Modeling of free convective gas flows in the RPV and their effect on the heatup of the core and the upper plenum structures

In relation to accident management actions, further improvement of the capabilities of reactor accident codes is required. For example, the description of the thermohydraulics used in such codes is not detailed enough. Its modeling features usually do not correspond to the advanced modeling of the solid structures, especially in the reactor core region. Therefore, significant efforts have been undertaken recently to couple codes for a detailed analysis of fuel rod behavior and advanced thermohydraulics in order to simulate, for example, free convective gas flows in the reactor pressure vessel and the reactor cooling system of a PWR. Following dryout of the core, they may have a significant effect on the further heatup of the core and the surrounding structures, especially in the upper plenum region. At the Institut fuer Kernenergetik und Energiesysteme, a coupling of the core meltdown analysis code MELSIM and the code FRECON for the description of two-dimensional gaseous free convective flows in porous media has been established. In the paper, a representative calculation for a PWR high pressure case is presented. As a result, a large scale convective flow is established. This enhanced gas flow, compared to the case with once-through steam flow, due only to evaporation of the residual water, leads to a slower heatup of the core and to a later onset of oxidation and local core melting. On the other hand, a significant heatup of the surrounding structures occurs. Especially in the upper plenum, the melting temperatures of steel are eventually reached. During later stages of the accident sequence, limitations in clad oxidation may occur, owing to the increasing amount of hydrogen in the recirculating gas mixture. (author). Poster presentation. 7 refs, 4 figs