COOLANT MIXING PHENOMENA WITHIN THE LOWER PLENUM OF THE REACTOR PRESSURE VESSEL

The objective of this PhD is to asses the capability of Computational Fluid Dynamics (CFD) to predict the complicated mixing phenomena in the Lower Plenum of the reactor pressure vessel (RPV). This activity has been conducted in the frame of the TACIS-Project R2.02/02A: “Development of safety analysis capabilities for VVER-1000 transients involving spatial variations of coolant properties (temperature or boron concentration at core inlet”. The use of CFD calculations is intended to better understand the in-vessel mixing phenomena, in order to help identifying the system code deficiencies and to properly define future use of system codes for future safety analyses. With such purpose, a specific experimental program and extensive pre and post-test calculations were performed in order to provide material for validating the capability of the developed Russian system code (i.e. TRAP-KS, DKM and KORSAR-GP) and commercial CFD code (i.e. ANSYS CFX-10) against the obtained experimental data, representing expected physical situations after nuclear power plant (NPP) postulated accident initiating events. The adopted CFD code predicts properly flow patterns for all tests and exhibits a general tendency to a low degree of mixing. Selected system code prediction highlighted capabilities of such fast running analysis tools; at the same time deficiencies were identified, especially for the cases of start-up of one pump, restart of natural circulation as well as the case of stable operation of one pump. At present it is conluded that selected system codes are not yet usable for safety analisys report (SAR) purposes unless the development and use of a proper ‘bounding’ methodology allows using such codes in a conservative and reliable way for SAR purposes; on the other hand the use of CFD codes proved to be a valuable tool to support a system code validation process.