Stress Mitigation Design of Tubesheets With Consideration of Thermal Stress Inducement Mechanism

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. Recognized thermal stress mechanism of semi-spherical tubesheet is as follows. (1) Dominant thermal stress is hoop stress caused by temperature difference between the perforated region and surrounding region. (2) Thermal stress is insensitive to size of specific portion, although is dominated by interaction mechanism between perforated and surrounded regions. (3) Stress concentration around hole’s edge generates peak stress. (4) Amplitude of peak stress depends on the tubesheet penetration angle and stress concentration becomes high near the outermost hole. Based on the above stress generation mechanism, authors proposed a stress mitigated tubesheet. It is center flatted spherical tubesheet (FST) as improved configuration. Calculated peak stress of FST was smaller than that of semi-spherical tubesheet. Further investigation revealed the detailed stress generation mechanism of FST during thermal transient. There were two different comparable thermal peak stress mechanisms in FST. Both location and magnitude of maximum peak stress depend on sodium temperature histories at thermal transient. One depends on the range (ΔT) of sodium temperature change. This type of peak stress was radial stress caused by the structural discontinuity, and it was located at the outermost hole. The other depends on the rate (dT/dt) of sodium temperature change. This type of peak stress was hoop stress caused by interaction between perforated region and surrounding region, and it was located at the one inner layer hole from outermost layer holes.