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About the active role played by the UO 2 oxidation on irradiated fuel collapse temperature M. Barrachin (IRSN) P.Y. Chevalier, B. Cheynet, E. Fischer (THERMODATA/INPG/CNRS). Background (1).

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slide1

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)

International VERCORS Seminar,

October 15-16th, 2007 – Gréoux les Bains, France

slide2

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.

International VERCORS Seminar,

October 15-16th, 2007 – Gréoux les Bains, France

slide3

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

International VERCORS Seminar,

October 15-16th, 2007 – Gréoux les Bains, France

slide4

Experimental observations

VERCORS 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 ?

International VERCORS Seminar,

October 15-16th, 2007 – Gréoux les Bains, France

slide5

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)

International VERCORS Seminar,

October 15-16th, 2007 – Gréoux les Bains, France

slide6

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

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.

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

International VERCORS Seminar,

October 15-16th, 2007 – Gréoux les Bains, France

slide7

FPT1 PHEBUS test

Main 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 T1100 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

International VERCORS Seminar,

October 15-16th, 2007 – Gréoux les Bains, France

slide8

FPT1 PHEBUS test

Post-mortem examinations

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

1 m

607 mm

(U0.99 Zr0.01)O 2.23

Interaction Fuel/steam

Impact of the fuel oxidation on the fuel collapse temperature ?

473 mm

PROGRESSIVE RELOCATION OF UO2/ZrO2 MIXTURES FROM

0.4-0.6 m

International VERCORS Seminar,

October 15-16th, 2007 – Gréoux les Bains, France

slide9

FPT1 PHEBUS test

Fuel 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

International VERCORS Seminar,

October 15-16th, 2007 – Gréoux les Bains, France

slide10

FPT1 PHEBUS test

Fuel oxidation calculations

Thermodynamic evaluationsKinetic approach (Dubourg-2005)

O/M=2.08

PO2 (2673 K, 2.2 bars)

O/M=2.11

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

International VERCORS Seminar,

October 15-16th, 2007 – Gréoux les Bains, France

slide11

Impact of fuel oxidation on melting UO2 temperature

O-U phase diagram (atmospheric pressure)

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)

hypostoichiometric

hyperstoichiometric

Liquid

UO2+x+G Liquid

G

FCC

U3O8UO2+x+G

U3O8

International VERCORS Seminar,

October 15-16th, 2007 – Gréoux les Bains, France

slide12

Impact of fuel oxidation on melting UO2 temperature

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

International VERCORS Seminar,

October 15-16th, 2007 – Gréoux les Bains, France

slide13

Impact of fuel oxidation on melting UO2 temperature

New thermodynamic modelling of the

U-O phase diagram at high temperature (1)

International VERCORS Seminar,

October 15-16th, 2007 – Gréoux les Bains, France

slide14

Impact of fuel oxidation on melting UO2 temperature

New thermodynamic modelling of

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

Atmospheric pressure

High pressure

1atm : UO2+x+G Liquid 2694 K

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

International VERCORS Seminar,

October 15-16th, 2007 – Gréoux les Bains, France

slide15

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

International VERCORS Seminar,

October 15-16th, 2007 – Gréoux les Bains, France

slide16

Impact of fuel oxidation on interaction UO2+x/ZrO2

Modelling of the U-O-Zr phase diagram at high temperature (2)

International VERCORS Seminar,

October 15-16th, 2007 – Gréoux les Bains, France

slide17

Impact of fuel oxidation on interaction UO2+x/ZrO2

Modelling of the U-O-Zr phase diagram at high temperature (3)

International VERCORS Seminar,

October 15-16th, 2007 – Gréoux les Bains, France

slide18

Composition

TLcalc(K)

TScalc(K)

(U0.88Zr0.12)O2.000

3080

3020

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

3060

2860

Fuel collapse temperature

Come back to the PHEBUS tests

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)

International VERCORS Seminar,

October 15-16th, 2007 – Gréoux les Bains, France

slide19

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

Fuel collapse temperature

Come back to the PHEBUS tests

Calculation of impact of oxidation : significant

Hyperstoichiometric compositions

International VERCORS Seminar,

October 15-16th, 2007 – Gréoux les Bains, France

slide20

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).

International VERCORS Seminar,

October 15-16th, 2007 – Gréoux les Bains, France