[en] This paper describes the results of studying on system problems in the cooling system of the First Conceptual Design. For normal operation, advanced cooling system proposals have been offered, which are capable of controlling the secondary hot-leg helium temperature increase owing to the margin of the heat transfer surface of intermediate heat exchangers and steam generators. As a result of comprehensively evaluating the proposals from the viewpoints of performance, economy, and safety, it was found with the cooling system that the method for adding by-pass nozzle to the steam generator has been evaluated as the best. For accidental conditions, temperature performance analysis has been undertaken in relation to components thermal shock generated by changing-over the primary cooling system to the auxiliary cooling system, and it became evident that the component in a state of sudden temperature changes is a mixer of single-wall pipe. However, it was shown from the results of structural stress analysis that stress generated in the mixer of single-wall pipe is allowable enough. Progressed two adjustments as described above, the cooling system has been established, which satisfied cooling function required to meet the normal operation and the accidental conditions. (author)