Analysis of steam and hydrogen distributions with PAR mitigation in NPP containments

The 3-D-field code, GASFLOW is a joint development of Forschungszentrum Kar lsruhe and Los Alamos National Laboratory for the simulation of steam/hydro gen distribution and combustion in complex nuclear reactor containment geom etries. GASFLOW gives a solution of the compressible 3-D Navier-Stokes equa tions and has been validated by analysing experiments that simulate the rel evant aspects and integral sequences of such accidents. The 3-D GASFLOW sim ulations cover significant problem times and define a new state-of-the art in containment simulations that goes beyond the current simulation techniqu e with lumped-parameter models. The newly released and validated version, G ASFLOW 2.1 has been applied in mechanistic 3-D analyzes of steam/hydrogen d istributions under severe accident conditions with mitigation involving a l arge number of catalytic recombiners at various locations in two types of P WR containments of German design. This contribution describes the developed 3-D containment models, the applied concept of recombiner positioning, and it discusses the calculated results in relation to the applied source term , which was the same in both containments. The investigated scenario was a hypothetical core melt accident beyond the design limit from a large-break loss of coolant accident (LOCA) at a low release location for steam and hyd rogen from a rupture of the surge line to the pressurizer (surge-line LOCA) . It covers the in-vessel phase only with 7000 s problem time. The contribu tion identifies the principal mechanisms that determine the hydrogen mixing in these two containments, and it shows generic differences to similar sim ulations performed with lumped-parameter codes that represent the containme nt by control volumes interconnected through 1-D flow paths. The analyzed m itigation concept with catalytic recombiners of the Siemens and NIS type is an effective measure to prevent the formation of burnable mixtures during the ongoing slow deinertization process after the hydrogen release and has recently been applied in backfitting the operational German Konvoi-type PWR plants with passive autocatalytic recombiners (PAR). (C) 2000 Elsevier Sci ence B.V. All rights reserved.