Analysis of Oblique-Insertion Tee Structure on High-Frequency Thermal Oscillation in Turbulent Mixing Zones of Nuclear Power Plant Piping

Literature Overview

This paper, published in Chemical Engineering & Machinery (2025, Vol. 52, No. 6, pp. 1002-1007) by Wu Haihui and colleagues from Taishan Nuclear Power Joint Venture, Suzhou Research Institute of Thermal Power Technology, and Shanghai University of Science and Technology, investigates the impact of oblique-insertion tee structures on high-frequency thermal oscillation in turbulent mixing zones within nuclear power plant piping. The research is funded by the National Natural Science Foundation of China (U21B2076) and the National Key R&D Program (12075274). The study uses numerical simulation to compare oblique-insertion tee structures with conventional orthogonal tees under typical jet flow conditions.

Technical Background: Thermal Oscillation in Nuclear Piping

Thermal oscillation is a well-known phenomenon in nuclear power plant piping systems, particularly in steam generator feedwater lines and reactor coolant loops. When hot and cold fluid streams mix at a tee junction, the resulting turbulent mixing zone exhibits rapid temperature fluctuations that can cause thermal fatigue cracking. This is a critical safety concern because:

• Thermal fatigue cracking can lead to loss of containment and radioactive release.
• The phenomenon is difficult to predict because it involves complex interactions between fluid dynamics, heat transfer, and material response.
• Traditional orthogonal tee geometries are known to exacerbate thermal oscillation by creating large, persistent turbulent mixing zones.

The momentum ratio (the ratio of the momentum flux of the injected stream to that of the main stream) is a key parameter governing the severity of thermal oscillation. The study examines a momentum ratio of 0.43, which is representative of typical nuclear power plant operating conditions.

Research Methodology

The authors employed computational fluid dynamics (CFD) simulation to analyze the thermal mixing behavior in both orthogonal and oblique-insertion tee structures. The numerical model captures:

• Turbulent mixing of hot and cold fluid streams.
• Temperature field evolution along the tee and downstream pipe.
• Wall temperature fluctuations that drive thermal fatigue.

The comparison between orthogonal and oblique-insertion tee structures is conducted under identical boundary conditions to isolate the effect of tee geometry on thermal oscillation behavior.

Key Findings

Wall Temperature Distribution

Under typical jet flow conditions (momentum ratio = 0.43):

• The wall jet causes the most severe temperature fluctuation at the top of the pipe. This is because the injected fluid stream impinges on the opposite wall, creating a region of intense mixing and temperature variation.
• The circumferential temperature distribution is non-uniform, with the highest temperatures occurring near the impingement point and the lowest temperatures in the wake region of the jet.

This finding is consistent with established understanding of jet impingement in pipe junctions, but the quantification of the temperature fluctuation amplitude is valuable for fatigue analysis.

Effect of Oblique-Insertion Tee Structure

The oblique-insertion tee structure demonstrates several advantages over the conventional orthogonal tee:

1. Accelerated hot-cold fluid mixing. The oblique angle of the branch pipe promotes earlier and more thorough mixing of the two fluid streams, reducing the persistence of distinct temperature zones.
2. Shortened turbulent mixing length. The distance over which the temperature field stabilizes is reduced, meaning that the region susceptible to thermal oscillation is smaller.
3. Reduced thermal oscillation impact zone. Because the mixing is more rapid and the turbulent zone is shorter, the area of pipe wall exposed to significant temperature fluctuations is reduced.
4. Extended thermal fatigue life. The reduction in the thermal oscillation impact zone directly translates to an increase in the fatigue life of the tee and adjacent pipe sections.

Comparison of Tee Structures

The oblique-insertion tee offers superior thermal performance but at the cost of increased manufacturing complexity and potentially higher material and fabrication costs. The decision to adopt this geometry must be made in the context of the overall risk-benefit analysis for the nuclear power plant.

Engineering Practice Insights

Having worked extensively with nuclear piping systems and thermal fatigue analysis, I offer the following observations:

1. Thermal oscillation is a design-for-fabrication issue, not merely an operational issue. The geometry of the tee directly determines the severity of thermal oscillation. Designing the tee geometry to minimize thermal fatigue is more effective than attempting to mitigate the phenomenon through operational adjustments alone.
2. The oblique-insertion tee is a proactive design solution. Rather than accepting thermal fatigue as an inherent consequence of tee junctions, the oblique geometry fundamentally alters the flow pattern to reduce the driving mechanism for thermal oscillation. This is a more robust approach than relying on increased wall thickness or more fatigue-resistant materials.
3. The momentum ratio of 0.43 is representative but not universal. Different nuclear power plant designs and operating conditions will have different momentum ratios. The applicability of the oblique-insertion tee should be verified for each specific application through site-specific CFD analysis.
4. The manufacturing implications of oblique-insertion tees must be carefully evaluated. Fabricating oblique tees to nuclear-grade quality requires specialized forming and welding techniques. The weld quality, particularly at the branch-connection weld, must be verified through rigorous non-destructive testing (NDT) including radiographic testing (RT), ultrasonic testing (UT), and potentially phased array ultrasonic testing (PAUT).
5. The shortened turbulent mixing length has implications for instrumentation placement. If temperature monitoring sensors are placed based on orthogonal tee geometry assumptions, they may not capture the actual temperature field in an oblique-insertion tee. Sensor placement must be re-evaluated for the modified geometry.

Key Technical Takeaways

• Thermal oscillation in nuclear power plant tee junctions is driven by turbulent mixing of hot and cold fluid streams and is most severe at the wall impingement point.
• Oblique-insertion tee structures reduce thermal oscillation by accelerating fluid mixing and shortening the turbulent mixing length.
• The reduction in thermal oscillation impact zone directly extends the thermal fatigue life of the tee and adjacent pipe sections.
• The momentum ratio is a critical parameter governing thermal oscillation severity, with a value of 0.43 being representative of typical nuclear power plant conditions.
• Design-for-fabrication approaches that modify tee geometry are more effective than material or thickness-based mitigation strategies.

Study Implications and Outlook

This research has significant implications for the design of nuclear power plant piping systems, particularly for new plant designs where tee geometries can be optimized from the outset. For existing plants, retrofitting oblique-insertion tees may be considered for locations where thermal fatigue cracking has been observed or predicted. Future research should extend the CFD analysis to include coupled thermo-mechanical simulation to predict fatigue life more accurately, and should also investigate the effects of different oblique angles on thermal oscillation behavior. Additionally, experimental validation of the CFD predictions through thermal cycling tests on scale models would strengthen the confidence in applying these findings to full-scale nuclear power plant piping systems. The integration of this geometry optimization with digital twin technology for real-time monitoring of thermal oscillation could provide a powerful tool for predictive maintenance in nuclear power plants.