Understanding Steam Explosion Micro Interactions: Visualization and Analysis

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1 Understanding Steam Explosion Micro Interactions: Visualization and Analysis Roberta C. Hansson School of Engineering Science Department of Physics Division of Nuclear Power Safety Royal Institute of Technology, Stockholm, Sweden

2 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.

3 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 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

4 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 explode

5 Corium and Corium Simulant Experiments Period: Panic (of unknows, ) WASH-1400, thermodynamic, Board and Hall, CR 30-40%, SL-1: 15% Period: Realism ( ) Winfrith, SNL, 10-20%, UO 2 Thermite, Single Drop, Period: (false?) optimism ( ) KROTOS, FARO, PREMIX. Period: doubt and lost ( ) FARO-33 (real corium), SERENA-1, TROI Period: comprehension (to come?) MISTEE

6 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

7 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.

8 Objectives Single Drop Vapor Explosions Tests Well Controlled High Temperature ( 2000 o C 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

9 MISTEE Facility X-ray Tube Melt Release Plug External Trigger Induction Furnace X-ray Detector/ Intensifier High Speed Camera Well-controlled Test apparatus Controlled triggering system Hightemperature melt generator Measurement Dynamic pressure High-speed photography High-speed radiography

10 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 Laser Induction Coil Crucible Test Chamber Shock Generator Release Plug Lexan Trigger Photo Sensor

11 The SHARP System Light Source X-ray Images Mirror X-ray Source Digital High-Speed Camera (100,000 fps) X-ray Detector Digital High-Speed Camera (8,000 fps) Photography Images

12 Image Processing Reference Phantom SE images Photography X-ray Radiography Photography X-ray Radiography Background subtraction Segmentation (edge detection) Center of mass Projected area Coordiantes offset Rescaling factor Matched Images

13 Radiography and Photography Photography X-ray Melt: tin T melt =1000 T water =72

14 Matched Images

15 Data Bubble diameter (mm) ,0 Bubble 0,9 Melt Pressure 0,8 0,7 External Trigger Pressure 0,1 0,0 0-0, Time (ms) 0,6 0,5 0,4 0,3 0,2 Pressure (MPa)

16 Steam Explosion Energetics 5,0 4,5 4,0 45 C 50 C () η t = W ( t) 0 E melt 3,5 3,0 73 C 1,8 14 D/D 0 2,5 2,0 80 C 1,6 1,4 12 1,5 1,2 10 1,0 0, t(ms) 3 R R&& 3 2 R R R + & W () t = 4πρl dr R0 2σR + + 4μRR& ρ l CR c (%) 1,0 0,8 0,6 0,4 4 0,2 0, t t (ms) (ms) 8 6 R (mm)

17 Bubble Dynamics 1 st Cycle 1st expansion: after the vapor film is destabilized, direct contact heat transfer and vapor is generated due to nucleation 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. 1,2 1 st expansion 2,0 3,2 1 st contraction 1,0 1,8 1,6 2,8 Conversion ratio (%) 0,8 0,6 0,4 0,2 0,0 1,4 1,2 1,0 0,8 0,6 0,4 0,2 d(deq/deq 0 )/dt max (Deq/Deq 0 )/dt 2,4 2,0 1,6 1,2 0, T coolant ( C) 0, T coolant (C) Conflicting remark Current wisdom: Low subcooling leads to a less energetic steam explosion

18 Bubble Dynamics - 2 nd cycle 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?) Deq/Deq nd expansion 6,0 5,6 5,2 4,8 4,4 4,0 (Deq/Deq 0 )/dt max (Deq/Deq 0 )/dt 6,0 5,6 5,2 4,8 4,4 4,0 2 nd expansion Pv/Pv 0 0 3, T coolant (C) 3, T coolant (C) Coolant subcooling shows dual-effect in respect to bubble dynamics How to explain the steam explosion energetics?

19 Melt Dynamics Undisturbed molten droplet Prior external trigger arrival Explosive vaporization fine fragmentation of the molten droplet 1st bubble expansion melt non-uniform pre-fragmentation/ deformation 2nd bubble collapse mixing Bubble collapse water entrainment Final Explosive vaporization total fine fragmentation of the molten droplet The dynamics of the first cycle and the molten material ability to deform/pre-fragment fragment will dictate the explosivity of the steam explosion

20 Understanding Steam Explosions To obtain theoretical prediction of the explosion pressure and propagation p velocity, it is required a detailed knowledge of fuel-coolant mixing vapor coolant Coolant /melt contact Melt prefragmentation/ deformation Coolant impingement bubble collapse Coolant explosive vaporization Fragmentation Model 1,8 14 1,6 1,4 12 and energy transfer CR c (%) 1,2 1,0 0,8 0, R (mm) is needed. 0,4 4 0,2 0, t (ms)