Thermal Stratification in Piping Systems
SIB-96-159

Since the U.S. Nuclear Regulatory Commission (USNRC) issued Bulletins 88-08 and 88-11, there has been an increased awareness of thermal stratification and related thermal cycling that were previously unidentified in piping systems. These thermal conditions can lead to piping displacements exceeding those considered in original design, and can produce significant thermal stresses in the pipe wall.

In addition, cyclic changes in stratification height can cause significant thermal stress cycling, as shown above for location P. This cycling can lead to thermal fatigue crack initiation and crack growth. At two pressurized water reactor (PWR) plants, valve leakage toward the reactor coolant system from high-pressure external sources resulted in leakage in reactor coolant system piping. This in-leaking cold fluid interacted with periodic turbulence from the reactor coolant system flow. The alternate heating and cooling at the elbow-to-pipe weld caused thermal fatigue cracking.

Structural Integrity Associates (SI) has played an integral role in the mitigation or elimination of thermal stratification in piping components, both on plant-specific and industry-wide basis. SI's expertise has helped utilities rapidly evaluate root cause of thermal cycling or stratification-related fatigue failures and to assist in developing rapid repairs.

SI has assisted the Electric Power Research Institute (EPRI) in evaluating this problem, and was a major contributor to the Thermal Stratification, Cycling, and Striping (TASCS) program (EPRI TR-103581). SI has continued their support to EPRI and developed the Fatigue Management Handbook (EPRI TR-104534). This handbook contains detailed screening and evaluation methodology that can be used to evaluate the potential for and the significance of thermal stratification in piping systems.

SI has performed detailed system reviews to determine piping geometries and systems that would be susceptible to thermal stratification. This has assisted utility engineers in performing focused inspection and implementation of monitoring programs.

In particular, feedwater nozzles in steam generators of PWRs have experienced cracking due to low-temperature auxiliary feedwater injection. SI has developed a unique replacement feedwater nozzle safe-end design, and has acted as prime contractor for replacement of the safe-end component.

At a B&W-designed PWR, SI performed a root cause evaluation to determine why a nominally-stagnant drain line had developed a leak during plant operation. SI's knowledge of thermal stratification, turbulence penetration, and thermal cycling issues allowed them to predict the source of the problem and to recommend an economical fix to prevent further occurrences of the cracking.

SI has assisted several utilities in requalifying piping systems to the requirements of the ASME Code when new loadings were identified. This has included an evaluation of plant operating conditions to develop new loading specifications, performance of ASME Code Class 1 piping fatigue analysis, and preparation of stress reports. Additionally, SI has developed several specialized computer programs that allow for the efficient evaluation of stratified piping, including TOPBOT which determines the stress distribution in a stratified piping system, and PIPEFAT which performs a rapid ASME fatigue piping analysis. Where complex piping geometries exist, SI's expertise in finite element analysis allows for the rapid determination of steady-state temperature and stress distributions in stratified components using ANSYS or other specialized programs.

If you would like more information regarding SI's capabilities in the area of thermal cycling, or any other stratification-related issue, please contact SI.