The aim of this article is to briefly introduce the probabilistic safety assessment (PSA) of nuclear power plants (NPPs), its scope, main concepts and application to a real case. The results of analysis presented here have been obtained by the Probabilistic Safety Analysis Group (GPSA) at the National Centre for Nuclear Research (NCBJ, Otwock) as a part of the work done for the Polish National Atomic Energy Agency (NAEA). As a reference, NPP Surry Unit 1 (USA), equipped with 800 MWe Westinghouse triple-loop PWR (pressurized water reactor), has been chosen. The emergency coolant injection (ECI) function availability following the small break loss of coolant accident (SBLOCA) was thoroughly analyzed. The approach and data, which were adopted for the selected part of the SBLOCA sequences, were those used in the U.S. NRC Reactor Safety Study (WASH-1400). As a result of this study, the SBLOCA event tree, including ECI systems, i.e. high pressure injection system (HPIS) and auxiliary feedwater system (AFWS) reliability models, was developed and quantified. The probability of each accident sequence was evaluated using Saphire v.8, the PSA software by U.S. NRC. The choice of the software was based on earlier PSA software study. The failure probability of at least one of the considered safety systems – P(FAIL) is equal to 5.76E-3 and the most pessimistic accident branch (unavailability of both HPIS and AFWS) is about 0.05% of P(FAIL). These results were obtained based on assumption that the SBLOCA has occured. The most significant failure components are those corresponding to charging pumps unavailability, loss of electric power and human errors.