Stress mitigation design of tubesheets with consideration of thermal stress inducement mechanism

Ando, Masanori ; Takasho, Hideki*; Kawasaki, Nobuchika ; Kasahara, Naoto

Adoption of double-wall-straight tube steam generators made of Mod.9Cr-1Mo steel is planned for next generation fast breeder reactors in Japan. One of the major concerns relevant to the SG is structural integrity of tubesheets. In the reactor transient operation, thermal stress is induced by the temperature distribution in tubesheet and the magnitude of it depends on configurations of tubesheet. Stress generation mechanism of tubesheets was revealed through Finite Element analysis. Semi-spherical tubesheet models were investigated for the first survey of the thermal stress mechanism. As calculated results, semi-spherical tubesheet model gave the extensive peak stress around the outermost hole. Based on the those stress generation mechanism, authors proposed a stress mitigated tubesheet. It is center flatted spherical tubesheet as improved configuration.