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Session 4 : Use of acquired knowledge for safety studies

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, R&D, Clamart. Contents.

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Session 4 : Use of acquired knowledge for safety studies

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  1. 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, R&D, Clamart International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France

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

  3. 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, …) International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France

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

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

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

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

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

  9. Fission Product Groups • 12 groups International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France

  10. Diffusion (intra / inter granular) Saturation Gas + FP FP Fuel pellet grain Gas flowrate 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 International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France

  11. dm m(t) ( ) ( ) ( ) = - = - t K t . m t and F(t) 1 dt m 0 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 International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France

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

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

  14. VERCORS tests benchmark with MAAP • Main characteristics • 3 pellets = 1 cell, • Atmosphere composition = injected flow • No activation of cladding oxidation model Fuel pellet International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France

  15. MAAP / VERCORS comparison (1/3) • Volatile Fission products : Noble gas Total release Good agreement for ORNL-Booth model International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France

  16. MAAP / VERCORS comparison (2/3) • Volatile Fission products : Iodine International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France

  17. MAAP / VERCORS comparison (3/3) • Low Volatile Fission products : Baryum • Good agreement in reducing and mixed atmosphere • Underestimation in oxidizing conditions International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France

  18. 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 : International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France

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

  20. MAAP / VERCORS comparison : conclusion • EDF/R&D 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 International VERCORS Seminar, October 15-16th, 2007 – Gréoux les Bains, France

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

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