A design guide for high-temperature gas-cooled reactor components is proposed and applied to the design and construction of the l.5-MW(thermal) helium heat exchanger test loop for nuclear steelmaking. To assure that the design method covers all conceivable failure modes and has a large enough safety margin, a series of lifetime tests of partial model may be needed. For this project, three types of model tests are performed. A lifetime test of an in-scale model of the center manifold pipe and eight heat exchanger tubes is described. Applied load is the combination of the simulated thermal expansion stress (deformation controlled quantity) and primary stress by internal pressure of tubes. The level of both loads is much higher than the corresponding values in the actual plant, which causes failure of the model in a shorter time. The eight tubes are arranged so that each is subjected to different damage conditions. The lifetime tests ran for 48 days, and six tubes out of eight failed during the test at the highest stressed stub tubes. Other parts of the components were found to be sound after the test. A damage criterion with a set of material constants and a simplified method for stress-strain analysis for a stub tube under a three-dimensional load are newly developed and used to predict the lives of each tube. The predicted lives are compared with the experimental lives and good agreement is found. The lifetime test model is evaluated according to the proposed design guide, and it is found that the guide has a safety factor of ∼200 in life for this particular model.