14th International Nuclear Graphite Specialist Meeting (INGSM ̵ 14) September 16-18, 2013, Hilton Seattle, Washington State, USA

Oxidation Effects on The Thermal Emissiv-ity of Selected Nuclear Graphite

Se-Hwan Chi1, Seung-Kuk Seo 2, Jae-Seung Roh 2 and Min-Hwan Kim1

1Nuclear Hydrogen Development and Demonstration Project, Korea Atomic Energy Research Institute (KAERI),

Dae Deok-daero 989-111, Yuseong, Daejeon 305-353 Rep. of Korea2 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)

Contents1. 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. IntroductionThermal 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-tem-perature gas-cooled reactor.

HTTR

Heat transport via thermal radiation across the gap between the graphite core

and the steel core barrel.

Thermal Emissivity is defined as the ratio of energy ra-diated 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 mate-rial, the emissivity of a given graphite will largely de-pend upon the component surface condition and the operating environment.

5

Results of GAMMA+ Calculation*GAMMA+ Results of MHTGR-350 Depressurized Conduction Cooldown Ac-cident

Graphite Emissiv-ity

Peak Fuel Temp (℃) Time to Peak Fuel Temp (hr) Peak RPV Temp (C)

0.55 1569 76 3750.65 1552 75 3770.75 1535 73 3800.85 1518 72 382

* Dr. Nam-il Tak, takni@kaeri.re.kr, +82-42-868-8082

0.45 0.55 0.65 0.75 0.85 0.951500

1520

1540

1560

1580

1600

Graphite Emissivity

Peak

Fue

l Tem

pera

ture

(C)

- Fuel & Replaceable Reflector : H451 (density=1740 kg/m3)

- Non Replaceable Reflector: Grade 2020 (density=1780 kg/m3)

Δ51℃/ Δ0.30

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.

2.1 Materials and Oxidized Specimen Preparation

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/cm3)

1 IG-11 Toyo Tanso Iso-stat. Molded

Petroleum coke 20 20.0 1.76

2 IG-110 Toyo Tanso Iso-stat. Molded

Petroleum coke

20 fine-grain 18.4 1.77

3 PCEA GrafTech Int. Extruded Petroleum

coke~360

med-grain 18.3 1.83

4 IG-430 Toyo Tanso Iso-stat. Molded Pitch coke ∼10

fine-grain 18.0 1.82

5 NBG-18 SGL Vibra. Molded Pitch coke ∼300med-grain 17.8 1.85

40

403

Specimen size (mm)

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 - 550 oC.All spectra were obtained as 128 integration times at 4 cm-1 resolution.

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 Id / Ig 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

Average Roughness (Ra)

Ra = Average of distance from Mean-line to Peak and valley

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)

3. Results (1)3.1 Oxidation and Temperature Effects on TE

100 200 300 400 5000.5

0.6

0.7

0.8

0.9

IG-11 0% 5% 10%

Ther

mal

Emiss

ivity

Temperature(oC)100 200 300 400 500

0.5

0.6

0.7

0.8

0.9

Temperature(oC)

IG-110 0% 5% 10%

Ther

mal

Emiss

ivity

100 200 300 400 500

0.5

0.6

0.7

0.8

0.9

NBG-18 0% 5% 10%

Ther

mal

Emiss

ivity

Temperature(oC)

Oxidation increases TE (12% - 24%). Little differences are seen between the 5% and 10% oxidized spec-imen. 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. KingeryProc. 4th Conference on Carbon (Buf-falo, 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

100 200 300 400 500

0.6

0.7

0.8

0.9

IG-430 0% 5% 10%

Temperature(oC)

Ther

mal

Emiss

ivity

100 200 300 400 5000.5

0.6

0.7

0.8

0.9

PCEA 0% 5% 10%

Ther

mal

Emiss

ivity

Temperature(oC)

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

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)

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).

4. Conclusion

Under the present limited test conditions, the thermal emissivity (TE) of nu-clear 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.