[en] The Japan Atomic Energy Research Institute has been engaged in the research on PTA of nuclear power plants since F Y 1980. It consists of: Development of PTA methodologies, including reliability analysis, core melt accident analysis and external event risk analysis, and Application of these methodologies, including PTAs of Japanese light water reactors. The past progress and future schedule is illustrated in Figure 1. This paper summarizes the important progress of the recent research as well as the topics in the current research plan. Currently a reliability benchmark test on fault tree analysis of the core spray system of a BWR is progressing under the cooperation of four contractors. As generally known, there is a high degree of freedom left to the analysis in the process of fault tree development. This characteristics, on the other hand, can be a source of uncertainty in the results. The objectives of the benchmark test are therefore to identify the causes of uncertainties and to quantify their effects. The project was systematically organized in order to distinguish the effects of various contributors to the uncertainty. At present, all the analyses by the contractors were completed and the results are reviewed at JAERI. Figure 2 shows the overall spread of the TOPO event probabilities calculated by the four contractors who established the boundary conditions, developed the fault trees and selected the reliability database independently in this analysis