Approach to Phebus containment nodalization for lumped-parameter codes

Containment of the Nuclear Power Plant is the last barrier preventing the release of radioactive fission products to the environment in case of an accident. Therefore investigation of the fission product transport and deposition processes is a very important task. Phebus test facility and the performed fission product program provides the most extensive experimental results on severe accident phenomena in a Light Water Reactor, starting from the nuclear fuel bundle degradation, continuing to radioactive material transport through the reactor cooling circuit and release to containment and ending with fission product deposition in containment. In order to understand and explain the obtained experimental results different computer codes are used. Though the Computational Fluid Dynamics (CFD) codes are already well advanced the lumped-parameter codes remain the main tool for safety assessment of NPPs and validation of these codes is an important task. The lumped-parameter codes have certain limitations that require a special attention while performing safety analyses of the nuclear power plants. A special attention has to be paid to development of the containment nodalization to be used for analysis. This nodalization has to provide possibilities to take into account the phenomena that are expected in the course of the accident progression. This paper presents analysis of aerosol deposition and distribution processes in PHEBUS containment using lumped-parameter code COCOSYS. For the analysis two models of 19 and 16 nodes were developed. The calculated thermal-hydraulic parameters are in good agreement with the measured results. The correct simulation of thermal-hydraulic phenomena is a precondition for simulation of transport and deposition of the materials released from the degrading core. Analysis of aerosol deposition showed, that the calculated deposition velocity is in good agreement with measured. Different ways of nodalization give different flow paths inside the containment and lead to different aerosol deposition distribution. In both cases deposition on the containment walls is significantly smaller than measured. Further investigation of diffusive deposition model is needed. (authors)