1. Stratified Flow Phenomena, Graphite Oxidation, and Mitigation Strategies of Air Ingress Accident
INL / Chang Oh, U.S. PI

2. Background and Motivation
What happens following LOCA ?
Depressurization
Stratified Flow
Diffusion
Natural Convection
T/H Safety Issues
Core maximum temperature
Potential core collapse
Technical Requirements
Accurate stratified flow modeling
Accurate graphite oxidation and collapse modeling
Accurate power distribution with neutronics model

3. Objectives
To conduct experiments to supply information to model important phenomena in air-ingress accident, and code V&V.
Effect of density-driven stratified flow on the air-ingress
Oxidation and density variation of the graphite structures
Internal pore area density of the graphite structures
Effect of burn-off on the structural integrity of the graphite structures
To develop a coupled neutronics and thermal-hydraulic capability in the GAMMA code
Development of core neutronics model
Coupling neutronic-thermal hydraulic tools
Coupled core model V&V
To evaluate various methods for the mitigation
of air-ingress

4. Project Organization Schematic diagram of all tasks involved GAMMA
Stratified Flow Study
Task 1 (INL)
Validation
Task 2 (INL)
Stratified Flow Analysis
(CFD & GAMMA)
Stratified Flow
Experiment
Advanced Graphite
Oxidation Study
Models and
Parameters
Core Neutronic
Model
Models and
Parameters
Task 7 (KAIST)
Task 3 (INL)
Coupling Neutronic
Thermal Hydraulic Tool
Advanced Graphite
Oxidation Study
GAMMA
Code
Models and
Parameters
Task 8 (KAIST)
Task 5 (KAIST)
Experiment of Burn-off
In the Bottom Reflector
Analysis
Core Neutronic Model
Task 9 (KAIST)
Task 6 (KAIST)
Task 4 (INL)
Coupled Core Model
V&V
Structural Test of Burn-
off Bottom Reflector
Full Air-ingress Analysis
Air-ingress Mitigation Study

5. New Assumption on the Air-ingress Analysis
Stratified Flow in Air-ingress
Previous air-ingress analyses are all based on the assumption that the main air-ingress is dominated by molecular diffusion.
Previous analyses were performed using
1-D and a vertical geometry
A new issue has been raised for the possibility of a convective flow driven by local density gradient.
After depressurization, there is large density differences between inside(Helium) and outside(Air) of vessel.
The density driven stratified flow can highly accelerate the whole air-ingress scenarios.
Diffusion Assumption
(40000 sec)
Stratified Flow Assumption
(60 sec)

6. Density-Gradient Driven Stratified Flow
(1) Depressurization
(2) Onset-of Flow
(3) Density-driven Flow
Air ingress velocities by density driven flow
Volumetric flowrates of air and helium are the same.
At the interface, the shear stresses are the same between Air and Helium.

7. Density Driven Stratified Flow
Water and Salted Water Experiment
Helium
Air
Salted Water
Water

8. New Scenario for Air Ingress
Helium

9. CFD Analysis on the Stratified Flow in VHTR
51,566 nodes
Mesh (GAMBIT / FLUENT)

10. Porous Media Parameters
Porous Media Approach
Core and Plenum were assumed to be porous media.
Porosity and Permeability should be determined.
Porous Zones
Additional Momentum Source
permeability
Inertia resistance

11. Porous Media Parameters
Porosity
In the Core
Core Hole Pattern
(d = 1.58 cm, p = 3.27 cm)
In the Lower Plenum
Geometry of Lower Plenum
(d = m, p = 0.36 m)

12. Porous Media Parameters
Flow Resistance Parameters in the Core
Empirically determined based on the friction correlations
Flow resistance in the radial was assumed to be infinitely large.
In the Core
Friction Factor in the Circular Channel

13. Porous Media Parameters
Flow Resistance Parameters in the Lower Plenum
Flow resistance in the radial direction was calculated based on the friction data in the staggered array.
Flow resistance in the axial direction was calculated based on the friction data in the circular pipe flow.
For axial direction
In the Lower Plenum
For radial direction
Friction Factor in the Staggered Array

14. Simulation of Stratified Flow
Stratified Flow Simulation (by FLUENT 6.3)
Initial Conditions
Temperature
Air-Mole Fraction
Natural convection started about 160 sec after simulation.

15. Turbulence Models

16. Multi-step Approach for Air-ingress Analysis
Stratified flow phase was solved by CFD code (FLUENT).
Depressurization and Diffusion/natural convection phase were solved by GAMMA code.
CFD Code
System Code
1. Depressurization Analysis
Data Transfer
2. Stratified Flow Analysis
Data Transfer
3. Natural Convection Analysis
FLUENT Simulation
GAMMA Simulation

17. Air Ingress Analysis - Results
Temperature
(Bottom Reflector)
Temperature
(Core)
Corrosion
(Lower Plenum)
DDA-1
Diffusion Dominated Air-Ingress
In the Infinite Vault (1X1010 m3)
DDA-2
In the Finite Vault (25,000 m3)
SFDA-1
Stratified Flow Dominated Air-Intress
SFDA-2
In the finite Vault (25,000 m3)
Stratified Flow Assumption
Diffusion Assumption

18. Isothermal Experiment in the Horizontal Circular Pipe (TEST-1)
Experimental Plan - 1
Isothermal Experiment in the Horizontal Circular Pipe (TEST-1)
Focused on the separate effect of stratified flow phenomena
A simple scaling method used for pipe sizing and test conditions.
1~1.5 m 1m
0.5~1.0 m
Tube diameter = 20 cm

19. Scaling Analysis of Stratified Flow in a Simple Channel
Experimental Plan - 1
Scaling Analysis of Stratified Flow in a Simple Channel
Summary of Scaling Results (ratio = scaled down/full-scale)
Countercurrent stratified flow behavior in the VHTR hot duct
(Turner(1973))
By Reynolds number similitude
By gas law

20. Non-isothermal Test (TEST-2)
Experimental Plan - 2
Non-isothermal Test (TEST-2)
Focused on the coupling effect of stratified flow and natural convection.
On-set of natural convection is the main measuring parameter.
0.8~1.0 m
Pipe diameter 5 cm

21. Onset of natural convection occurred at around 400 sec.
Experimental Plan - 2
Fluent Simulation for Density Driven Air-ingress Experiment
10 sec
20 sec
40 sec
80 sec
300 sec
400 sec
410 sec
420 sec
Onset of natural convection occurred at around 400 sec.

22. Experimental Plan - 2
FLUENT Results for Density Driven Air-ingress Experiment (at Valve-3)
Time vs. Flow-rate
Time vs. Temperature
onset of
natural convection
onset of
natural convection
Flow rate and Temperature can be used as signals for onset-natural-circulation.

23. Experimental Plan – 3 and 4
Non-isothermal Test (TEST-3, TEST-4)
Focused on the coupling effects
Stratified Flow + Natural Convection + Porous Media + Chemical Reaction
Basic Experimental Procedures are the same as TEST 2
Metal (TEST-3)
or Graphite (TEST-4)
Non-isothermal test with Pebbles
Non-isothermal test with Structures

24. Experiment on the Oxidized Graphite Fracture
Experimental Set-up
The experiment was performed at 650 oC for uniform oxidation.
The test procedure and set-up is based on ASTM standard test method.
IG-110 and H451 graphite was used for testing.
Sample load and holder

25. Experiment on the Oxidized Graphite Fracture
Normal Compressive Stress vs. Burn-off
old data
old data
H-451
IG-110
New data
New data

26. Graphite Surface Area Density
Graphite Surface Area Density (Unoxidized Initial Value)
The graphite surface area density was calculated from the BET surface area measured by previous investigations.
Density
[g/m3]
Specific Surface Area
[m2/g]
Surface Area Density
[m2/m3]
NBG-18
(Contescu (2008))
1790
0.21
375.9
NGB-10
0.29
519.1
PCEA
20-20
0.46
823.4
IG-11
(Eto and Growcock (1981))
1750
2.8
4900
IG-110
(Nakano et al. (1997))
1780
0.5
890
H451
(Pawelko et al. (2001))
1760
0.75
1320
PGX
1730
0.7
1211

27. Effect of Graphite Burn-off on the Oxidation Rate
Graphite Oxidation Rate and Burn-off
The reaction rate increases with the increasing burn-off in the beginning because of the increase of pore size and porosity open.
The reaction rate decreases at high burn-off because the pores join together, decreasing the reaction surface area.

28. Modeling of Graphite Oxidation and Fracture in Air-ingress
Reference Reactors (GT-MHR 600 MWt)
Core Graphite Structure

29. Estimation of Corrosion Depth by GAMMA code
Burn-off refers to the oxidation of the graphite’s internal body, causing reduction of density, leading to reduction of stiffness and mechanical strength.
Corrosion refers to oxidation taking place on the outer surface exposed to airflow. The corrosion decreases the cross-sectional area available to support the weight leading to stress concentration.
Most Seriously Damaged!

30. Detailed Geometry of the Supporting Block
Coolant channels from core
Lower reflector blocks

31. Oxidized State Results
Compressive stress distribution on plenum head, 6.5 days after ONC
Corrosion Progression
1/6 cyclic symmetry unit of the modified plenum head for each day
* The specified time is the elapsed time after natural convection.

32. Oxidized State Results
Failure occurs 5.5 to 6 days after the start of natural convection

33. Experimental facility
Task 5 - Experimental facility
Task Progress
Kinetics
Completed activity
Planned activity
Activation energy
Order of reaction
Graphite selection
Experimental facility
Bottom reflector
Burn-off model
IG-110
IG-430
NBG-18
NBG-25
Mass transfer
Design
Installation
Heat/mass analogy
Other effects
Geometry
Burn-off
Moisture
IG-110, IG-430, NBG-18, NBG-25 were selected sample graphite. The test facility was installed.
This facility was designed to test kinetics, mass transfer, combined-effect, effect of burn-off and effect of moisture. Kinetics of IG-110 and IG-430 was studied this year.
Schematics of Experimental Facility 33

34. Task 5 - Test Condition Graphite Temperature (oC) Flow rate (SLPM)
First Kinetics of IG-110 and IG-430 was measured. Temperature range is 540~800 degree C. oxygen fraction was maximum 32%.
Picture of the Test Section
Temperature (oC)
540 ~ 800
Flow rate (SLPM)
~ 10 SLPM (0.04 m/s)
Oxygen fraction (%)
~ 34 %
Graphite 34

35. Task 5 – Kinetics (I) IG-110 IG-430 Activation energy (kJ/mol) 218 ± 4
Activation energy and order of reaction of IG-110 and IG-430 were measured. Figure show the data of IG-430
Effect of Temperature on Oxidation Rate
Effect of Oxygen Concentration on Oxidation Rate
IG-110
IG-430
Activation energy (kJ/mol)
218 ± 4
158.5 ± 1.5
Order of reaction, n
0.75 ± 0.15
0.37 ± 0.04 35

36. Task 5 – Kinetics (II) IG-110 IG-430 Material Author T (℃)
Oxygen Mole Fraction
Flow rate
(SLPM)
Ea (kJ/mol) n
Method
IG-110
Fuller
450~750
0.2
0.496
201 -
TGA
Kawakami
550~650
210
Gas Analysis
Ogawa
700~1500
0.05~0.19
0.2~4.5
200
KAIST
540~630
0.03~0.32
7~18
218
0.75
IG-430
KAERI
608~808 10
161.5
540~800
0~0.34
8~10
158.5
0.37 36

37. Task 6 - Task Progress Task Progress Fresh graphite Failure test
Completed activity
Planned activity
Fresh graphite
Failure test
Oxidized graphite
Mechanical test
Structural analysis
Uniform oxidation
Non-uniform oxidation
Structure
Support column
Support block
Other components
GAMMA
Failure Model
Development
Data collection
Estimation of burn-off 37

38. Task 6 - Bottom structures
Bottom reflector components and condition
Components
Condition
Relative temperature
Support column
A graphite column encounters oxygen first when an air-ingress event occurs
Low (cooling effect)
Support block
A support block has a lot of channels. External surface is relatively large.
High (close to core)
There are two bottom structures mainly issued. One is support column and another is support block. Support column was historically considered that it is the weakest structure. A support block also is easy to be oxidized because a support block has a lot of channels; large external surface. This year, the study was focused about failure of graphite column. (simplified graphite column figure)
Oxidized graphite column is to be decreased in bulk density and to be slender. Long column is easy to fail under axial load.
Schematics of oxidation trend in graphite support columns
Graphite support columns, GT-MHR 600MWth 38

39. Task 6 - Test facility Picture of the electric furnace
Hardened steel plate
Machine cross head
Air outlet
Vent
Thermocouple
Clear Plastic safety shield
Test specimen
Graphite specimen
Insulating material
Compression block
Spherical block
Support plate
Air inlet
Distributor
Picture of the electric furnace
Picture of the failure test facility 39

40. Task 6 – Compressive and buckling strength of
fresh IG-110 column
Compressive strength of IG-110
IG-110
Φ15mm ×30mm
80.16 ± 1.97MPa
Φ25mm ×50mm
78.75 ± 2.48 MPa
Average
79.46 MPa
Buckling point:
Measured compressive strength was listed in table. The measured bucking strength versus slenderness ratio was showed in figure . It shows that the critical strength of graphite columns is reduced with an increase of the slenderness ratio. buckling point is estimated by the calculating the intersection point of compressive strength and buckling strength correlation.
Buckling strength 40

41. Task 6 - Compressive and buckling strength of oxidized IG-110 column
Normalized compressive strength of oxidized graphite
Normalized compressive and buckling strength of oxidized graphite columns.
Samples of graphite were oxidized in the electric furnace. Furnace temperature was 600 degree C and air is sufficiently supplied.
Figure shows that normalized compressive strength of oxidized graphite. The relation between the compressive strength and bulk density can be estimated by the Knudsen relation. Another figure shows the degradation of oxidized graphite which have various geometry. The relation between the strength and bulk density also can be estimated by the Knudsen relation; there is no remarkable trend.
The strength of oxidized graphite column under axial load: 41

42. Average fracture load (Unit: Kgf)
Task 6 - Fracture load changes of
oxidized complicated-shape samples
Table of complicated-shape sample test
Sample 1
Sample 2
Sample 3
Geometry (Unit: mm)
Top: Φ10 × 10
Bottom: Φ20 × 10
Top: Φ10 × 20
Bottom: Φ20 × 20
Cylindrical column: Φ15 × 60
Cap: 15.1 × 10 (7 in depth)
Average fracture load (Unit: Kgf)
590.1
531.3
1297.3
Complicated-shape sample were tested for applicability of the correlation. The correlation can be applied for the complicated-shape samples. It turns out that the strength degradation trend of oxidized graphite structure is independent of geometry while the strength degradation of a uniformly oxidized graphite structure is dependent on the initial strength of structure and the bulk density change.
Fracture load changes of
oxidized complicated-shape samples 42