Analysis of loss of coolant accidents is one of the most important issues in the Design Basis Accident assessment. Since a number of LOCAs is defined in terms of break size and location of the break, the variety of analyses for postulated accidents should be performed. According to the international practice the best-estimate analysis is used for evaluation of safety margins or limits, required for safety analysis. The best-estimate analysis consists of deterministic analysis using best-estimate code with sensitivity and uncertainty analysis of calculation results. This paper presents the results of analysis of the worst case of LOCA – break of Main Circulation Pump pressure header with failure to close check valve of one Group Distribution Header. Performed analysis allowed to estimate number of fuel channels, where cladding could be affected. This information further can be used for the analysis of radiological consequences.