Overview of Experimental Programs on Core Melt Progression and Fission Product Release Behaviour

1
Overview of Experimental Programs on Core Melt
Progression and Fission Product Release Behaviour

• B.J. Lewis, Royal Military College of Canada
• R. Dickson, Atomic Energy of Canada Limited
• F.C. Iglesias, Candesco Corporation
• International VERCORS Seminar
• Gréoux les bains, France
• October 15-16, 2007

2
Outline

• Experiment Review
• Integral Severe Accident and Single Effect Tests
• Degraded Core Accident Phenomena
• Fission Product Release (FPR) Behavior

3
In-Pile Tests

• Source Term Experiments Project (STEP 1,2,3,4)
• Fission product release (FPR) and aerosol
  chemistry
• Source Term Tests (ST 1,2)
• FPR aerosols from highly-irradiated fuel
  (reducing conditions)
• Damaged Fuel (DF 1,2,3,4) Relocation Experiment
• Coolant flow rate, system/fuel-rod pressure,
  degree of initial clad oxidation
• Severe Fuel Damage Tests (SFD ST, 1-1, 1-3, 1-4)
• Fuel bundle FPR (transport/deposition)
  behavior, H2 generation

4
In-Pile Tests Contd

• Full Length High Temperature Tests (FLHT 1,2,4,5)
• Oxidation H2 generation in full-length rods
• Loss-of-Fluid Test Facility Fission Product Test
  (LOFT FP-2)
• Large-scale test on FPR, steam supply/reflood
• Melt Progression (MP 1,2)
• Ceramic pool behavior in blocked-core accidents
• Blowdown Test Facility (BTF-104, -105A, -105B,
  -107)
• CANDU fuel and FPR behaviour
• Phebus SFD ? Phebus Fission Product Tests
  (FPT-0,-1,-2,-3-4)
• Core, cooling system containment response
  including FPR transport/deposition
• Semi-volatile actinide release from UO2/ZrO2
  rubble bed

5
Out-of-Pile Tests (Electrically Heated)

• CORA (19 tests) ? Quench
• Temporal behavior of core melt progression
  reflood
• PARAMETER (UO2 pellets and VVER cladding (1 Nb))

6
Bundle Configurations
7
Out-of-Pile Annealing Tests

• Single Effects Fission Product Release (FPR)
  Experiments
• FPR from spent fuel (hydrogen, steam, air)
• ORNL Horizontal Induction (HI 1-6), Vertical
  Induction (VI 1-7)
• CEA-CENG HEVA 1-8, Vercors 1-6, High Temperature
  (HT 1-3), Release of Transuranics (RT 1-8)
• AECL-CRL gt300 tests, e.g., MCE1-1,-6,-7,
  MCE2-13,-19, HCE2-BM3,-CM4, UCE12-8
• JAERI Verification Experiments of radionuclides
  Gas/Aerosol release (VEGA 1-10)

8
ORNL Experiments

• Test conditions
• Highly-irradiated Zircaloy-clad UO2 fuel samples
  15-20 cm long (100-200 g)
• Atmospheric pressure up to 1700-2700 K (time at
  temperature 2-60 min)
• Test atmosphere steam in VI-3, hydrogen in VI-5,
  hydrogen followed by steam in VI-6, and air and
  steam in VI-7 (investigate atmospheric effect on
  FPR)
• Major differences between VI and HI tests
• VI tests (vertical) vs HI tests (horizontal)
• VI test conducted at higher burnup and
  temperature (2300-2700 K)
• Measurements obtained
• Sample temperature vs time measured by optical
  pyrometry
• Thermal gradient tube (TGT) downstream to collect
  condensing vapors
• Graduated filters impregnated charcoal
  cartridges to collect particulates and volatile I
  species
• Charcoal cold trap to measure fission gases
• On-line measurements of fuel location and Cs-137
  in TGT and Kr-85 in gas traps
• Post-test analysis of all components by gamma-ray
  spectrometry, NAA, spark-source mass spec,
  emission spectrometry

9
ORNL Experiments Contd

• Major test results
• Similar release rates for noble gases, Cs and I
• Difference Cs transport behavior in steam
  relative to hydrogen.
• Reactive vapor forms of Cs predominate in
  hydrogen conditions and transportable aerosols in
  steam
• Similar Te and Sb release from UO2 as for
  volatile FPs but retained by metallic Zircaloy
  until nearly complete clad oxidation
• Both Eu and Sb showed sensitivity to oxygen
  potential at high temperature.
• Limitations
• Segmented furnace tube did not provide good
  containment of test environment (oxidation of
  graphite susceptor)
• Samples at temperature for short period of time
  (20 min) which may not be long enough for
  oxidative release

10
CEA-CENG Experiments

• HEVA program (1983-1989)
• Heated Zircaloy-clad specimens of irradiated PWR
  fuel in mixtures of steam/H2 and pure H2 from
  1800-2370 K (8 tests)
• Gamma spectrometry measured FPR from fuel and
  transport
• Aerosols collected in heated (temperature varied)
  cascade impactor and filters
• Control rod materials used in HEVA-07 (Ag-In-Cd
  exclusively) and HEVA-08 (control rod and fuel
  material)

11
CEA-CENG Experiments Contd

• VERCORS program (1989 to 1994)
• Spent fuel samples heated to maximum temperature
  of 2620 K (6 tests)
• 3 PWR pellets with 2 half pellets (depleted U at
  ends) in unsealed Zrly clad
• Re-irradiated in SILOE reactor to restore
  short-lived FPs (I, Te, Mo, Ba, La)
• Post-test gamma scanning (including gamma
  tomography) (complete FP mass balance)
• Results of FP behaviour
• Nearly complete release of volatiles (Cs, I, Te
  and Sb)
• Te and Sb initially trapped in unoxidized
  cladding
• Semi-volatiles (Mo, Rh and Ba) (1/2 that of
  volatile release depending on atmospheric
  conditions)
• Increased Mo release in oxiding conditions ? 92
  release (VERCORS 5) vs 47 (VERCORS 4)
• Increased Ba and Rh release in reducing
  conditions ? 45 and 80 of Rh and Ba, (VERCORS 4)
  vs 20 and 55 (VERCORS 5)
• Low-volatile FPs and actinides between 3 to 10
  (Ru, Ce, Np, Sr and Eu)
• Increased Np and Ce release under reducing
  conditions (VERCORS 4)
• No release of non-volatile FPs (Zr, Nb La and Nd)
  
• No significant enhancement in release in VERCORS
  6 (early fuel collapse and partial liquid corium)
• Similar problems due to flow bypass (as for ORNL
  tests)

12
CEA-CENG Experiments Contd

• VERCORS HT and RT program (1996 to 2002)
• Study FP and actinide release during later phase
  of accident with fuel liquefaction
• Study FPR behaviour as influenced by
• Fuel type (UO2 versus MOX)
• Fuel morphology (intact pellets versus debris
  fragments)
• Fuel burnup
• Presence of control materials (Ag, In, Cd and
  boric acid)
• Environmental conditions (oxidizing or reducing
  conditions)
• Nb and La release in severe VERCORS HT and RT
  tests
• Fuel collapse temperature (2400 to 2600 K for
  fuel burnups of 47-70 GWd/tU) 500 K below UO2
  melting temperature

13
AECL-CRL Experiments

• CRL Program (gt 300 annealing tests)
• FPR from clad unclad spent fuel samples (800 to
  2350 K in Ar/H2, steam and air atmospheres)
• Bare UO2 fragments (0.2-1.5 g each) and cladded
  specimens (Zircaloy foil bags and short segments
  of Zircaloy-clad fuel with end caps)
• Presence of Zircaloy can inhibit/delay release of
  volatile FPs
• Associated with time to oxidize Zircaloy
  cladding.
• Volatile FP release rates almost independent of
  temperature from 1670 to 2140 K after complete
  clad oxidation
• FPR Behaviour
• Deposition and transport of FPs studied.
• Volatiles release (Kr, Xe, I, Cs and Te) low in
  inert/reducing atmospheres but increase
  significantly after clad oxidation in oxidizing
  atmospheres
• Large fuel volatilization in high temperature
  tests with bare fuel (highly oxidizing
  environments) ? low-volatile release (Zr, La,
  Ba, Ce, Pr, Eu) via matrix stripping process
• Eu, Ba released in hydrogen-rich atmospheres vs
  Mo, Ru, Nb in steam
• Oxygen potential of environment well
  characterized
• Models developed for steam and air oxidation of
  UO2
• Significant release of fission products in air
  (Ru, Nb)

14
Degraded Core Accident Phenomena
15
Degraded Core Accident Phenomena Pressure

• Comparable behavior
• Phebus FP (0.2 MPa), LOFT FP-2 (1 MPa), CORA
  (0.2 to 1 MPa), TMI-2 (5 to 15 MPa)
• Enhanced clad ballooning failure (low pressure)
• FPT-0 (trace-irradiated) at 735?C
• Gap release measured (SFD, LOFT FP-2, Phebus FP)
• Aerosol composition
• Phebus FPT-0 and FPT-1 control rod (Ag,In,Cd),
  thermocouple (Re), fuel rod (Sn,U) materials
  (0.2 MPa)
• PBF SFD 1-4 FPs more important role (7 MPa)

16
Degraded Core Accident Phenomena Control Rod
Effects

• Pressure-Dependent Phenomena
• (i) Low Pressure
• SS clad/Zry guide tube contact with ballooning
  (high Cd vapour pressure) ? liquid phase 1150?C
• Ejection of molten control rod material ?
  chemically dissolves guide tube/ clad of
  surrounding rods well below Zry MP (1760?C) (CORA
  tests)
• (ii) High Pressure
• Failure of SS clad at MP (1450?C)
• Phebus FP tests consistent with low pressure
  scenario
• Control rod failure at 1120C (FPT-0) and 1350C
  (FPT-1)

17
Degraded Core Accident Phenomena Metallic Melt
Formation

• Interaction of spacer grids/Zry cladding/control
  materials
• Relocation
• PBF SFD TMI-2 accident (below coolant level)
• LOFT FP2, Phebus FP CORA (cooler bundle
  regions)
• Freezing temperature of melt
• 1070 K (Ag-In-Cd alloy) to 1220 K (Zr-Fe
  eutectic), 1230 K (Zr-Ni eutectic elemental
  silver) and 1460 K (Zr-Ag eutectic)
• Metallic blockages in integral tests similar to
  TMI-2 but not as extensive (shorter duration)
• FPT-0 metallographic examination
• Demonstrates role control rod plays in early melt
  formation
• Attack of Zry clad by molten Ag-In-Cd alloy
• Zr (20-40 wt), Ag (10-50wt), In (10-40wt), U
  (lt15wt), O (lt10wt), SS (lt5wt)

18
Degraded Core Accident Phenomena Zircaloy
Oxidation

• Exothermic reaction (6.5 kJ/g-Zr oxidized)
• Accelerated heatup rates (?10 K/s) at T gt 1500 to
  1700 K
• In-pile tests (PBF SFD, LOFT FP-2 and Phebus FP)
  and out-of-pile tests (CORA)

19
Degraded Core Accident Phenomena Hydrogen
Generation

• Related to steam availability

20
Degraded Core Accident Phenomena Fuel
Liquefaction

• UO2 ZrO2 dissolved by metallic Zircaloy/?-Zr(O)
  (1760 to 2000C)
• Fuel liquefaction in integral tests
• PBF SFD tests (15-18), LOFT FP-2 (15)
• TMI-2 core (45)
• Phebus FPT-1 (20), FPT-0 (50)
• Separation between ceramic and metallic blockage
• (U,Zr)O2 freezes at higher temperature

21
Degraded Core Accident Phenomena Molten Pool
Formation

• Ceramic heatup
• Steam diversion around blockage poor thermal
  conductivity
• FP decay heat (TMI-2) or increased
  fission/electrical heating (integral tests)
• Molten pool (surrounded by U-rich crust) under a
  cavity
• TMI-2 (U,Zr)O2 ceramic with transition metal
  oxides (Cr2O3, Fe3O4) 2700 K
• Phebus FPT-0 (U,Zr)O2 lattice with U (62wt),
  Zr (22 wt) O(14wt), Fe(0.6wt) 2720 K
• Fuel movement
• TMI-2 Thermo-mechanical failure of crust ? 20 t
  lower plenum
• FPT-0 Downward motion of pool (from lower grid
  spacer) (18100 s)
• Comparable to MP tests

22
Degraded Core Accident Phenomena Debris Bed
Formation

• TMI-2
• Top of molten pool lower plenum region
• Integral tests
• Upper debris bed formed by coolant injection
  (fragmentation) (SFD-ST LOFT FP-2)
• Less steam-rich transients - decladded
  fuel/fragments in upper part of bundle due to
  melting/relocation of clad (SFD 1-4, Phebus FP)

23
FPR Behavior

• Comparison
• PBF SFD-ST (steam-rich)/SFD (steam-starved),
  Phebus FPT-1 (steam-rich), TMI-2
• Ce actinides (typically lt 0.01)
• Ru,Sr,Sb (lt1)
• Ba (few )
• Mo (up to 50)
• Te (between 1 to 83)
• I,Cs, Noble Gas (up to 90)
• Comparable to annealing tests (ORNL, CEA, CRL)

24
FPR Phenomena

• FP trapping
• Te release (clad-oxidation state) Sn segregation
  ? SnTe release
• Sb sequestered in metallic melts (Ni, Ag alloys)
• Burnup
• Enhanced volatile FPR in SFD 1-4 (high-burnup) vs
  SFD 1-1 (trace-irradiated)
• Swelling (irradiated rods) in Phebus FPT-1
• Oxygen potential (H2/H2O ratio)
• Low Ba, Sr (Eu) release in steam tests (low-
  volatile oxides/hydroxides) vs higher release in
  reducing tests (ST, VI, HEVA, VERCORS, CRL)
• Enhanced Ba release in FPT-0 during escalation
  phase (H2 generation)
• Low Ru release (PO2 too low to form high-volatile
  oxides)
• Low actinide release (fuel volatilization (UO3)
  )

25
FPR Phenomena Contd

• FP behaviour in molten pool
• No enhancement with fuel liquefaction
  (non-coherent process)
• Volatile FPs bubble nucleation,
  coalescence/growth release via buoyancy
• Bubble trapping at pool/surrounding crust
• Volatile FPs (I,Cs) in previously molten ceramics
  in PBF SFD, Phebus FP TMI-2 reactor
• TMI-2 Iron oxides in melt (lower limit of -120
  kJ/mol)
• La, Ce, Sr as oxide (soluble in (U,Zr)O2)
• Ru, Sb metal immiscible in ceramic melt
• Cooldown/reflood
• Small release (fuel relocation) in Phebus FPT-0
  vs large release in LOFT FP-2 (12 volatile
  release) (local heating)

26
Ba Release in ORNL, CEA and Phebus Tests
Test Temp. (K) Duration (min) Atmosphere Ba Release ()
HI-4 HI-5 VI-2 VI-3 VI-4 VI-5 HEVA-4 HEVA-6 VERCORS-1 VERCORS-4 VERCORS-5 VERCORS HT-1 Phebus FPT-0 PEBUS FPT-1 2200 2025 2300 2700 2440 2720 2270 2370 2130 2570 2570 3070 2700 2500 20 23 60 20 20 20 7 30 17 30 30 7 - - H2O H2O H2O H2O H2 H2 H2O H2 H2 H2O H2 H2 H2O H2 H2O/H2 H2O/H2 lt1 lt1 19 30 27 76 6 27 4 80 55 49 1 1
27
Difference in Ba Release from Out-Pile and Phebus
Tests

• Short duration of temperature escalation in
  in-pile tests
• No high temperature plateau as in annealing
  tests but rather temperature escalation (with
  formation of molten pool) in Phebus test
• Ba volatility is reduced with significant amount
  of ZrO2 in fuel melt (47 mol) and small amounts
  of iron oxide in in-reactor test
• Thermochemical analysis with GEMINI2
• Reduced Ba vapor pressure in solidus-liquidus
  transition zone in the U-Ba-O phase diagram
  (2400-3100 K)

28
Concluding Remarks

• In- and out-pile experiments on severe accident
  melt progression FPR behaviour reviewed
• Melt progression non-coherent process
• Phebus FPT-0 and -1 tests performed for longer
  high-temperature period than earlier in-pile
  experiments ? information on late-phase behaviour
• Local propagation of core melt due to control rod
  failure (at lower temperature)
• Metallic blockages result from interactions of
  spacer grids, fuel rod cladding material and
  control rod materials that flow down bundle and
  solidify at lower (cooler) position
• Separation between metallic and ceramic blockages
  with freezing of (U,Zr)O2 melt at higher
  temperature
• Observed melting temperature of ceramic blockage
  in Phebus FPT-0 test (2720 K) slightly lower
  than pure ceramic (2800 K) due to eutectic
  interaction (consistent with TMI-2 examination)
• Molten pool formed due to increased fission heat
  generation and reduced heat transfer in several
  in-pile experiments (held in place by ceramic
  crust)

29
Concluding Remarks Contd

• Consistent release behaviour of volatile (Xe, Kr,
  I, Cs, Te and Sb), semi-volatile (Mo, Rh, Ba),
  low volatile (Ru, Ce, Np, Sr and Eu) and
  non-volatile (Zr, Nb, La and Ne) FPs observed in
  annealing (ORNL, CEA, CRL) and in-pile tests
• Reduced volatility of Ba for in-reactor
  experiments (thermochemical effects with presence
  of iron and zirconium oxides)
• Local atmospheric condition/oxygen potential
  influences low-volatile fission product release
  behaviour
• Non-coherent melt progression masks individual
  release mechanisms identified in out-of-pile
  experiments
• Enhanced release due to fuel liquefaction not
  typically observed in separate effects experiments

30
Acknowledgements

• Discussions with P. Elder, L. Dickson, M.
  Schwarz, R. Zeyen, B. Clement, M. Kissane and G.
  Ducros
• Funded by NSERC/COG Collaborative Research and
  Development (CRD) Grant