1. 14 th International Nuclear Graphite Specialist Meeting (INGSM ̵ 14 ) September 16-18, 2013, Hilton Seattle, Washington State, USA Oxidation Effects on The Thermal Emissivity of Selected Nuclear Graphite Se-Hwan Chi 1, Seung-Kuk Seo 2, Jae-Seung Roh 2 and Min-Hwan Kim 1 1 Nuclear Hydrogen Development and Demonstration Project, Korea Atomic Energy Research Institute (KAERI), Dae Deok-daero 989-111, Yuseong, Daejeon 305-353 Rep. of Korea 2 School of Advanced Materials and Systems Engineering, Kumoh National Institute of Technology, Gumi, Gyeoungbuk 730-701, Rep. of Korea (shchi@kaeri.re.kr, 042-868-2385)

2. Contents 1. Introduction: Background and purpose 2. Experiment: Far-Infrared radiation spectra measure- ment/ Surface roughness and crystallinity measurement/Porosity effects by APSM. 3. Results - Oxidation and Temperature Effects on TE - Surface roughness and Crystallinity Effects on TE - Porosity effects on TE 4. Conclusion

3. 3 1. Introduction Thermal Emissivity (TE) is an important thermal property that controls the transfer of heat out of core to the final heat sink during an off- normal event in a graphite-moderated high- temperature gas-cooled reactor. HTTR Heat transport via thermal radiation across the gap between the graphite core and the steel core barrel.

4. Thermal Emissivity is defined as the ratio of energy radiated by a material to that radiated by a theoretical black body (emissivity = 1) at the same temperature and environment. Since graphite is nearly the perfect black body material, the emissivity of a given graphite will largely depend upon the component surface condition and the operating environment.

5. 5 Results of GAMMA+ Calculation* GAMMA+ Results of MHTGR-350 Depressurized Conduction Cooldown Accident Graphite Emissivity Peak Fuel Temp ( ℃ ) Time to Peak Fuel Temp (hr)Peak RPV Temp (C) 0.55156976375 0.65155275377 0.75153573380 0.85151872382 * Dr. Nam-il Tak, takni@kaeri.re.kr, +82-42-868-8082takni@kaeri.re.kr - Fuel & Replaceable Reflector : H451 (density=1740 kg/m 3 ) - Non Replaceable Reflector: Grade 2020 (density=1780 kg/m3) Δ 51 ℃ / Δ0.30

6. In the present study, the TE of selected nuclear graphite grades for HTGR core components have been determined under both as-received (un-oxidized) and oxidized conditions to see the changes in TE owing to the surface condition (roughness, crystallinity, porosity), and Temperature.

7. 2.1 Materials and Oxidized Specimen Preparatio n 2. Experiment (1), Material: IG-11, IG-110, IG-430, PCEA, NBG-18 Specimen: As-received condition (0% oxidation) Oxidized specimens (weight loss: 5%, 10% in air at 600 ℃ box furnace) No.Grade Manufa- cturer Forming Method Source Coke Grain size ( ㎛ ) Porosity (%) Density (g/cm 3 ) 1IG-11Toyo Tanso Iso-stat. Molded Petroleum coke 2020.01.76 2IG-110Toyo Tanso Iso-stat. Molded Petroleum coke 20 fine-grain 18.41.77 3PCEA GrafTech Int. Extruded Petroleum coke ~360 med-grain 18.31.83 4IG-430Toyo Tanso Iso-stat. Molded Pitch coke ∼ 10 fine-grain 18.01.82 5NBG-18SGLVibra. MoldedPitch coke ∼ 300 med-grain 17.81.85 40 3 Specimen size (mm) Ra: < 0.5 ㎛

8. 8 2. Experiment (2) 2.2 Thermal Emissivity Measurement Far-IR measurement equipment : JOOWON Industrial CO., LTD. Detector: Liquid N 2 cooled MCT (Midac 4400, USA) Measured wavelength range: 2-25 ㎛ (5000–400 cm -1 ). (The wave length in between 5 - 20 ㎛ was processed for emissivity determination). Measured temperature range: 100–500 o C Reference Ideal Black Body Furnace : Infrared Sys. Dev. Corp, Model 563, Hyperion R, Copper, Thermal stability: ±0.1 o C, 0.995 at 30 - 550 o C. All spectra were obtained as 128 integration times at 4 cm -1 resolution.

9. 2.3 Surface Roughness and Crystallinity Measurements (α-Step, SEM, Raman spectroscopy) Raman Spectroscopy (inVia System, Renishaw) Wavelength: 514.5 nm Ar Laser(Green), Beam size: 1nm, Resolution: down to 0.4cm±1-2 cm -1 (x500). Averaged I d / I g ratio at 5 locations for crystallinity estimation 2. Experiment (3) α-Step Model: Dektak 6M ± 250 ㎛,  1000 ㎛ scan : without pore  5000 ㎛ scan : with pore Scan speed : 30sec

10. Average Roughness (Ra) Ra = Average of distance from Mean-line to Peak and valley

11. 2.4 Evaluation of Effects of Pore on TE by Artificial Pore Simulation Method (APSM): - Both the Roughness and Pore affect TE simultaneously. (A)Effects of Pore ? Simulation of pores with artificial holes (Φ:500 ㎛, Depth: 250 ㎛ ) (B) Effects of Pore and Roughness ? Number of holes: a) 0, b)32, c) 64, d) 128 a)Hole: 0, Roughness: 0.5 ㎛ b)Hole: 32, Roughness: 0.5 ㎛ c)Hole: 32, Roughness: 2 ㎛ 2. Experiment (4)

12. 3. Results (1) 3.1 Oxidation and Temperature Effects on TE Oxidation increases TE (12% - 24%). Little differences are seen between the 5% and 10% oxidized specimen. Grade specific but TE tends to decrease for 5 – 20% with temp. with some exception (NBG- 18, IG-11) Ref: J. D. Plunkett and W. D. Kingery Proc. 4 th Conference on Carbon (Buffalo, 1960) pp. 457-472 AUC graphite, oxidized at 900 ℃, 12 min. 0.2 - 10 ㎛ At 500 ℃, AUC : 0.554 to 0.772 PCEA: 0.596 to 0.696

13. 3.2 Surface Roughness Effects on TE 3. Results (2) : α-step (without pore-1000 ㎛ scan length) (with pore-5000 ㎛ scan length) Emissivity- Roughness (Ra) Ra of IG-11, IG-110, and PCEA (Petroleum coke) show a peak at 5 %, however, IG-430 and NBG-18 (Pitch coke) show an increase without a peak with weight loss (oxidation). Emissivity increases from 0.558 to 0.800 when Ra increases from 0.143 to 7.839 ㎛.

14. 14 3.2 Crystallinity Effects on TE Ig increases with oxidation (weight loss), and Emissivity increases with Ig. The increase in Ig with oxidation (weight loss) is attributed to the selective oxidation of the binder phase resulting in an increase of crystalline grains exposure. 3. Results (3)

15. 3.3 Porosity effects on TE 3. Results (4) Emissivity peak appears to exist against the number of holes (pore), and Emissivity appears to increases with holes (pores) and surface roughness at 100 ℃, but decreased a little at 500 ℃ (different temperature effect).

16. 4. Conclusion Under the present limited test conditions, the thermal emissivity (TE) of nuclear graphite grades for (V) HTR appear to increase with oxidation (5%, 10 %) and largely decrease with temperature (- 500 ℃ ). These changes in the TE of oxidized specimens were attributed to the changes in the graphite surface condition owing to a selective oxidation of binder phase resulting in an increase in surface roughness, porosity, and crystallinity. Though not as critical as the other thermal properties, such as the heat capacity or thermal conductivity, the changes in TE during an off-normal condition are expected to contribute to the safety of (V) HTGR positively (decrease in a fuel temperature of 15 - 18 ℃ per 5% oxidation).

17. 17 Thank you for your attention.