1 international vercors seminar, october 15-16th, 2007 – gréoux les bains, france about the...

International VERCORS Seminar,October 15-16th, 2007 – Gréoux les Bains, France 1

About the active role played by the UO2 oxidation on irradiated fuel collapse temperature

M. Barrachin (IRSN)

P.Y. Chevalier, B. Cheynet, E. Fischer (THERMODATA/INPG/CNRS)


Background (1)

In case of a severe accident, one challenge of the safety analysis is to evaluate the amount of materials in the lower plenum and the composition of the molten pool.

This depends on the degradation scenarios, in particular on the interaction between UO2 fuel and Zry cladding.

Large spectrum of conditions : temperatures between 900 and 2800 K and atmosphere from highly oxidising to reducing.

Expected different values for the fuel collapse temperature (SA code parameter), function of these different conditions.


Background (2)

Two extreme situations

Highly reducing atmosphere

Zry extracts oxygen from UO2 fuel and can dissolve it

Interaction solid-solid (1273-1973 K) (Hofmann-1984)

Interaction liquid Zry-UO2 solid (> 2023 K) (Olander-1994-96)

Expected fuel collapse temperature ~1000 K below fuel melting temperature

Highly oxidising atmosphere (this presentation)

Zry quickly oxidised, transformation in ZrO2

In this situation, no UO2 fuel reduction

Interaction ZrO2 solid-UO2 solid (UO2-ZrO2 phase diagram,Lamberston-1953)

Expected fuel collapse temperature > 2800 K, i.e. slightly below fuel melting temperature


Experimental observationsVERCORS tests

• UO2 irradiated fuel (from 38 GWd/tU to 70 GWd/tU)

• Complete oxidation of the cladding at low temperature : complete transformation Zr ZrO2

• After this period, high temperature phase in oxidising atmosphere

• Detection of the fuel collapse by gamma signal

There was evidence of fuel collapse temperature at about 2500-2600 K

It means that the observed fuel collapse temperature is 300 K below than the expected fuel collapse temperature (2800 K)

SO WHY ?


Proposed interpretation for the reduction of fuel collapse temperature (1)

VERCORS tests (Pontillon et al.-2005)

They mentioned the effect of the burn-up (BU), i.e. the UO2-Fission Products interactions

MATPRO correlation

melting Temperature of UO2 (BU=50 Gwd/tU) reduced by 200 K

NOT CONSISTENT WITH THE ANALYSIS OF THE EXPERIMENTAL DATA

Experimental data (apparently contradictory)

-Christensen (1964)

melting temperature decrease with BU

-Yamanouchi (1970)

melting T. of UO2 (30 Gwd/tU) = melting T. UO2 (fresh)

Christensen technique = tungsten crucible, possible interaction, composition change during the measurement

unsignificant up to 50 GWd/tU (on the basis of the Yamanouchi’s results)


Proposed interpretation for the reduction of fuel collapse temperature (2)

VERCORS tests

Effect of non fully oxidised cladding during the low temperature plateau

could be a satisfactory explanation

Difficult to go further in the VERCORS test analysis : no PIE examination only qualitative interpretation.

Another explanation is possible on the basis of the PHEBUS FPT1 post-mortem examinations

BUT

No consistent with the VERCORS RT4 test observation :

UO2-ZrO2 debris bed initial configuration,

It means no cladding pre-oxidation,

highly oxidising atmosphere,

fuel collapse temperature at 2500 K.


FPT1 PHEBUS testMain events

Fuel bundle : 1 m high, 18 irradiated rods (23 GWd/t), 2 instrumented fresh fuel rods, 2 zircaloy grids (0.24 and 0.76 m), SIC control rod (steel clad)

Oxidising atmosphere (P=2.2 atmospheres)

Main degradation events :

Cladding burst ~5600-5800 s at T1100 K (inner rods)

Rupture of control rod ~9690 s at T=1623 K (steel-Zr interaction) :

RELOCATION OF METALLIC MATERIALS TOWARDS THE BOTTOM OF THE TEST SECTION

Oxidation period (11060 s-13200 s) :

LARGE RELOCATIONS OF FUEL MATERIALS DUE TO THE INTERACTION BETWEEN MOLTEN ZIRCALOY CLADDING/UO2

High temperature period (11060 s-13200 s) :

PROGRESSIVE RELOCATION OF UO2/ZrO2 MIXTURES (as in VERCORS TESTS) from 15380 s for 0.4-0.6 m elevation

Complete formation of the molten pool at 16900 s between 0.16 and 0.23 m elevations


FPT1 PHEBUS testPost-mortem examinations

607 mm

473 mm

1 m

The composition of irradiated fuel remnant was measured after the test

(U0.86 Zr0.12Fe 0.01)O 2.42

Interaction Fuel/Cladding

Interaction Fuel/steam

(U0.99 Zr0.01)O 2.23

Interaction Fuel/steam

Impact of the fuel oxidation on the fuel collapse temperature ?

PROGRESSIVE RELOCATION OF UO2/ZrO2 MIXTURES FROM 0.4-0.6 m


FPT1 PHEBUS testFuel oxidation experimental evidences

Measurements : oxygen contents measured by EPMA : not reliable

Molybdenum FP absent in the 5-metal FP precipitates (Mo-Ru-Tc-Rh-Pd) oxidation of the molybdenum during the test


FPT1 PHEBUS testFuel oxidation calculations

Thermodynamic evaluations Kinetic approach (Dubourg-2005)

TMI-2 post mortem analyses : O/M=2.14 (Bottomley-1989)

0,00E+00

1,00E-02

2,00E-02

3,00E-02

4,00E-02

5,00E-02

6,00E-02

7,00E-02

8,00E-02

0 2000 4000 6000 8000 10000 12000 14000 16000 18000

Time (s)F

ue

l d

ev

iati

on

sto

ich

iom

etr

y

0

500

1000

1500

2000

2500

3000

Te

mp

era

ture

(K

)

Fuel Deviation stoichiometry

TEMP[K]

O/M=2.08

O/M=2.11

PO2 (2673 K, 2.2 bars)


Impact of fuel oxidation on melting UO2 temperature

O-U phase diagram (atmospheric pressure)

O-U

1800

2000

2200

2400

2600

2800

3000

3200

3400

0.06 0.12 0.18 0.24 0.30 0.36 0.42 0.48 0.54x (mol)

T/K

Liquid

FCC

G

hyperstoichiometric

hypostoichiometric

U3O8

U3O8 UO2+x+G

UO2+x+G Liquid

UO2+x+G Liquid

-Transition fixed by the shape of the liquidus of UO2+x at high temperature

-Up to very recently, only experimental data of Latta was available (1970), W contamination.

-Different published values

3077 K (Chevalier-2002)

2873 K (Roth-1981)

2700 K (Guéneau-2002)



New Manara’s data on liquidus-solidus on UO2+x (2005)

Laser heating enabling fast melting and freezing

Container-less conditions

High pressure to prevent non-congruent

Evaporation

Thermal arrest method


Phases Thermodynamic model Experimental information

Reference

liquid phase

UO2+x solid solution (FCC)

associate model (U,O,UO2)

3-Sublattice model (U)1(O, □)2(O, □)1

Liquidus-solidus

RTlnPO2 = f(O/U)

Manara (UO2+x)

Latta (UO2-x)

Chevalier-2002

U3O8

UO3

U4O9

G(T) Cp, Hf, S Chevalier-2002

Gas phase O,O2,O3,UO3,UO2,OU,U

G(T) COACH database Cheynet-2002


New thermodynamic modelling of the

U-O phase diagram at high temperature (1)



New thermodynamic modelling of

the U-O phase diagram at high temperature (2)Atmospheric pressureHigh pressure

1atm : UO2+x+G Liquid 2694 K

2atm : UO2+x+G Liquid ~2600 K


Impact of fuel oxidation on fuel collapse temperature

Modelling of the U-O-Zr phase diagram at high temperature (1)

Rich oxygen part of the U-O-Zr phase diagram

1atm : Liquid at T>2500 K

2atm : Liquid at T>2400 K


Impact of fuel oxidation on interaction UO2+x/ZrO2



Fuel collapse temperatureCome back to the PHEBUS tests

Composition TLcalc(K) TScalc(K)

(U0.88Zr0.12)O2.000 3080 3020

(U0.87Zr0.12Fe0.01)O2.00 3060 2860

Calculation of impact of structurals materials : relatively limited

Stoichiometric compositions

In agreement with the experimental measurements of Ronchi (2002) on (U,Zr)O2 (23Gwd/tU)

In agreement with the experimental data of Uetsuka (1993) on (U,Zr,Fe)O2 (TMI2 core simulating material)


Fuel collapse temperatureCome back to the PHEBUS tests

Calculation of impact of oxidation : significant

Hyperstoichiometric compositions

Composition TLcalc(K) TScalc(K)

(U0.88Zr0.12)O2.08-2.11 2980-2960 2760-2660

(U0.87Zr0.12Fe0.01) O2.08-2.11 2960-2920 2560-2460


Conclusions

Experimental evidences of fuel collapse temperature at 2500-2600 K in oxidising conditions (PHEBUS FP FPT0 and FPT1 tests, VERCORS tests).

New experimental data on liquidus/solidus on UO2+x was produced by Manara (ITU), more precise than the past one by Latta.

New thermodynamic modelling of U-O phase diagram, and U-O-Zr phase diagram taking into account these new data.

On the basis of this new modelling, the oxidation of fuel could quantitatively explain the observed low fuel collapse temperatures.

Evidence of lower fuel collapse temperature in oxidising conditions (VERCORS HT2) than in reducing conditions (VERCORS HT1 and HT3).

1 international vercors seminar, october 15-16th, 2007 – gréoux les bains, france about the...

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