An experimental and numerical study were carried out on the single-phase natural circulation phenomena in the pool-type LMFBR. This study is concerned with the primary and intermediate system which are relevant to the pool-type LMFBR. The main purposes of this work are to understand the qualitative behavior of the natural circulation phenomena in the pool through experiment and to develope the simplified fast running computer code to simulate the transient behavior with the one-dimensional lumped parameter model. Two purposes of the experiment are to understand the qualitative behavior of the natural circulation phenomena in the pool and to validate the developed fast running system code-SIMFARP. And the experimental systems were scaled down to have about 1/10-size of the primary and intermediate system of the Super-Phenix-I. The purposes of the development of the fast running system code SIMFARP are to simulate the qualitative behavior of the natural circulation in the pool-type LMFBR and to apply to the operational transient analysis for the real time basis as well as the conservative safety analysis and sensitivity study for licensing. For the development of the SIMFARP, eight time-dependent governing equations were derived based on the one-dimensional lumped parameter model. And the Runge-Kutta algorithm of order 4 is applied to integrate the eight differential equations. Comparisons of the experimental data with the computational predictions and plant design data are presented in this study.