International VERCORS Seminar,

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Session 4 Use of acquired knowledge for safety
studies

• Use of VERCORS results for EDF safety studies

F. Andreo Electricity of France, SEPTEN,
Villeurbanne J.S. Lamy Electricity of France,
RD, Clamart
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Contents

• Nuclear Safety context in France and industrial
  approach regarding Severe Accident
• Position of VERCORS Program in Safety analysis
• Use of VERCORS results in EDF Safety analysis
• Determination of bounding releases
• Development of MAAP models
• Conclusion

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Nuclear safety context in France and SA approach

• SA situations low probability but analysed from
  safety viewpoint
• EDF started to draw up a synthesis of
  requirements associated with SA in 2001
• Definition of  high level  objectives based on
  probabilistic and radiological criterions
• Assessment of NPP under SA conditions
• Based on safety analysis, modifications are
  specified on nuclear sites
• Venting procedure (U5),
• Installation of passive autocatalytic
  recombiners,
• Reliability enhancement of pressurizer safety
  valves,
• Severe Accident (Low Pressure situations) are
  taken into account on Generation III power plants
  (EPR, AP1000, ESBWR, )

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Position of VERCORS Program in Safety analysis

• If PSA allows to evaluate the frequency of each
  releases category, it is necessary to evaluate
  the associated radiological consequences and the
  behaviour of equipments under SA conditions
• For these evaluations, numerous phenomena have to
  be taken into account
• In vessel releases,
• Ex vessel releases,
• Late releases from RCS,
• Transport in RCS,
• Iodine chemistry,
• Deposition of FP in containment and auxiliary
  buildings,
• Interaction with spray system (if available),
• VERCORS program is focused on in-vessel releases
  and allows to have elements on deposition in core
  and transport in RCS

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Use of VERCORS results

• Analytical tests as VERCORS allow to elaborate a
  large data base on fission product releases
• Atmosphere conditions (oxidizing/reducing),
• Burn-up,
• Configurations (debris bed, intact fuel),
• These different configurations are of great
  interest for reactor case because conditions in
  core are time- and location- dependant
• These numerous conditions allow to
• Define reasonable bounding releases for safety
  analysis for decoupling step,
• Improve models in code for validating step

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Determination of bounding releases (1/2)

• Numerous scenarios are possible with a complex
  phenomenology
• Decoupling step aims at having good predictions
  without scenario dependant
• In 2002, EDF defined these assumptions to
  re-assess reference source term in the
  environment (called  S3 ) on 900 MWe power
  plants serie, based on
• Analytical tests HI-VI, VERCORS,
• Integral tests PHEBUS FPT0, FPT1,
• TMI2

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Determination of bounding releases (2/2)

• Four classes of fission products have been
  defined
• Volatile FP Xe, Kr, I, Cs, Te, Sb, Rb
• Totally released in case of total core melt down
• Semi-volatile FP Mo, Ba
• Releases are dependant of oxido-reduction
  conditions
• During the accident, conditions vary in function
  of time and location
• Release rates taken into account a half of core
  inventory
• Ba/Fe interactions not taken into account because
  PHEBUS results was not understood yet
• Low volatile FP La, Ce,
• Low releases
• High deposition near the fuel
• Few percents of core inventory
• Non volatile FP Zr, Nd,
• Very low releases 1 i.c.i. as bounding value

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Development of MAAP models

• Developped by FAI for EPRI
• Able to describe core degradation
• MAAP is the reference code at EDF for severe
  accident studies
• EDF develops complementary development and
  qualification activities
• Simplified models
• calculation time relatively limited
• compatible with industrial use
• Benchmark possible

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Fission Product Groups

• 12 groups

N Group Elements Volatility
1 Noble gas Xe, Kr volatile
2 Iodin CsI, RbI volatile
3 Tellurium TeO2 volatile
4 Strontium SrO low volatile
5 Molybdenum MoO2 low volatile
6 Cesium CsOH, RbOH volatile
7 Baryum BaO low volatile
8 Lanthanides La2O3, Pr2O3, Nd2O3, Sm2O3, Y2O3 low volatile
9 Cerium CeO2 low volatile
10 Antimony Sb low volatile
11 Tellurium Te2 volatile
12 Uranium UO2, NpO2, PuO2 low volatile
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Release phenomenology

• Fission product releases 2 main steps
• Intra and inter granular diffusion,
• Saturation
• Limiting Step
• Diffusion for volatile FP
• Saturation for low and semi volatile FP

PF




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MAAP models (1/3)

• MAAP release models
• Models based on Arrhenius law
• Kelly model, CORSOR-O, CORSOR-M
• ? Atmospheric composition is taken into account
  in CORSOR-O model
• Scale Factor (1 fH2) . Oxydising Factor fH2
  . Reducing Factor
• NUREG-0772
• ? Retention of Te in claddings taken into account

• m(t) FP mass
• m0 FP initial mass
• K(t) Release rate

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MAAP models (2/3)

• MAAP release models
• Diffusion models
• Cubicciotti model
• ?FP migration linked with fuel oxidation rate
• ORNL-Booth model
• Based on 2nd Fick law resolution
• Atmospheric composition is taken into account

• C FP concentration
• D Diffusion coefficient

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MAAP models (3/3)

• Saturation model
• FP releases are limited by the FP quantity
  transported in the gas
• Necessity to determine maximum concentration
  thanks to saturated pressure
• Modelling
• Gas is gradually charged in FP

Fuel


Gas Saturation

FP

FP

FP


Gas flow

crown
i

FP


Channel j
FP Concentration
ij

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VERCORS tests benchmark with MAAP

• Main characteristics
• 3 pellets 1 cell,
• Atmosphere composition injected flow
• No activation of cladding oxidation model

Test Temperature Atmosphere
02 2150 K mixed
04 2570 K reducing
05 2570 K oxydizing
06 2620 K mixed
HT1 2900 K reducing
HT3 2680 K reducing
RT7 2890 K reducing
Fuel pellet
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MAAP / VERCORS comparison (1/3)

• Volatile Fission products Noble gas
• Total release
• Good agreement for ORNL-Booth model

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MAAP / VERCORS comparison (2/3)

• Volatile Fission products Iodine

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MAAP / VERCORS comparison (3/3)

• Low Volatile Fission products Baryum

02 04 05 06
Experiment 4 80 55 28
Corsor-O 1.7 87.7 14.1 39.6
ORNL-Booth 2.3 58 5.8 28.4

• Good agreement in reducing and mixed atmosphere
• Underestimation in oxidizing conditions

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Main improvements (1/2)

• Claddings retention
• ? Te and Sb retention in claddings for all
  models
• Improvement of low volatile fission product
  releases in certain atmosphere conditions for
  ORNL-Booth and CORSOR-O models
• For instance, Ba releases improvements

02 04 05
Experiment 4 80 55
Corsor-O 1.7 87.7 14.1
Corsor-O modified 4.3 87.6 53.1
ORNL-Booth 2.3 58 5.8
ORNL-Booth modified 8.9 99.2 50
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Main improvements (2/2)

• Problem with U group

• Group Composition U, Pu, Np
• Similar behaviour of Np and U but Pu release is
  order of magnitude lower
• Necessity to divide U group

expérience expérience expérience calculs calculs
U Pu Np ORNLBooth Cor-O
04 ? 2 ? 0,2 6 0,021 0,063
05 ? 2 ? 0.2 lt 4 0,22 0,584
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MAAP / VERCORS comparison conclusion

• EDF/RD work on MAAP/VERCORS comparison allow
• To acquire knowledge on MAAP modelling,
• To improve models
• It leads to new recommendations concerning the
  use of release models in MAAP
• ORNL-Booth for volatile fission products,
• CORSOR-O with modification for low volatile
  fission products

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Conclusion

• FP release from fuel relevant issue for Safety
  Studies
• VERCORS a large data base linked with FP
  releases and transport
• From VERCORS tests analysis
• Decoupled assumptions for FP release have been
  defined
• MAAP models have been improved
• Necessity to have more data on recent fuel
  management (MOX fuel, HBU UO2) and air ingress
  situation
• EDF support VERDON program as part of ISTP and
  modelling program achieved by IRSN/DPAM.