RBMK-1500 accident analysis using BE approach

This paper presents the results of analysis of three cases, which cover worst postulated RBMK-1500 accidents and transients: maximum design basis accident at RBMK-1500- break of Main Circulation Pump (MCP) pressure header with failure to close check valve of one Group Distribution Header (GDH); analysis of the postulated blocking of coolant flow rate in GDH event; partial GDH break. The first case with its consequences cover all Loss of Coolant Accident (LOCA) type accidents, while the second deals with transients. The third case (partial GDH break) can lead to flow stagnation in the group of channels, and consequences of this event are similar as in GDH blocking case. For all cases two types of analysis were performed: 'best-estimate' and 'partially-conservative'. Comparison of two approaches enables to select the most suitable one, since each of them has positive and negative sides. Analysis of three cases was performed using 'best-estimate' and 'partially-conservative' calculations: maximum design basis accident at RBMK-1500, break of Main Circulation Pump (MCP) pressure header with failure to close check valve of one Group Distribution Header (GDH); analysis of the postulated blocking of coolant flow rate in GDH event; partial GDH break. The performed comparison between two approaches shows that in general 'partially-conservative' approach provides higher peak temperatures. This approach could be recommended for safety calculations if obtained results do not violate safety criteria. Otherwise, 'best-estimate' approach would be appropriate. It is necessary to point that for the 'partially conservative' approach proper boundary and initial conditions should be selected. This selection requires experience from user, since for different phenomena, which are observed during the accidents, initial and boundary conditions can differ