14 th International Nuclear Graphite Specialist Meeting (INGSM ̵ 14 )

1. 14th 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 Chi1, Seung-KukSeo 2, Jae-SeungRoh 2 and Min-Hwan Kim1 1Nuclear Hydrogen Development and Demonstration Project, Korea Atomic Energy Research Institute (KAERI), DaeDeok-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. 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. Results of GAMMA+ Calculation* Δ51℃/ Δ0.30 * Dr. Nam-ilTak, takni@kaeri.re.kr, +82-42-868-8082

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. 40 3 40 Specimen size (mm) • 2. Experiment (1) 2.1 Materials and Oxidized Specimen Preparation 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) Ra: < 0.5㎛

8. 2. Experiment (2) 2.2 Thermal Emissivity Measurement Far-IR measurement equipment : JOOWON Industrial CO., LTD. Detector: Liquid N2 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 oC Reference Ideal Black Body Furnace : Infrared Sys. Dev. Corp, Model 563, Hyperion R, Copper, Thermal stability: ±0.1 oC, 0.995 at 30 - 550oC. All spectra were obtained as 128 integration times at 4 cm-1 resolution.

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

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

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

12. 3. Results (1) 3.1 Oxidation and Temperature Effects on TE Ref: J. D. Plunkett and W. D. Kingery Proc. 4th 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 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)

13. 3. Results (2) : α-step 3.2 Surface Roughness Effects on TE Emissivity- Roughness (Ra) (with pore-5000㎛ scan length) (without pore-1000㎛ scan length) 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. 3. Results (3) 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.

15. 3. Results (4) 3.3 Porosity effects on TE 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 Underthe 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. Thank you for your attention.