International VERCORS Seminar,

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Volatile FP release from VERCORS tests

• Preamble
• What have we learnt from VERCORS tests ?
• Volatile FP behaviour

Parameters affecting their release
VERCORS HT Loop
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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
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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 lt UO2 melting point
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What have we learnt Fuel collapse temperature
Same fuel rod Similar temperature evolution
histories
HT1 (47 GWd/t) HT2 (47 GWd/t) HT3 (47 GWd/t)
T (K) 2500 2300 2500
Atm reducing oxidizing reducing
Beginning of fuel collapse
Beginning of fuel collapse
Atmosphere effect
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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
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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 (lt1)

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What have we learnt FP classification
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Volatile FP Behaviour
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Volatile FP behaviour
1
2
3
gases Cs and I Te, Sb and Ag

• For each case
• Kinetics (release from the fuel)
• Global release
• (Transport G. Ducros, Tuesday, 16)

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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.

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Fission gas release Kinetics
T gtgt 1200C
1000C lt T lt1200C
Fuel relocation
Below 1000C
RT6, UO2, 70GWd/t
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Fission gas release Kinetics
Consistent with previously reported results
(Tlt1200C)
85Kr release
METEOR
UO2, 70 GWd/t

• MAIN PEAK (T gt 1000C)
• Bubbles interconnection and release
• Diffusion of intra-granular gas atoms lt 2

FIRST PEAK (600-800C) Grain boundary cracking
Y. Pontillon et al., Proceedings of the 2004
International Meeting on LWR Fuel Performance,
Orlando, USA, September 2004
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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
Temperature 1860-1880C 1860-1880C
Test VERCORS 2 VERCORS 1
Release 23 33
     
     
Temperature 2300C 2300C
Test VERCORS 4 VERCORS 5
Release 86 87
Temperature 1860-1880C 1860-1880C
Test VERCORS 2 VERCORS 1
Duration 13 minutes 17 minutes
Release 23 33
     
     
Temperature 2300C 2300C
Test VERCORS 3 VERCORS 4
Duration 15 minutes 30 minutes
Release 77 86
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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.

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Cs and I release Kinetics
From VERCORS program

• Parameters affecting their release rate
• Burn-up,
• Oxidizing or reducing conditions,
• Fuel nature
• MOX versus UO2
• Initial morphology

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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
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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
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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
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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
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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
Temperature 1860-1880C 1860-1880C
Test VERCORS 2 VERCORS 1
Release 30-40 30-40
     
     
Temperature 2300C 2300C
Test VERCORS 4 VERCORS 5
Release 87 - 93 87 - 93
Temperature 2300C 2300C
Test V_ 3 V_4 and V_5
Duration 15 minutes 30 minutes
Release 70 87-93
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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

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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

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Te, Sb and Ag release Kinetics
VERCORS 4 UO2, 38 GWd/T hydrogen
Tellurium retention in the cladding until the
latter was completely oxidised
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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
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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
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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