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FP release from VERCORS tests: semi-volatile, low-volatile FP and actinides

FP release from VERCORS tests: semi-volatile, low-volatile FP and actinides. Classification by volatility degree Semi-volatile FP: Mo, Ba, Rh, Pd, Tc Low-volatile FP: Ru, Nb, Sr, Y, La, Ce, Eu Non volatile FP: Zr, Nd, Pr Actinides: U, Np, Pu, Am, Cm

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FP release from VERCORS tests: semi-volatile, low-volatile FP and actinides

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  1. FP release from VERCORS tests:semi-volatile, low-volatile FP and actinides • Classification by volatility degree • Semi-volatile FP: Mo, Ba, Rh, Pd, Tc • Low-volatile FP: Ru, Nb, Sr, Y, La, Ce, Eu • Non volatile FP: Zr, Nd, Pr • Actinides: U, Np, Pu, Am, Cm • Analysis of the main parameters affecting their release • Temperature • Oxidising/Reducing conditions • Material interactions • Fuel burn-up • Fuel nature • UO2/MOX • steady state fuel/debris bed/molten pool International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France

  2. Semi-volatile: Molybdenum, main characteristics • Only one significant radioactive isotope: 99Mo … • 2,7 days of half life  acting in the short term • Low radiological effects, as well as low impact on residual power • … but several stable isotopes • High mass inventory and ability to be chemically associated with other FP (Cs to form Cs2MoO4) and to modify their volatility • Essentially under metallic precipitate within the fuel • Associated with Ru, Rh, Pd International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France

  3. Mo release: impact of oxidising conditions International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France

  4. Mo release: interaction with Cs (from VERCORS 6) • Same location of deposit on the upper part of the sleeve • Same location in the fuel-corium zone • And different from 95Zr distribution, representative of the fuel-corium location Lower sample (remained within the crucible) Upper sample (extracted from the crucible) Downstream deposit along the sleeve International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France

  5. Mo retention in the fuel: large metallic precipitates associated with Ru (from RT3) International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France

  6. Semi-volatile: Barium, main characteristics • Main radioactive isotope: 140Ba, parent of 140La • 12,7 days of half life  acting in the middle term • Important radiological effect • Released fraction • High impact on residual power • "Corium" fraction, but also deposit within the upper part of the RPV • 140Ba/140La holds 15-20% of the total residual power between 1 day and 1 month International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France

  7. Ba release: impact of reducing conditions International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France

  8. Ba release: interaction with the cladding (trapping by Zr) – From VERCORS 5 Fint clad Fext clad 15% of initial Inventory inside the cladding International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France

  9. Ba release: high upstream retention (from VERCORS 5) International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France

  10. Ba release without cladding – Debris bed configuration from RT3 • Higher kinetics than Mo (the only time) • Nearly total release at the end of the test (60% at the beginning of debris melting) International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France

  11. Semi-volatile FP: Rh, Pd, Tc - main characteristics • Very low radiological impact • Rh: only 105Rh • 1,5 days  acting in the short term • Pd, Tc: no radioactive isotopes • Essentially under metallic precipitate within the fuel • Associated with Ru and Mo International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France

  12. Rh release: similar behaviour than Ba International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France

  13. Pd, Tc: similar behaviour than Mo ? Data obtained by chemical analysis at TUI • Slightly lower release than Mo • Slightly lower release in reducing conditions International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France

  14. Semi-volatile FP release: conclusion • Significant released fraction • Can be as high as for volatile FP, but … • High sensitivity to oxidizing/reducing conditions • Mo more volatile in oxidizing conditions • And probably also Pd, Tc • Ba, Rh more volatile in reducing conditions • Materials interactions, affecting their release • Ba trapped by Zr of the cladding • Potential chemical interaction of Mo with Cs, limiting Cs release • Additional Burn-Up effect • Seems to increase Ba release in oxidising conditions International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France

  15. Low volatile: ruthenium, main characteristics • 2 important radioactive isotopes • 103Ru: 39 days  acting in the middle term • 106Ru: 1 year  acting in the long term • High inventory in MOX fuel • Metallic precipitate within the fuel • Potentially very high radiological impact • Volatile oxide forms can be produced in very oxidising environment, particularly under air ingress • Among them, RuO4 may stay gaseous at low temperature within the containment International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France

  16. Ru release: impact of oxidising conditions • VERCORS 4 and VERCORS 5 comparison (UO2 38 GWj/t) • Low Ru release in both tests: 6% • Deposit mainly located at high temperature, but … • For Vercors 5 (pure steam conditions), 10% of the released fraction reaches the impactor heated at 870K • Vercors HT3 and HT2 comparison (UO2 50 GWj/t) • Same low release (6%) for HT3 as for VERCORS 4, both performed in reducing conditions, with total deposit at high temperature • HT2 (pure steam conditions) exhibits a large release (65%), with 20% of the released fraction recovered at low temperature (400 K) • Significant release may also occur in steam (even mixed steam and hydrogen) conditions • Significant impact of burn-up is also evidenced International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France

  17. Ru release in HT2 test 53% of Ru deposited in a hot zone (12% downstream in a colder zone) International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France

  18. Ru release kinetics for HT2 and RT6 tests • Same kinetics up to fuel delocation: • Burn-up effect for RT6 • More oxidising conditions for HT2 International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France

  19. Low volatile: Niobium, main characteristics • Refractory element, with 2 radioactive isotopes, both daughter of a radioactive Zr isotope • 97Zr/97Nb (16,9/1,2 hours)  acting in the short term • 95Zr/95Nb (64/35 days)  acting in the middle term • No stable FP isotope  very low mass inventory • Under oxide forms within the fuel (dissolved and precipitates) • Nb is less refractory than Zr, with several oxides, one of them (Nb2O5) having a relatively low melting point (~ 1500°C) International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France

  20. Nb release: only detected in HT/RT grid at very high temperature International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France

  21. Large Nb release in debris bed configuration (RT3) International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France

  22. Nb release: main parameters • Firstly high temperature • Burn-up effect • No release measured below 50 GWj/t • Significant increase of the release at 70 GWj/t (RT6) • Debris bed configuration • Seems to be an important factor for Nb (RT3 test, highest release of all the Vercors grid) • Impact of oxidising conditions ? • Seems to be less important than expected • The comparison between HT2/HT3 highlights a potential compensation effect of a long duration time at high temperature before fuel collapse International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France

  23. Low volatile FP: Sr, Y, La, Ce, Eu – main characteristics (1/2) • The most refractory elements of this group of low volatile FP • Dissolved oxides within the fuel • High melting temperature of the oxides (> 2300°C), but low melting temperature of the metallic form (< 1000°C, except Y) • Like for Ba, we can expect an higher release in reducing conditions (but at higher temperature) • Sr: high radiological impact of 90Sr (30 years) • But also 91Sr (10 hours), acting in the short term • Y: low radiological impact • 93Y (10 hours), acting in the short term • All other isotopes have a behaviour imposed by their parent (91Sr/91Y, 92Sr/92Y), all acting in the short term International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France

  24. Low volatile FP: Sr, Y, La, Ce, Eu – main characteristics (2/2) • La: 140La (1,7 day), daughter of 140Ba (12,7 days), which imposes the inventory evolution • Less volatile than Ba • Difficult to quantify precisely • High impact on residual power for 140Ba/140La • Ce: very high impact on residual power (and activity) • 144Ce (285 days)  acting in the long term (20% of the core residual power after one month, 45% after one year) • 141Ce (32 days) and 143Ce (1,4 day)  acting in the middle and short term • Eu: low inventory, low radiological impact • Activity (Pres) always < 1% of total core activity (Pres) • 154Eu (8,8 years), 155Eu (5 years)  acting in the long term • 156Eu (15 days)  acting in the middle term International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France

  25. Sr, Y, La, Ce, Eu release: main results from VERCORS tests International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France

  26. Sr, Y, La, Ce, Eu … Zr, Nd, Pr: main conclusion • Sr: very low volatility, nearly non volatile • Y: could be measured only one time (VERCORS 3), because of very short half life of 93Y • Seems to confirm its low-volatile behaviour • La: • Higher release in reducing conditions • Higher release for high burn-up fuel • Ce: • Same tendency than La, but with lower amplitude of the release • Seems to have an additional effect of higher release in MOX fuel • Eu: higher release in reducing conditions • Non volatile elements: Zr, Nd, Pr International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France

  27. Actinide release: U, Np, Pu, Am, Cm • Main characteristics • Generally alpha emitters and very long half life isotopes … • High U mass inventory  impact on aerosol transport, even with low release • … Except Np, which has 2 short half life isotopes • 239Np and 238Np (2 days)  acting in the short term • 239Np loads 20% of the core residual power after 1 day • Np release • Very similar to Ce (La) release • Total release up to 10% • Favoured in reducing conditions • Favoured at high burn-up • U, Pu release: measured by chemical analyses (RT1 to 4) • U release can reached 10% in oxidising conditions (RT1 – RT4) • Pu release seems to be lesser than U release by a factor 10 International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France

  28. General conclusion: FP release classification • Volatile FP: (Kr, Xe), Cs, I, but also Te, Sb, Rb, Ag, Cd • Nearly complete release • Kinetics sensitive to oxidising/reducing conditions • Release delay for Te et Sb (trapping in the clad when not fully oxidized) • Semi-volatile FP: Mo, Ba, Rh, Pd, Tc • Release can be as high as for volatile FP, but : • High sensitivity to oxygen potential • Sensitivity to material interaction and burn-up (Ba) • Significant retention close to the fuel • Low volatile FP: Sr, Y, Nb, Ru, La, Ce, Eu • Release level from some % to 10%, BUT: • Potentially higher release (~30-40%) at high burn-up for some of them • Sensitivity to oxygen potential • Deposit very close to the fuel • Non volatile FP: Zr, Nd, Pr • No significant release measured up to now (<1%) • Actinides: U, Np similar to low volatile – Pu to non volatile International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France

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