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The helium/helium heat exchanger (i.e., intermediate heat exchanger: IHX) of a high-temperature gas-cooled reactor (HTGR) system with nuclear heat applications is installed between a primary system and a secondary system. IHX is operated at the highest temperature of 950°C and has a high capacity of up to 600 MWt. A plate-ˆn-type heat exchanger is the most suitable for IHX to improve construction cost. The purpose of this study is to develop an ultraˆne plate-ˆn-type heat exchanger with aˆner pitchˆn than a conventional technology. In theˆrst step, fabrication conditions of the ultraˆne plateˆn were optimized by press tests. In the second step, a brazing material was selected from several candidates through brazing tests of rods, and brazing conditions were optimized for plate-ˆn structures. In the third step, tensile strength, creep rupture, fatigue, and creep-fatigue tests were performed as typical strength tests for platen structures. The obtained data were compared with those of the base metal and plate-ˆn element fabricated from SUS316. Finally, the accuracy of the creep-fatigue life prediction using both the linear cumulative damage rule and the equivalent homogeneous solid method was conˆrmed through the evaluation of creepfatigue test results of plate-ˆn structures.