The analysis of uncertainties was performed for the significant parameters for conditions with loss of the spent fuel pool cooling using best estimate code SOCRAT-B1/B2. Based on the analysis results, the parameters which have the largest impact on significant results of the calculation are determined. It is also shown that simultaneous deviation of the initial parameters first to the conservative side (for getting maximal cladding temperature at the end of calculation higher so to obtain the worst results in terms of safety) and then to the optimistic side (to obtain the most favorable results in terms of safety) leads to interval of the results larger than the range of uncertainties under consideration. The results of the performed analysis show that this approach can be used for NPP safety justification in accident conditions with loss of spent fuel pool cooling in addition to the methods recommended in regulatory documents.