The Potential for Graphite Oxidation in Dry Wall ICF

Chambers

El S. Mogahed, I. N Sviatoslavsky, J. G. Murphy, H. Y. Khater, M. H. Anderson, and

G. L. Kulcinski

Fusion Technology Institute University of Wisconsin

May 31, 2001

Outline of Presentation• Why are we concerned?• Fundamental assumptions and model of worst accident • Time dependent temperature of graphite first wall after

accident (without oxidation)• Time dependent availability of oxygen in the chamber • Potential first wall carbon oxidation rates• Time dependent temperature of graphite first wall after

accident (with oxidation)• Calculated erosion rates (with oxidation)• Methods to mitigate carbon oxidation• Conclusions and recommendations

3

Why are we concerned about possible oxidation of dry wall graphite structures?

• Woven graphite structures run at 800 to 1500 °C in a 0.5 torr Xe atmosphere in SOMBRERO.

• The introduction of air as a result of an accident could release radioactive tritium and result in structural degradation if oxidation were to take place.

• The fundamental question is “ Will the graphite first wall temperature be high enough, when the oxygen reaches it, to cause significant oxidation?”

4

Worst Accident Scenario• A 1 m2 hole is introduced in the 1.3 m thick outer building wall

while the reactor is operating.• The air rushes through the 1 m2 opening and through the laser

ports in the outer chamber, and finally through the beam ports in the inner chamber.

• The reactor and all mechanical equipment shuts down and the Li2O coolant drains from the chamber and upper manifold by gravity.

• The first wall radiates heat to the rest of the chamber and it also loses heat out the back of the blanket and shield.

• The ultimate heat sink is the wall of the outer building

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Model of SOMBRERO Building as it is used in the MELCOR code

1 m2 HoleAtmospheric

PressureOUTER BUILDING INNER

BUILDING CHAMBER9.54 m2 Hole

2.28 m2 Hole

60 x 0.159 m2

VOLUME = 9x105 m3

SOMBRERO BUILDING

60 x 0.038 m2

AtmosphericPressure

1 m2

HoleOUTER BUILDING OUTER BUILDING

INNERBUILDING

VOLUME = 1.73x103 m3

CHAMBER

VOLUME = 2.6x104 m3

50m 15m

10 m

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-The pressure in the chamber increases with time and reaches one atmosphere in about 4000 s (1.1 Hr) (MELCOR Calculations).

Air Pressure History in SOMBRERO Chamber After the Accident

0

2

4

6

8

10

12

0 1 2 3 4 5

Pressure In the Chamber Time History

Pres

sure

(104 P

a)

Time (103 s)

Atmospheric Pressure

Air Pressure

- The oxygen partial pressure follows same build-up pattern if there is no oxidation.

Maximum O2

Partial Pressure

(No Oxidation )

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SOMBRERO Finite Element Thermal ModelAssumptions:

Axi-symmetric model. Transient solution,that allows conduction, convection, and radiation heat transfer that varies with time. Back of chamber radiates to the inner building wall that is initially at 350°C. The back of the inner building wall radiates to the outer building wall at 20°C.

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• Assumptions:

Cooling due to Li22O drainage during the first 130 s after the accident

Only conduction and radiation heat transfer are in force after 130 s.

Midplane

Max.

Min.

SOMBRERO CHAMBER

Transient Thermal Analysis of SOMBRERO First Wall With No Oxidation Li2O, T= 550 °C

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• Assumptions:

Cooling due to Li22O drainage during the first 130 s after the accident

Only conduction and radiation heat transfer are in force after 130 s.

Transient Thermal Analysis of SOMBRERO First Wall With No Oxidation

600

700

800

900

1000

1100

1200

1300

1400

1500Temperature History of SOMBRERO FW

Max. (°C)Midplane (C°)Min. (C°)

0.1 1 10 100 1000

SOMBRERO FW Temperature (°C)

Time (s)

Midplane

Max.

Min.

SOMBRERO CHAMBER

Maximum Initial Temp. = 1450°C

•Conduction•Radiation

•Convection Cooling•Conduction•Radiation

TemperatureEqualization

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Cooling due to Li22O Drainage During the First 130 s After the Accident.

Only Conduction and Radiation Heat Transfer is in effect after 130 s.

- The Back of the Chamber Remains Hot (≈ 780°C) During the First 130 s.

Transient Thermal Analysis of SOMBRERO FW/Chamber With No Oxidation

200

400

600

800

1000

1200

1400

10-1 100 101 102 103 104 105 106 107

Max

imum

Fir

st W

all T

empe

ratu

re (°

C)

Time (s)

Midplane

Max.

Min.

SOMBRERO CHAMBER

130

s

1 D

ay

1 M

onth

Max.

Mid.

Min.

Cooling Due toLi

2O Drainage

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200

400

600

800

1000

1200

1400

0

2 104

4 104

6 104

8 104

1 105

1.2 105

10-1 100 101 102 103 104

Maximum First Wall Temperature (No Oxidation) (°C) Pressure (Pa)

Time (s)

FW Temperature Air Pressure

Oxygen PartialPressure

The Oxygen Partial Pressure is Low While the First WallTemperature is High

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10 -9

10 -8

10 -7

10 -6

10 -5

10 -4

10 -3

200 400 600 800 1000

Oxidation Rate (g/cm

2s)

First Wall Maximum Temperature (°C)

Union Carbide (CIT)

GraphNOL N3M

1500

ExperimentalDataExtrapolation

Union Carbide (CIT)

GraphNOL N3MPfizer Pyrolytic

ExperimentalDataExtrapolation

0

5 10 5

1 10 6

1.5 106

2 10 6

2.5 106

3 10 6

Time (s)

Union Carbide (CIT)

GraphNOL N3M

1 Month

One Day

One Week

Pfizer Pyrolytic

ExperimentalDataExtrapolation

FW Max.Temperature

Different Graphites Have Very Different Oxidation Rates

Experimental data is available for temperatures over 800°C.

- Union Carbide graphite oxidation rate is two times higher than GraphNOL N3M.- Pfizer pyrolytic graphite oxidation rate is much less than both Union Carbide Graphite and GraphNOL N3M.

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The Choice of the Wrong Graphite for an IFE First Wall Could Cause Problems if The Building is Breached

200

400

600

800

1000

1200

1400

1600

10-3

10-2

10-1

100

101

102

103

104

10-1 100 101 102 103 104 105 106 107

Maximum First Wall Temperature (°C) Oxidation thickness( cm)

Time (s)

Temp. of FWwithout Oxidation

Temp. of GraphNOL N3Mwith Oxidation

• FW graphite temperature rises as a result of exothermic reaction with oxygen.

Temp. of Pfizer Pyrolyticwith Oxidation

Oxidation thickness(Pfizer Pyrolytic)

• Oxidation of pyrolytic graphite is very low and raises FW temperature about 10°C at most.

Oxidation thicknessGraphNOL N3M

• Oxidation of GraphNOL graphite is very rapid, it will oxidize the 1 cm FW in about 3.5 Hr.

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Methods of Mitigating Air Ingress Accident Effect

• Shutters on chamber beam-ports triggered by pressure imbalance

• Reduce inner building wall temperature• SiC coating everywhere except first wall• Thin (≈ 30 ) SiC undercoating on the first wall• Flood chamber with CO2

• Plug hole in building!

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Conclusions• The survival of the graphite first wall, in the event of

an unlikely ingress of air, is critically dependent on the type of graphite used.

• If the wrong type of graphite is chosen, the first wall could be completely oxidized in a few hours

• If pyrolytic, or H-451 graphite is chosen, there should be little oxidation (even after a few months)

• An experiment is needed to measure oxidation rates at reduced oxygen levels and at 200<T< 800 °C.