Computer codes for coupled calculations of WWER-440 and WWER-1000 reactor core thermohydraulic and fuel rod thermomechanical behaviour under DBA LOCA conditions

Safety considerations and limiting parameters, as well as computer codes and physical models for analysing the reactor core thermohydraulics and fuel element integrity under DBA LOCA conditions are discussed. The authors consider the well known conservative and the recent 'best estimate' approaches as well as the widely used computer codes to predict the reactor core and the fuel rod performance and to estimate the ECCS efficiency of WWER reactors. Some mostly used thermohydraulic computer codes, the physical models involved and different versions of the RELAP code are viewed and compared. The choice of the RELAP4/Mod6 version to perform calculations for the WWER reactors is well grounded. The necessity to analyse the processes in the fuel rods with special tools is also proved. Some computer codes and the important physical models for prediction of the fuel rod thermomechanical behaviour are discussed and the program system SSYST-2 is chosen as the most convenient code available to perform calculations for the WWER reactors. As a part of the 'best estimate' approach, the coupled thermohydraulics, the fuel rod thermomechanical analysis and its consequent stages are presented and their application for WWER reactors is suggested. 66 refs., 3 figs. (author)