A great expansion in the number of new advanced nuclear power plants (NPPs) being under design increases the importance of probabilistic safety assessment (PSA) carried out during the pre-operational stage and its role in the design process. The book is devoted to the safety assessment of nuclear power plants at the design stage using PSA methodology and integrated risk informed decision making (IRIDM) process. The book consists of three papers. The first paper addresses issues of probabilistic safety assessment for reactors of new design in Russia. The second paper considers how the concept of "fail-safe" design is being practiced by the designers of nuclear power plants. The consequences of particular application of "fail-safe" design principle at Fukishima Diachi Unit 1 are also discussed. IRIDM process is proposed for resolving issues related to "fail-safe" design in NPPs systems. The third paper discusses specific features of human reliability analysis conducted at the stage of the development of the preliminary safety analysis report.