Thermal transient test and strength evaluation of a tubesheet structure made of Mod.9Cr-1Mo steel. Part I: Test model design and experimental results

The tubesheet structure is one of the components that suffer the most severe loading in fast reactors, and it is one of the most difficult components to design because of such severe operation conditions and its complex three-dimensional structure with an arrangement of numerous penetration holes. In this study, to clarify the failure mode of a semispherical tubesheet structure originally designed for the steam generator in the Japan sodium-cooled fast reactor, a cyclic thermal loading test was performed using a tubesheet model test structure. The tubesheet model made of Mod.9Cr-1Mo steel was subjected to 1873 cycles of severe thermal transient loads using a large-scale sodium loop, in which sodium heated to 600 and 250 °C was flowed repeatedly with periods for each transient of 2 and 1 h, respectively. After the test, the test model was inspected by performing liquid penetrant testing. Then, observation using a scanning electron microscope and hardness testing were performed to characterize the failure mechanism in the structural model tested under the thermal transient loading with holds at elevated temperature. A thermal-hydraulic analysis was also performed to validate the measured temperature history during the thermal transient. Through these examinations and evaluation with thermal-hydraulic analysis, the manner of failure in the tubesheet under cyclic thermal loading is discussed.