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Interpretation of Melt Oxidation Observations in QUENCH-09 test

M.S. Veshchunov *

Nuclear Safety Institute (IBRAE)

Russian Academy of Sciences * Visiting Scientist at FZK (August-September 2005)

11th International QUENCH Workshop

FZK, Karlsruhe

25-27 October 2005

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Observations of melt oxidation

in previous FZK tests

(QUENCH, CORA and crucible tests)

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T=2200ºC

Zr:O= 42:58 Zr:O= 39:61 Zr:O= 37:63

T=10 min. T=15 min. T=25 min.

Zr melt oxidation in ZrO2 crucible tests tests (J.Stuckert)

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Cross-section of QUENCH-03 test bundle at elevation 750 mm. “Bulk” oxidation of melt.

Zr-O melt oxidation in QUENCH tests

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Position 1 Position 3

Ceramic phase(U,Zr)O2x

Voids Metal phase(U,Zr,O)

Cross section of CORA-W2 test bundle

U-Zr-O melt oxidation in CORA tests

“Bulk” oxidation of melt

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• Completely oxidised melt consisted of grown up bulk ceramic precipitates and the peripheral oxide crust, was formed in the AECL and FZK crucible tests on Zr melt interactions with ZrO2 crucible walls

during long-term (1500 s) oxidation stage at high temperatures (2100-2200C) under a temperature gradient between the crucible walls and melt

• A close similarity of corium melt appearance is revealed in the bundle QUENCH and CORA tests, where bulk precipitation of ceramic particles up to complete conversion into ceramic phase was observed

• According to the present interpretation, ceramic structure of the central corium was formed under non-equilibrium test conditions in the course of melt oxidation by precipitation of ceramic phase in the oversaturated metallic melt along with the growth of peripheral oxide crust.

Main conclusions

from comparison of FZK bundle and crucible tests

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Corium melt oxidation

model

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.

TB

TI

T

CO(I) CO* CO

Liquid

O(I)

Solid (U,Zr)O2

O

(st)

O

* Solid

Liquid

Transition layer << M, L

TTI

TBCO(I)

O

CO(B) f

M xL

CO*

Fragment of quasi-binary phase diagram Spatial distribution of temperature and oxygen concentration

SVECHA model for U-Zr-O melt oxidation under non-equilibrium conditions (1/2)

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Main findings:

Two processes of pellet dissolution and melt oxidation in the convectively stirred

melt cannot be considered separately and should be modelled self-consistently.

Under non-equilibrium conditions, melt oxidation and UO2 dissolution depend on

temperature difference between solid and liquid phases and can proceed after

attainment of the melt saturation, resulting in the ceramic phase precipitation in

the bulk of the melt.

Depending on test conditions, the precipitation process can be accompanied

with the peripheral oxide layer (crust) growth or dissolution.

The source of temperature gradients is oxidation heat at the oxide crust-melt

interface and fission heat in the fuel pellet. Estimations show that these

temperature drops can attain several tens K.

SVECHA model for U-Zr-O melt oxidation under non-equilibrium conditions (2/2)

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Melt downward relocation in the bundle tests

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.

Progression of temperature front during melt relocation in CORA tests

Axial mass distribution after the test and axial temperature distribution during the transient phase in CORA-W1 test

Estimated velocity of relocation front: 1mm/s

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.

Progression of “flame” and “droplets/rivulets” fronts in CORA tests

Analysis of on-line video inspections in CORA-5 test (W. Hering, thesis)

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.

Progression of temperature front during melt relocation in Q-09 test

Temperature evolution at various elevations before quenching.

Estimated velocity of relocation front: 1mm/s

1050 mm

850 mm750 mm 650 mm

Time, s

T, K

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• Temperature front with T2000ºC in CORA tests relocated downward with a characteristic velocity v1  1-2 mm/s, which was extremely small in comparison with the characteristic velocities of metal rivulets and droplets (v2  0.5 m/s)

• “Flamefront” in CORA tests relocated coherently either with a “droplet/rivulets front” or with Zr melting isotherm, i.e. fairly associated with the melt progression front

• A similar melt progression (v11mm/s) apparently took place in QUENCH-09 test

• A new SVECHA model for melt oxidation/dissolution during relocation of a massive melt “slug” is currently under development (ISTC Project #2936)

Main conclusions from CORA tests observations

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Analysis of melt oxidation in

QUENCH-09 test

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• extremely high temperatures during quenching

0 1000 2000 30000

50

100

150

200

250

300

Te

mp

era

ture

, °C

Time, s

TCI 9/270 [°C] TCI 10/270 [°C] TCI 11/270 [°C] TCI 13/270 [°C] TCI 1/180 [°C] TCI 4/180 [°C] TCI 7/180 [°C] TCI 11/180 [°C] TCI 12/180 [°C] TCI 13/180 [°C] TCI 15/180 [°C] TCI 9/90 [°C] TCI 10/90 [°C] TCI 11/90 [°C] TCI 13/90 [°C] TCI 1/0 [°C] TCI 4/0 [°C] TCI 7/0 [°C] TCI 11/0 [°C] TCI 12/0 [°C] TCI 13/0 [°C] TCI 15/0 [°C] TCO 9/270 [°C] TCO 4/180 [°C]

QUENCH-03

0 1000 2000 3000 4000 5000

400

600

800

1000

1200 TCI 9/270 [KAN: 91] TCI 10/270 [KAN: 92] TCI 11/270 [KAN: 93] TCI 13/270 [KAN: 94] TCI 1/180 [KAN: 96] TCI 4/180 [KAN: 97] TCI 7/180 [KAN: 98] TCI 11/180 [KAN: 99] TCI 12/180 [KAN: 100] TCI 13/180 [KAN: 101] TCI 9/90 [KAN: 104] TCI 10/90 [KAN: 105] TCI 11/90 [KAN: 106] TCI 13/90 [KAN: 107] TCI 1/0 [KAN: 109] TCI 7/0 [KAN: 111] TCI 11/0 [KAN: 112] TCI 12/0 [KAN: 113] TCI 13/0 [KAN: 114]

Te

mp

era

ture

, K

Time, s

QUENCH-09

Comparison of temperatures of cooling jacket by survived thermocouples in two tests Q-03 and Q-09

Special features of Q-09 test (1/3)

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• extremely high hydrogen generation (~ 300 g) during quenching with increasing rate within ~ 100 s

Hydrogen release (~ 16 g) during molten pool oxidation

in FPT1 (~ 1000 s)

Special features of Q-09 test (2/3)

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• practically complete melt oxidation during quenching within ~ 100 s with formation of “foaming” ceramic structure

Cross section at 590 mm bundle elevation

FPT1 bundle cross-section

Special features of Q-09 test (3/3)

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Microstructure of oxidised local molten pools

Pool (ceramic): equiaxed fine grainsPellet: coarser grainsFlow channel scale: columnar grains

(from analysis of G. Schanz et al.)

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Comparison of melt oxidation in bundle tests

  Melt cross-section

(diameter)

Melt temperature

Period of melt oxidation

H2 release (during melt oxidation)

H2 rate

(during melt oxidation)

FPT 1 75 mm 2670 K 1000-3000 s(MP phase)

16 g decreasing

Q-09 100 mm 2670 K 100 s (quenching)

300 g increasing

Conclusions:

•The new model of molten pool (MP) oxidation based on the bulk precipitation mechanism (valid for interpretation of molten corium oxidation in FP tests) can explain post-test observations of melt microstructure in Q-09 test;

•However, additional mechanisms which can further enhance melt oxidation rate during quenching, should be considered for interpretation of Q-09 measurements.

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Void formation after relocation of melt

Non-oxidised melt Oxidised melt

Melt dispersion

Elevation 950 mm

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Oxidation of relocated melt

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Oxidation of dispersed melt

(through open pores formed after melt relocation)

Elevation

507 mm

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Elevation

590 mm

Oxidation of dispersed melt

(through channels formed after melt relocation)

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Conclusions from post-test observations (1/2)

1. Melt dispersion:

In the course of slow relocation of molten “slug” (massive melt), parts of metallic melt relocate downward (droplets, rivulets) from

the slug, leaving debris (oxide scales and precipitates) and

forming open voids and channels in the melt

Compare with visual observations in the CORA tests of the “front of rivulets” relocated coherently with the “flamefront” (i.e. with the melt progression front)

Typical size of voids and channels is comparable with the typical size of droplets and rivulets, that can be characterized by the capillary length of the corium melt:

Instability of melt progression front (local melting through the supporting crust and rapid downward relocation of droplets and rivulets)

mmg

ac 512/1

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2. Oxidation of dispersed melt:

a) relocated non-oxidised melt with “fresh” metallic surfaces is further attacked by steam;

b) steam immediately penetrates into the formed open voids and channels and additionally attacks fresh internal surfaces of the non-relocated melt (forming oxide scale around the channels);

• majority of the channels and voids in the slug are open (filled with epoxy) !

Both processes provides stepwise and enhanced oxidation of dispersed melt.

Conclusions from post-test observations (2/2)

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Small MP: incomplete pellet dissolution + complete precipitation

Simulation of local MP oxidation (1/5)

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Larger MP: incomplete pellet dissolution + incomplete precipitation

Simulation of local MP oxidation (2/5)

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Larger MP: complete pellet dissolution + complete precipitation

Simulation of local MP oxidation (3/5)

High temperature

scenario

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Oxidation of dispersed melt: - low temperature scenario

Simulation of local MP oxidation (4/5)

“effective” MP

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Oxidation of dispersed melt: - high temperature scenario

Simulation of local MP oxidation (5/5)

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Conclusions

•The new model of molten pool oxidation based on the bulk precipitation mechanism, can explain post-test observations of melt microstructure in Q-09 test.

•In order to explain extremely high oxidation rates of metallic melt during high-temperature quenching observed in the Q-09 test, additional consideration of a strong dispersion (or fragmentation) of the downward relocating massive melt (slug) is proposed.

•Subsequent oxidation of relocated away “fresh” portions of melt (droplets and rivulets) and “internal” oxidation of remaining melt through the formed channels and voids, should be self-consistently considered.

•This complicated mechanism can be taken into consideration in the new slug relocation model (under development in the ISTC Project #2936).

•Preliminary calculations with the stand-alone model for melt oxidation in application to dispersed melt (forming small-sized local pools) allows qualitatively consistent interpretation of Q-09 test observations.