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

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

3 International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France 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 GWd/t: Relocation at T < UO 2 melting point

4 International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France What have we learnt : Fuel collapse temperature HT1 (~47 GWd/t) HT2 (~47 GWd/t) HT3 (~47 GWd/t) T (K)~2500~2300~2500 Atmreducingoxidizingreducing Same fuel rod Similar temperature evolution histories Atmosphere effect Beginning of fuel collapse

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

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

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

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

9 International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France Volatile FP behaviour For each case :  Kinetics (release from the fuel)  Global release  (Transport : G. Ducros, Tuesday, 16) {gases}{Cs and I} {Te, Sb and Ag} 123

10 International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France 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 85 Kr); 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 85m Kr (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 133m Xe, 133 Xe and 135 Xe); active in the short term.

11 International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France Fission gas release: Kinetics Fuel relocation Below 1000°C 1000°C < T <1200°C T >> 1200°C RT6, UO 2, ~70GWd/t

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

13 International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France 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 : Temperature °C TestVERCORS 2VERCORS 1 Release23%33% Temperature2300°C TestVERCORS 4VERCORS 5 Release86%87% Final temperatureDuration of high T° plateau Temperature °C TestVERCORS 2VERCORS 1 Duration13 minutes17 minutes Release23%33% Temperature2300°C TestVERCORS 3VERCORS 4 Duration15 minutes30 minutes Release77%86%

14 International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France 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 134 I to 8 days for 131 I); 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 138 Cs and 136 Cs with respective half-lives of 30 min and 13 days) stretch into long term over several decades.

15 International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France Cs and I release: Kinetics From VERCORS program Parameters affecting their release rate:  Burn-up,  Oxidizing or reducing conditions,  Fuel nature:  MOX versus UO 2  Initial morphology

16 International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France Cs and I release: Kinetics - BU effect Comparison between RT1 (reference test) and RT6 (High BU test): Significant increase in release rates for RT6 compared to RT1 VERCORS RT1: UO 2, 47 GWd/T Mixed H 2 0/H 2 VERCORS RT6: UO 2, 70 GWd/T Mixed H 2 0/H 2

17 International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France Cs and I release: Kinetics - Atm effect Comparison between HT2 and HT3 (same fuel used): Significant increase in release rates for HT2 compared to HT3 VERCORS HT2: UO 2, 50 GWd/T steam VERCORS HT3: UO 2, 50 GWd/T hydrogen

18 International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France Cs and I release: Kinetics – Fuel nature (MOX versus UO 2 ) Significant increase in release rates for RT2 compared to RT1 Comparison between RT1 (reference test) and RT2 (MOX test): VERCORS RT1: UO 2, 47 GWd/T Mixed H 2 0/H 2 VERCORS RT2: MOX, 46 GWd/T Mixed H 2 0/H 2

19 International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France Cs and I release: Kinetics – Fuel nature (Initial morphology) Comparison between RT1 (reference test), RT3 and RT4 : VERCORS RT1: UO 2, 47 GWd/T Mixed H 2 0/H 2 VERCORS RT3: UO 2, debris bed reducing VERCORS RT4: UO 2, debris bed oxidising RT1 RT3 RT4 RT1 RT3 RT4 Release rate

20 International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France 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 : Temperature °C TestVERCORS 2VERCORS 1 Release30-40% Temperature2300°C TestVERCORS 4VERCORS 5 Release % Final temperatureDuration of high T° plateau Temperature2300°C TestV_ 3V_4 and V_5 Duration15 minutes30 minutes Release70%87-93%

21 International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France Te, Sb and Ag release: Generalities Te:  Main isotopes 132 Te (3.26 d) and 131m Te (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 :  125 Sb (2.76 y), acting in the long term  127 Sb (3.85 d), acting in the short term Ag:  Main isotope 110m Ag (250 d), acting in the middle/long term

22 International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France 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 132 Te (best tracer isotope for Te) with the use of thoria in the furnace component after VERCORS 6 Data available up to VERCORS 5

23 International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France Te, Sb and Ag release: Kinetics Tellurium retention in the cladding until the latter was completely oxidised VERCORS 4: UO 2, 38 GWd/T hydrogen

24 International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France 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 % respectively for the RT grid and VERCORS 4 to 6) Partial retention in the solidified corium

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

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