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Volatile FP release from VERCORS tests

Volatile FP release from VERCORS tests. Preamble : What have we learnt from VERCORS tests ? Volatile FP behaviour. Parameters affecting their release. VERCORS HT Loop. What have we learnt ?. VERCORS program. VERCORS- 6 tests (from 1989 and 1994). VERCORS HT/RT-11 tests

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Volatile FP release from VERCORS tests

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  1. Volatile FP release from VERCORS tests • Preamble : • What have we learnt from VERCORS tests ? • Volatile FP behaviour Parameters affecting their release VERCORS HT Loop International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France

  2. What have we learnt ? VERCORS program VERCORS- 6 tests (from 1989 and 1994) VERCORS HT/RT-11 tests (from 1996 to 2002) FP classification by volatility degree Fuel collapse temperature International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France

  3. What have we learnt : Fuel collapse temperature Since the beginning of the RT/HT grid : Systematic fuel collapse for T between 2400/2600 K without significant difference for high burn up fuel in the range of 45-70 GWd/t: Relocation at T < UO2melting point International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France

  4. What have we learnt : Fuel collapse temperature Same fuel rod Similar temperature evolution histories Beginning of fuel collapse Beginning of fuel collapse Atmosphere effect International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France

  5. What have we learnt : FP classification From VERCORS program Volatile : gases, I, Cs, Te, Sb, Ag, Rb, Cd Semi-Volatile : Mo, Ba, Rh, Pd, Tc Low-Volatile: Ru, Nb, Sr, Y, La, Ce, Eu Non-Volatile: Zr, Nd, Pr + actinides : U, Np, Pu, Am, Cm International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France

  6. What have we learnt : FP classification • FPs Volatility for irradiated nuclear fuel • Volatile FP : • Present lecture • Semi-volatile FP : • Release can be as high as for volatile FP, but : • High sensitivity to oxidizing/reducing conditions • Mo very volatile in oxidizing conditions (MoO3) • Ba more volatile in reducing than in oxidizing conditions • Significant retention close to the fuel • Low volatile FP: • Release from few % to 10% BUT potentially higher release (~30-40%) at high burn-up and/or very oxidizing conditions • Deposit very close to the fuel • Non volatile FP: • No significant release (<1%) International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France

  7. What have we learnt : FP classification International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France

  8. Volatile FP Behaviour International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France

  9. Volatile FP behaviour • For each case : • Kinetics (release from the fuel) • Global release • (Transport : G. Ducros, Tuesday, 16) 1 2 3 {gases} {Cs and I} {Te, Sb and Ag} International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France

  10. Fission gas release: Generalities • Fission gases (Kr and Xe) are composed of isotopes whose half-lives have a very different radiological impact over time under severe PWR accident conditions: • Long half-life for krypton (10.71 years for 85Kr); active over the mid and long term. The other tracer isotopes of the element have sufficiently short half-lives for having no significant impact in the hours following reactor shutdown, with the exception of 85mKr (half-life of 4.48h) whose effects are felt for a little longer. • Short half-lives for the main isotopes of xenon (2.19 days, 5.24 days and 9 h respectively for 133mXe, 133Xe and 135Xe); active in the short term. International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France

  11. Fission gas release: Kinetics T >> 1200°C 1000°C < T <1200°C Fuel relocation Below 1000°C RT6, UO2, ~70GWd/t International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France

  12. Fission gas release: Kinetics Consistent with previously reported results (T<1200°C) : 85Kr release METEOR UO2, ~70 GWd/t • MAIN PEAK (T > 1000°C) • Bubbles interconnection and release • Diffusion of intra-granular gas atoms < 2% FIRST PEAK (600-800°C) Grain boundary cracking Y. Pontillon et al., Proceedings of the 2004 International Meeting on LWR Fuel Performance, Orlando, USA, September 2004 International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France

  13. Fission gas release: Global release • Since VERCORS 6: • Total release (100% of the initial inventory) • From VERCORS 1 to 5: • Released fraction is a function of : Final temperature Duration of high T° plateau International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France

  14. Cs and I release: Generalities • FP of great importance with regard to the radiological consequences following a severe accident in a PWR core. They are composed of isotopes with very different half-lives: • Short half-life for iodine (from 1 hour for 134I to 8 days for 131I); the short-term radiological effects are very high in the first few days following an accident, but are negligible after 1 month. Iodine carries 15% of the core's decay heat 1 day after the emergency shutdown; • Long half-life for caesium (30 years for 137Cs); the radiological effects, which are more or less negligible in the short term (there are nevertheless 138Cs and 136Cs with respective half-lives of 30 min and 13 days) stretch into long term over several decades. International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France

  15. Cs and I release: Kinetics • Parameters affecting their release rate: • Burn-up, • Oxidizing or reducing conditions, • Fuel nature: • MOX versus UO2 • Initial morphology From VERCORS program International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France

  16. Cs and I release: Kinetics - BU effect Comparison between RT1 (reference test) and RT6 (High BU test): VERCORS RT6: UO2, 70 GWd/T Mixed H20/H2 VERCORS RT1: UO2, 47 GWd/T Mixed H20/H2 Significant increase in release rates for RT6 compared to RT1 International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France

  17. Cs and I release: Kinetics - Atm effect Comparison between HT2 and HT3 (same fuel used): VERCORS HT2: UO2, 50 GWd/T steam VERCORS HT3: UO2, 50 GWd/T hydrogen Significant increase in release rates for HT2 compared to HT3 International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France

  18. Cs and I release: Kinetics – Fuel nature (MOX versus UO2) Comparison between RT1 (reference test) and RT2 (MOX test): VERCORS RT1: UO2, 47 GWd/T Mixed H20/H2 VERCORS RT2: MOX, 46 GWd/T Mixed H20/H2 Significant increase in release rates for RT2 compared to RT1 International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France

  19. Cs and I release: Kinetics – Fuel nature (Initial morphology) Comparison between RT1 (reference test), RT3 and RT4 : VERCORS RT1: UO2, 47 GWd/T Mixed H20/H2 Release rate RT4 RT4 RT3 RT3 RT1 RT1 VERCORS RT3: UO2, debris bed reducing VERCORS RT4: UO2, debris bed oxidising International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France

  20. Cs and I release: Global release • Since VERCORS 6: • release almost complete whatever the nature of the test • From VERCORS 1 to 5: • Released fraction is a function of : Final temperature Duration of high T° plateau International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France

  21. Te, Sb and Ag release: Generalities • Te: • Main isotopes 132Te (3.26 d) and 131mTe (1.25 d).The short-term radiological effects are very high in the first few days following an accident. Parent of the corresponding Iodine. • Sb, main isotopes composed of isotope with very different half-lives : • 125Sb (2.76 y), acting in the long term • 127Sb (3.85 d), acting in the short term • Ag: • Main isotope 110mAg (250 d), acting in the middle/long term International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France

  22. Te, Sb and Ag release: Kinetics • Results obtained are relatively restricted because of: • Problems with detecting antimony and silver in all the VERCORS tests This made it impossible to monitor their release from the fuel over time • The loss of detectability of 132Te (best tracer isotope for Te) with the use of thoria in the furnace component after VERCORS 6 Data available up to VERCORS 5 International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France

  23. Te, Sb and Ag release: Kinetics VERCORS 4: UO2, 38 GWd/T hydrogen Tellurium retention in the cladding until the latter was completely oxidised International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France

  24. Te, Sb and Ag release: Global release • For tellurium and silver : • Global release was comparable and almost total for all of the most severe VERCORS tests, i.e. from VERCORS 6 onwards • The main difference between these two FP was in terms of the quantities deposited in the hot zones of the experimental loop (transport effect) • For antimony: • Release delay by trapping into the clad • For the entire RT grid, the release rates were generally lower than those obtained for VERCORS 4, 5 and 6 (typically around 80-95% and 97-100% respectively for the RT grid and VERCORS 4 to 6) Partial retention in the solidified corium International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France

  25. Sb release: Global release This retention sometimes (tests VERCORS RT1, RT2 and RT7) involved the dissociation of this element from the solidified corium: Zr after the test = corium position Sb before the test Sb after the test International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France

  26. Conclusion • Volatile FP: • Nearly complete release since VERCORS 6, whatever the nature of the test • Up to VERCORS 5: the release is a function of the final Temperature and duration at high temperature plateau • Sensitive to : • Burn up • Atmosphere of the test • Fuel nature Global release Kinetics International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France

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