Thematic network for a Phebus FPT-1 international standard problem (THENPHEBISP)

FISA 2003, 10-13 novembre 2003, Luxembourg
B. Clément, T. Haste (1), E. Krausmann (2), S. Dickinson (3), G. Gyenes (4), J. Duspiva (5) , F. de Rosa (6), S. Paci (7), F. Martin-Fuertes (8), W. Scholtyssek (9) , H-J Allelein (10), S. Güntay (11), B. Arien (12), S. Marguet (13), M. Leskovar (14), A. Sartmadjiev (15).

The THENPHEBISP 2-year thematic network started in December 2001, and was concerned with OECD/CSNI International Standard Problem 46, itself based on the Phebus FPT1 core degradation/source term experiment. The aim was to assess the capability of computer codes to model in an integrated way the physical processes taking place during a severe accident in a pressurised water reactor, from the initial stages of core degradation, the fission product transport through the primary circuit and the behaviour of the released fission products in the containment. ISP-46, coordinated by IRSN/DRS Cadarache, attracted 33 participating organisations, from 23 countries and international bodies, who submitted 47 base-case calculations and 21 best-estimate calculations, using 15 different codes.

The thermal behaviour of the fuel bundle and the hydrogen production were generally well captured, and good agreement for the core final state could be obtained with a suitable choice of bulk fuel relocation temperature, however this is unlikely to be representative of all plant studies so sensitivity calculations are needed with the modelling in its current state. Total volatile fission product release was simulated, but its kinetics, and the overall modelling of semi-volatile, low-volatile and structural material release (Ag/In/Cd, Sn) needs improvement. Overall retention in the circuit is well predicted, but calculations underestimate deposits in the upper plenum and overestimate those in the steam generator, also the volatility of some elements could be better predicted. Containment thermal hydraulics and depletion rate of aerosols are well calculated, but with difficulties related to partition amongst the deposition mechanisms. Calculation of iodine chemistry in the containment turned out to be more difficult. Its quality strongly depends of the calculation of release and transport in the integral codes. The major difficulties are related to the existence of gaseous iodine in the primary circuit and to the prediction of the amount of organic iodine in the gas phase. This paper summarises the results achieved and the implications for plant calculations.

(1) IRSN, Cadarache (Fr)
(2) JRC, Petten (Nl)
(3) AEA Technology (UK)
(4) AEKI, Budapest (Hu)
(5) NRI, Rez (Cz)
(6) ENEA, Bologna (It)
(7) UPI, Uv. Pisa (It)
(8) UPM, Uv. Madrid (Sp)
(9) FZK
(10) GRS
(11) PSI
(12) AVN
(13) EDF