Understanding Steam Explosion Micro Interactions:

Visualization and Analysis

Roberta C. Hansson

School of Engineering ScienceDepartment of Physics

Division of Nuclear Power SafetyRoyal Institute of Technology, Stockholm, Sweden

Steam Explosions

Vapor explosion (MFCI)High temperature liquid contacts with

cold and volatile liquid.

Rapid heat transfer between the high temperature liquid (molten material)

and cold liquid (water)•Explosive vapor generation, strong shock waves.•Hydrodynamic loading to the surrounding system.

Nuclear Reactor Case

• In-vessel FCIs (a-mode failure)– much experimental work has been done– deterministic and probabilistic methods

provided the consensus by (SERG-2, 1995)• the conditional probability of containment

failure is less than 0.001– Conservatively predicted dynamic loading is

bellow structural fragility (especially true for BWR: melt relocation, forest of penetrations)

• Ex-vessel FCIs– possible if accident management strategy

involves establishing a water pool under the vessel (e.g, AP-600, SBWR etc.) and supplying water to the melt.

• Water is highly subcooled• Large discharge rates

Ex-vessel MFCIs

1. Can we predict steam explosion energetics ?

2. Corium (low) explosivity?

3. Effect of material properties?

4. How to extrapolate to prototypic reactor conditions?

High temperature melts are known to explodeHigh temperature melts are known to explode

Corium and Corium SimulantExperiments

Period: Panic (of unknows, 1970-1980) WASH-1400, thermodynamic,Board and Hall, CR 30-40%,SL-1: 15%

Period: Realism (1980-1990) Winfrith, SNL, 10-20%,UO2 Thermite, Single Drop,

Period: (false?) optimism (1990-2000) KROTOS, FARO, PREMIX.

Period: doubt and lost (2000-2005) FARO-33 (real corium),SERENA-1, TROI

Period: comprehension (to come?) MISTEEMISTEE

Vapor Explosions Phases

• Vapor explosion consists of various sequential multiphase and multi-component phenomena in scales of– Mixing phase

• Jet impingement (Jet breakup and penetration) in air and coolant

– Triggering phase• bubble dynamics (interfacial instability)

– Propagation/Escalation phase• shock wave generation, propagation and escalation (detonation)• jet fragmentation

– Expansion phase• expansion of multiphase, multi-component mixture • structure response by impact

Triggering/Fine Fragmentation

• Definition– Phenomenon of rapid (explosive) vaporization and expansion

with detonation characteristics due to sudden direct contact between extremely hot liquid and volatile, cold liquid

• Rapid heat transfer resulting from dynamic fine fragmentation of the high temperature liquid.

• Fine fragmentation process is a key to understand the explosion phase of FCI– However, quite limited support from experimental observation.

Objectives

• Single Drop Vapor Explosions Tests – Well Controlled– High Temperature ( 2000 oC or more)

• Visualization of Triggering and Fine Fragmentation Process.– Continuous High-Speed X-ray radiography– High-Speed Regular Photography

• Quantitative data for Triggering and Fine Fragmentation– Phase Dynamics and Distributions (Melt, Vapor, Liquid)– Energetics of Vapor Explosions

• Effects of Materials on Vapor Explosions – Limiting Mechanism

MISTEE Facility

InductionFurnace

MeltRelease Plug

X-ray Tube

ExternalTrigger

X-rayDetector/Intensifier

High SpeedCamera

Well-controlled• Test apparatus

– Controlled triggering system

– High-temperature melt generator

• Measurement– Dynamic

pressure– High-speed

photography– High-speed

radiography

Test Chamber

• Test section– 180 x 130 x 250 mm

(~ 6 liter, 10 mm thick)– Plexiglas

• Furnace– Induction Furnace (300V, 40A)

• Trigger system– Piston Shock Generator by Rapid

Capacitor Discharge

InductionCoil

Crucible

TestChamber Lexan

ReleasePlug

ShockGenerator

Trigger

PhotoSensor

Laser

The SHARP System

X-ray Source

Light Source

Mirror

Digital High-SpeedCamera

(100,000 fps)

Digital High-SpeedCamera

(8,000 fps)

X-ray Detector

X-ray Images

PhotographyImages

Image Processing

Photography X-ray Radiography

SE images

Background subtraction

Segmentation (edge detection)

X-ray Radiography

Photography

Matched ImagesMatched Images

Reference Phantom

Center of mass

Rescaling factor

Coordiantesoffset

Projected area

Radiography and Photography

Photography X-ray

Melt: tin Tmelt=1000℃ Twater=72℃

Matched Images

Data

223 224 225 226 227 228 229 230 231 232 233 2340

1

2

3

4

5

6

7

8

9

10

11

12

223 224 225 226 227 228 229 230 231 232 233 234-0,1

0,0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1,0 Bubble Melt

B

ubbl

e di

amet

er (m

m)

Pressure

External Trigger Pressure

Pre

ssur

e (M

Pa)

Time (ms)

Steam Explosion Energetics

-5 -4 -3 -2 -1 0 1 2 3 4 50,5

1,0

1,5

2,0

2,5

3,0

3,5

4,0

4,5

5,045 C

50 C

D/D

0

t(ms)

80 C

73 C

-5 -4 -3 -2 -1 0 1 2 3 4 5

2

4

6

8

10

12

14

-5 -4 -3 -2 -1 0 1 2 3 4 5

0,0

0,2

0,4

0,6

0,8

1,0

1,2

1,4

1,6

1,8

R (m

m)

t (ms)

CR

c (%)

( ) dRRRR

RRRRtW

R

R

l

l ∫⎥⎥⎥⎥

⎦

⎤

⎢⎢⎢⎢

⎣

⎡

++

++=

0 4223

4

223

&

&&&

μρσπρ

( ) ( )0meltEtWt =η

-5 -4 -3 -2 -1 0 1 2 3 4 5

2

4

6

8

10

12

14

-5 -4 -3 -2 -1 0 1 2 3 4 5

0,0

0,2

0,4

0,6

0,8

1,0

1,2

1,4

1,6

1,8

R (m

m)

t (ms)

CR

c (%)

Bubble Dynamics – 1st Cycle

30 35 40 45 50 55 60 65 70 75 80 85 900,0

0,2

0,4

0,6

0,8

1,0

1,2

1,4

1,6

1,8

2,0

0,0

0,2

0,4

0,6

0,8

1,0

1,2

Tcoolant (°C)

d(D

eq/D

eq0)/d

t

1st expansion

Con

vers

ion

ratio

(%)

30 35 40 45 50 55 60 65 70 75 80 85 900,8

1,2

1,6

2,0

2,4

2,8

3,2

1st contraction

max

(D

eq/D

eq0)/d

t

Tcoolant (C)

1st expansion: after the vapor film is destabilized, direct contact heat transfer and

vapor is generated due to nucleation

Conflicting remarkConflicting remark

Current wisdom: Low subcooling leads to a less energetic steam explosion

Contraction: bubble reaches its maximum and collapses (inertia).

The dynamic impact of the vapor bubble contraction, after the nucleation, on molten material leads to

coolant entrainment.

Bubble Dynamics - 2nd cycle

30 35 40 45 50 55 60 65 70 75 80 85 90

0

1

2

3

4

5

30 35 40 45 50 55 60 65 70 75 80 85 90

3,6

4,0

4,4

4,8

5,2

5,6

6,0

Deq

/Deq

0

2nd expansion

(Deq

/Deq

0)/dt

Tcoolant (C)

3,6

4,0

4,4

4,8

5,2

5,6

6,0

30 35 40 45 50 55 60 65 70 75 80 85 90

-2

0

2

4

6

8

10

2nd expansion

max

(D

eq/D

eq0)/

dtTcoolant (C)

Pv/

Pv 0

2st expansion:

entrained water leads to explosive vaporization.--fine fragmentation of the molten material

-High conversion ratio-fast transient (does coolant temperature play an important role?)

Coolant subcooling shows dual-effect in respect to bubble dynamics

How to explain the steam explosion energetics?

Melt Dynamics

Undisturbed molten dropletPrior external trigger arrival

1st bubble expansion

melt non-uniform pre-fragmentation/ deformation

Bubble collapse

water entrainment

Explosive vaporization

fine fragmentation of the molten droplet

2nd bubble collapse

mixing

Final Explosive vaporization

total fine fragmentation of the molten droplet

The dynamics of the first cycle and the molten material ability The dynamics of the first cycle and the molten material ability to to deform/predeform/pre--fragment will dictate the fragment will dictate the explosivityexplosivity of the steam explosionof the steam explosion

Understanding Steam Explosions

To obtain theoretical prediction of the explosion pressure and pTo obtain theoretical prediction of the explosion pressure and propagation ropagation velocity, it is required a detailed knowledge of fuelvelocity, it is required a detailed knowledge of fuel--coolant mixingcoolant mixing

and energy transfer and energy transfer is needed.is needed.

vapor coolant Coolant /melt contact

Melt prefragmentation/

deformation

Coolant impingement

bubble collapse

Coolant explosive

vaporization

-5 -4 -3 -2 -1 0 1 2 3 4 5

2

4

6

8

10

12

14

-5 -4 -3 -2 -1 0 1 2 3 4 5

0,0

0,2

0,4

0,6

0,8

1,0

1,2

1,4

1,6

1,8

R (m

m)

t (ms)

CR

c (%)

Fragmentation

Model