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OVERVIEW OF FISSION PRODUCT BEHAVIOUR IN THE PHEBUS FP TESTS,INCLUDING LATEST EXPERIMENTAL RESULTS ON THE FPT2 TEST


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M. ZABIÉGO, J.J. COCHAUD, B. CORNU, Y. GARNIER, J.M. GIRARD, G. GRÉGOIRE, D. JACQUEMAIN , L. PANTERA, E. RAGAGLI, P. ZAVALETA, R. ZEYEN“OVERVIEW OF FISSION PRODUCT BEHAVIOUR IN THE PHEBUS FP TESTS,INCLUDING LATEST EXPERIMENTAL RESULTS ON THE FPT2 TEST”FSRSM, JAERI Tokai, Japan, March 4-5, 2002

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The Phebus FP program is a wide international effort to investigate LWR severe accident phenomena, through a series of in-pile integral experiments, dealing with fuel degrada-tion, hydrogen production, fission product release and subsequent transport to the con-tainment. Three tests simulating a low-pressure cold-leg break have already been suc-cessfully performed: -FPT0, in December 1993, with trace-irradiated fuel and steam-rich atmosphere; -FPT1, in July 1996, with re-irradiated fuel (23-GWd/tU fuel burn-up) and steam-rich atmosphere; -FPT2, in October 2000, with re-irradiated fuel (32-GWd/tU fuel burn-up) and a significant reducing phase. These tests provided experimental data of high interest, particularly regarding: -Fuel degradation with prototypic bundle material; -Fission product, fuel and structure material release from the test bundle; -Transport and deposition in the model Reactor-Coolant-System (RCS, including a steam-generator model); -Fission product behaviour in the model containment (including a sump model and reactor-representative painted surfaces), emphasising iodine source-term issues. The data analysis of FPT0 and FPT1 is now terminated (and available in public domain literature), while that of FPT2 is still under progress. The following observations can be made from these tests: -All three tests resulted in the formation of a melted UO2/ZrO2 pool. The re-location temperature was significantly lower than expected in FPT0 and FPT1 (< 2550 °K). -Main release/transport phases were found to correlate with specific bundle deg-radation events (zircaloy cladding oxidation and material re-locations). -Volatile fission products (I, Cs, Te and Mo) and noble gases (Kr and Xe) were strongly released from the degraded fuel. Large fractions of these elements reached the containment vessel. -Deposition in the RCS essentially occurred in the circuit hot-leg (in sections above the degraded fuel and at the entrance of the steam-generator), with larger deposits in FPT2 (also characterised by significant deposits on the moderately-degraded upper-part of the fuel bundle) owing to the lower steam injection rate than in previous tests. -Aerosols conveying fission products through the RCS were multi-component, with structure material dominant in mass. -Gaseous iodine was transported in significant quantities to the containment ves-sel during zircaloy oxidation phases. -Sump chemistry exhibited marked iodine trapping by silver (originating from the degradation of the Ag-In-Cd control rod), preventing the formation of volatile io-dine by radiolysis and hydrolysis. -On the long term, containment gaseous iodine species were dominated by or-ganic iodides released from painted surfaces in FPT0 and FPT1, and by molecu-lar iodine in FPT2. These differences in gaseous iodine speciation may result from differences in release processes associated with trapping surfaces (painted condensers in FPT1 versus stainless-steel vertical wall in FPT2). The results of the Phebus FP programme are currently used by the different partners for validating and improving severe accident codes (ICARE/CATHARE, ATHLET-CD, SCDAP/RELAP5, ASTEC, MELCOR, ...). An example is given in the paper, regarding the interpretation of fission-products and control-rod-material release during the FPT1 experiment, with code ELSA (with bundle degradation data from ICARE2).

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