG B-A / Δa B-A ) J/m 3 4T ? 6.5T ?

4.5 Comparison of G-Δa curves G- Δa from the extruded (or vibration molded) medium particle size grades is higher than the iso-molded fine particle size grades.

4. Observation 1.More or less smaller value due to small dimension 2.Observed anisotropy in K IC except IG-110 and Observed the smallest value for IG-110, the largest value for NBG G IC : NBG-17, 18, PCEA > IG-110, -430, NBG Differences between the fine grain, isostatic molding and medium grain, vibration molded or extruded in G-Δa “global behavior” is far larger and apparent than the differences in “local parameter” K IC. 6. No large differences in SCL. 7. No correlation between the K IC and G-Δa.

The results show that, though the K IC value from 6.5T was largely a little higher than the K IC from 4T for 2-28%, the 6.5 T K IC values appear a little smaller (15-30 % for IG-110, NBG-18) or larger (12.0 – 15.6 % for PCEA) than the reported values from larger size specimens. Overall, the K IC and G IC from the extruded (or vibration molded) medium particle size grades were a little higher than the iso-molded fine particle size grades. Differences in G IC were larger than the differences in K IC between grades. While the changes in K IC were small or negligible, a large increase in the G IC was observed with increasing specimen size from 4T to 6.5T. Detailed analysis of G-Δa curves, showed that, on average, SCL was 53% and 44%, and ΔG was 152.5% and 187.5% for 4T and 6.5T specimens, respectively. Present results show that both the local (K IC ) and gross (G) fracture characteristics of the Extruded (or vibration molded) medium particle size grades may largely be better than the iso-molded fine particle size grades, and the fracture parameter G seems to be appropriate in describing the fracture characteristics of nuclear graphite after crack initiation. 5. Summary

23 Surface Filometry (Model: Dektak 150) IG-110NBG-18 G-Δa curves will be compared with surface filometry measurements

NBG-18 IG-430