Nuclear Engineering and Design 239 (2009) 833–839 Contents lists available at ScienceDirect Nuclear Engineering and Design journal homepage: www.elsevier.com/locate/nucengdes Numerical analysis of thermal striping induced high cycle thermal fatigue in a mixing tee Jeong Ik Lee, Lin-wen Hu ∗ , Pradip Saha, Mujid S. Kazimi Center for Advanced Nuclear Energy Systems, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA article info Article history: Received 20 November 2007 Received in revised form 3 June 2008 Accepted 18 June 2008 abstract Thermal striping, characterized by turbulent mixing of two flow streams of different temperatures that result in temperature fluctuations of coolant near the pipe wall, is one of the main causes of thermal fatigue failure. Coolant temperature oscillations due to thermal striping are on the order of several Hz. Thermal striping high-cycle thermal fatigue that occurs at tee junctions is one of the topics that should be addressed for the life management of light water reactor (LWR) piping systems. This study focuses on numerical analyses of the temperature fluctuations and structural response of coolant piping at a mixing tee. The coolant temperature fluctuations are obtained from Large Eddy Simulations that are validated by experimental data. For the thermal stress fatigue analysis, a model is developed to identify the relative importance of various parameters affecting fatigue-cracking failure. This study shows that the temperature difference between the hot and cold fluids of a tee junction and the enhanced heat transfer coefficient due to turbulent mixing are the dominant factors of thermal fatigue failure of a tee junction. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Prediction of high cycle fatigue in mixing tees is a challenging subject for life management of nuclear power reactor piping sys- tems. Thermal striping is one of the phenomena that resulted in thermal fatigue failure, and is characterized by turbulent mixing of hot and cold flow streams that result in temperature fluctuations of the coolant near a piping wall. The coolant temperature fluctuations may cause cyclical thermal stresses and subsequent fatigue crack- ing of the pipe wall. Coolant temperature oscillations due to thermal striping are of relatively high frequencies, reported previously to be on the order of several Hz (Wakamatsu et al., 1995).Studies of thermal striping were initially carried out for liquid–metal-cooled fast breeder reactor (LMFBR) in 1980s because of the high thermal conductivity of the liquid metal coolant (Muramatsu and Ninikata, 1996). Areas susceptible to thermal striping include components in the core outlet region, such as core upper plenum, flow guide tube, and control rod upper guide tubes. Outside the core region, components where hot and cold streams come in contact, such as tee junctions, elbows, and leakage from valves, may also be affected. The issue of thermal striping shifted to light water reac- tors (LWRs) after several incidents of piping failure at some nuclear power plants (Kim et al., 1993; Fukuka et al., 2003; Claude, 2003). ∗ Corresponding author. Tel.: +1 617 258 5860; fax: +1 617 253 7300. E-mail address: lwhu@mit.edu (L.-w. Hu). The piping systems that are most susceptible to thermal striping fatigue cracking are mixing tees of the residual heat removal (RHR) systems in both BWR and PWR. Evaluation of thermal striping can be performed by mock- up experiments or three-dimensional, unsteady computational fluid dynamics (CFD) simulations. The primary goal is to iden- tify the temperature fluctuation magnitude and frequencies. The local thermal stresses of the pipe are proportional to the temper- ature oscillations in the pipe wall, normally a fraction of those in the coolant. The accumulative thermal stress cycles are then applied for stress and strain analysis. In experiments where coolant and pipe wall temperatures were measured, both frequencies and magnitudes of temperature fluctuations can be obtained at selected locations in the pipes. Thermal striping was modeled previously using large eddy simulation (LES) and direct numer- ical simulation (DNS) (Muramatsu and Ninikata, 1996; Roubin, 1998). However, these studies were conducted for only a small control volume around the mixing zone due to computing power constraints. Therefore, careful selection of the subdomain and associated boundary conditions play an important role in such numerical analysis. Our benchmark study, discussed in this paper, demonstrates that LES can predict accurately the temperature fluctuations asso- ciated with thermal striping. A fatigue cracking analysis was conducted using the thermal stress and stress intensity factor (SIF) for comparison of important variables. The objective of this study is to identify the effects of these variables through sensitivity anal- 0029-5493/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.nucengdes.2008.06.014