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About the active role played by the UO 2 oxidation on irradiated fuel collapse temperature

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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About the active role played by the UO 2 oxidation on irradiated fuel collapse temperature

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

  2. 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

  3. 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

  4. 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

  5. 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

  6. 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

  7. 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

  8. 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

  9. 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

  10. 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

  11. 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

  12. 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

  13. 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

  14. 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

  15. 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

  16. 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

  17. 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

  18. 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

  19. 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

  20. 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

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