Radionuclide transport and retention in reactor cooling systems during severe accidents

During degraded core accidents, fission product release, transport and deposition depend on the prevailing thermal-hydraulic conditions within the reactor coolant system (RCS). The self-heating of the suspended and deposited fission products can substantially increase the gas and structure temperatures along the RCS. These increased temperatures can potentially alter the RCS flow patterns, induce local failure of thin structural elements, redistribute previously deposited fission products (via revaporization and transport), and will ultimately impact the release of these radioactive fission products to the containment. Therefore, both the thermal-hydraulic and fission product transport processes are highly coupled and must be simulated simultaneously. The Severe-accident Integrated Analysis Methodology (SIAM) computer code is a computational tool capable of simulating the fully-coupled fission product transport and deposition, and thermal-hydraulic response of the RCS. The SIAM code couples the CORMLT, PSAAC and RAFT computer codes, which simulate melt progression, RCS thermal-hydraulics, and aerosol/vapor transport and deposition, respectively. This paper provides an overview of these computer codes, and outlines the methodology developed to perform the fully coupled calculations. Sample results of the PWR TMLB scenario are shown for both the Westinghouse and B and W designs