Abstract
Thermal stratification-induced stresses could lead to a serious failure and fatigue crack on piping systems. U.S. NRC Bulletin 88-08 [1] requires to investigate which unisolable pipings are subjected to the thermal stratification and to demonstrate compliance with applicable code limits during the piping design stage by incorporating the thermal stratification-induced stresses into the fatigue evaluation.
In this paper, the computational fluid dynamic (CFD) analyses considering both the out-leakage case by turbulent penetration and the in-leakage case by valve leakage were performed for the unisolable portion of the Direct Vessel Injection (DVI) piping between the reactor vessel nozzle and the first check valve to determine the change of temperature gradient on the pipe wall as a function of time due to the thermal stratification. And then the CFD-based temperature distributions on the pipe wall at each time interval were transformed as input data for the structural analysis to evaluate the stresses induced by the global bending moments and local stresses by the thermal stratification of the DVI piping.
The localized thermal stratification stress intensities were directly extracted from the 3-D model using the ANSYS program and were categorized as the three stress terms induced by ΔT1, ΔT2, and Ta - Tb defined in NB-3600 of ASME B&PV Sec. III [2], but including thermal stratification effects herein for the fatigue analysis. To evaluate the air environment- and LWR environment-based fatigue damages for the DVI piping, the bending moments and three local stress terms due to the thermal stratification were incorporated into the fatigue analysis. NB-3200/-3600 of ASME B&PV Sec. III- and Regulatory Guide 1.207-based cumulative usage factors [3, 4] were compared with each other to investigate the effects of fatigue damages considering the thermal stratification in the air and light water reactor (LWR) environments.
Thermal fatigue damage evaluation of a PWR NPP steam generator injection nozzle model subjected to thermal stratification phenomenon
