Evaluation of the RBMK-1500 accident confinement system

The main safety tasks of the Accident Confinement System (ACS) of a RBMK-1500 plant are to provide confinement of steam, water and radioactive products released by a break in the primary coolant circuit or by opening of the main steam relief valves or fast-acting pressure-reducing valves, to prevent excessive pressure building up within section of confinement system, to provide adequate structural strength of each compartment to withstand the pressure and temperatures which may occur in a loss-of-coolant-accident (LOCA) and to limit release of radioactivity to the environment in a design basis accidents to specified limits. The results of the thermohydraulic and structural response of the ACS of Ignalina NPP to LOCAs is presented. This is the first analysis that considers both short and long term responses of the ACS for full range of LOCAs employing state-of-the-art best estimate methods. An adequate modeling of complex multi-compartment and multi-suppression pool system of the Ignalina NPP ACS is performed using new CIIAF version of the CONTAIN code. The analysis employs mass and energy source from calculations performed using RELAP5 code. The calculated atmospheric temperatures and pressure inside ACS compartments as well as temperatures of structures are uses as input data for structural analyses of the ACS. The results of a finite element analysis are compared with those of the traditional technique of limit analysis. The bending moment and axial force of the cross-section are founded from the analytical expression for plated subjected to bending and under different support conditions. The study covered the evaluation of performance parameters: loads initiating the cracks; sizes of cracks initiated; possibilities of closing the cracks. Numerical results of strength evaluation for most loaded compartments are presented. (J.P.N.)