Introduction to Thermal Fatigue

Keywords: Fluctuating Thermal Stresses; Corrosion Fatigue; Low Cycle Thermal Fatigue; Thermal Shocks; Fracture Mechanics

What is Thermal Fatigue?

Thermal fatigue causes costly unplanned outages in power generation facilities, with feedwater nozzle cracking alone resulting in extended shutdowns and expensive maintenance repairs. As nuclear and fossil plants age beyond their original design life, understanding and mitigating this degradation mechanism becomes critical for maintaining safe, reliable operations while managing regulatory compliance and maintenance budgets. Thermal fatigue is metallurgical crack growth caused by fluctuating thermal stresses [1, 2]. When temperature changes produce dimensional changes that are constrained—either mechanically (by piping supports) or by adjacent material at different temperatures—thermal stresses develop. Under cyclic loading, these stresses cause progressive microstructural damage including grain boundary cracking, void formation, and fatigue crack propagation that can ultimately lead to component failure.

Thermal fatigue manifests in two distinct regimes: low cycle thermal fatigue (thermal shocks) and high cycle thermal fatigue (thermal striping). In thermal striping, high-frequency temperature fluctuations occur when incompletely mixed fluid streams at different temperatures impinge on metal surfaces, as observed in liquid sodium mixing tees in fast breeder reactors [6, 7, 8].

Critical Applications in Power Generation

Nuclear Power Plant Components

Thermal fatigue is a primary aging mechanism in both boiling water reactor (BWR) and pressurized water reactor (PWR) systems, where it is classified under corrosion fatigue.

BWR Feedwater Nozzles: Cracking occurs when relatively cool feedwater bypasses the thermal sleeve/sparger assembly and mixes with hot reactor water, inducing thermal cycling at the nozzle corner and bore [3, 4]. The problem is amplified by stainless steel cladding on low alloy steel nozzles due to differences in thermal expansion coefficients between the two materials [4].

PWR Feedwater Systems: Similar thermal mixing phenomena affect PWR feedwater line components, leading to documented cracking in nozzles and piping [5]. PWR reactor vessels face low-cycle thermal shock events when cold safety injection water enters a hot vessel during emergency cooling scenarios, potentially causing significant cracking under certain postulated accident conditions 

Other Affected Systems

Components throughout power generation and process industries experience thermal fatigue damage, including:

• Pressure vessels subjected to cyclic thermal fluxes [9]
• Turbine blades under operational cycling
• Heat exchanger tubing
• Large diameter piping with stiffening rings and saddle supports during system startup and shutdown 

Should thermal fatigue result from thermal mixing or from constraints during system heating or cooling transients, the challenge is on the design engineer to account for thermal fatigue. However, proper material selection is required to minimize thermal fatigue. For example, austenitic stainless steel is quite sensitive to thermal fatigue because of its relatively low thermal conductivity and high thermal expansion [1].

Material Considerations

Material selection significantly influences thermal fatigue susceptibility. Austenitic stainless steel is particularly vulnerable due to its low thermal conductivity combined with high thermal expansion coefficient [1]. This combination creates larger thermal gradients and higher induced stresses compared to ferritic steels under identical thermal loading conditions.

Stainless steel cladding on ferritic base metals exacerbates thermal fatigue problems through two mechanisms: the material property mismatch described above, and the creation of a bi-metallic interface with differing stress distributions under thermal cycling.

Design and Operational Considerations

Thermal stresses fall into three primary categories, each requiring specific design attention:

1. Through-Wall Thermal Gradients in Thick Components

Rapid heating and cooling of thick-walled components—reactor vessels, heavy flanges, and large valves—creates through-wall temperature gradients and corresponding stress distributions. Typically, components must exceed 1/2″ to 2″ thickness before through-wall stresses become significant, though stiffening rings and saddles can add constraint that induces significant thermal stresses in thinner sections [1, 11]. Design controls include limiting heatup and cooldown rates and avoiding rapid temperature transients that exceed material stress capabilities.

2. Stratification-Induced Thermal Gradients

Flow stratification in horizontal piping creates top-to-bottom thermal gradients when fluids of different temperatures separate rather than mix. This condition produces cyclic bending stresses in the pipe wall as the temperature distribution shifts during transient operations.

3. Global Thermal Stresses from Constrained Components

Piping systems, vessels, and other equipment constrained by rigid supports or connecting components develop global thermal stresses during heating and cooling. The constraint prevents free thermal expansion, converting thermal strain into mechanical stress.

Gradient Mitigation Strategies

Effective thermal gradient reduction requires minimizing temperature differences between hot and cold fluid streams and ensuring adequate mixing occurs away from metal walls. Thermal sleeves, spargers, and flow distribution devices serve this purpose when properly designed and maintained.

Mitigation and Remediation Strategies

Design Phase Solutions

Finite element analysis (FEA) identifies critical stress concentrations and enables design optimization to minimize thermal fatigue damage. Detailed stress analysis should address all three thermal stress categories during the design phase.

Operational Controls

Operating procedures should include:

• Adherence to established heatup and cooldown rate limits
• Control of mixing conditions to avoid direct impingement on metal surfaces
• Monitoring and documentation of thermal cycles for fatigue tracking

Inspection and Life Assessment

Periodic inspection using surface examination methods—liquid penetrant testing or magnetic particle inspection—should target locations where thermal fatigue is suspected based on stress analysis or operational history. Quantification of thermal cycles and stress magnitudes provides essential input for fracture mechanics analysis. This analysis evaluates repair strategies and predicts remaining component life, supporting informed decisions about continued operation, repair, or replacement. Proper material selection, geometry optimization, and operational limit establishment during design prevent many thermal fatigue issues before they occur.

References

1. Szabo, B. A., et al, “An Analysis of Ductile Crack Extension in BWR Feedwater Nozzles”, EPRI NP-1311, Jan. 1980.
2. Watanabe, H., “BWR Feedwater Nozzle/Sparger Final Report”, General Electric Company, NEDO 21821-A, Feb. 1980.
3. Enrietto, J. F., Bamford, W. H., and White, D. H.,” Preliminary Investigation of PWR Feedwater Line Cracking”, Int. Journal of Pressure Vessels and Piping, Vol. 9, pp. 421- 443.
4. Bhandari, S. K., “Thermal Fatigue-Thermal Striping”, Fracture, Fatigue, and Advanced Mechanics, Proceedings of the 1985 Pressure Vessels and Piping Conference, PVP-Vol. 98-8, ASME, p. 135.
5. Pradel, P., “The Main Objectives of Thermal Striping Studies in Progress for French LMFBR Thermal Hydraulic and Design Aspects”, Ibid., p. 143 – 146.
6. Clayton, A. M. and Irvine, N. M., “Structural Assessment Techniques for Thermal Striping”, Ibid., pp. 147 – 152.
7. Morrow, D. L., “Component Simulations of a Pressure Vessel Subject to Thermal Fluxes”, Thermal and Environmental Effects in Fatigue: Research-Design Interface Presented at 4th National Congress on Pressure Vessel and Piping Technology, ASME, Portland, Oregon, June 19 – 24, 1983,PVP-Vol. 71, pp. 59 – 73.
8. Kussmaul, K. and Sauter, A., “Application of Ductile Fracture Mechanics to Large Scale Experimental Simulation and Analyses for Pressurized Thermal Shock Behavior of LWR RPV’s”, The Mechanism of Fracture, Presented at the International Conference and Exposition on Fatigue, Corrosion Cracking, Fracture Mechanics, and Failure Analysis, Dec. 2 – 6, 1985, Salt Lake City, Utah, ASM, c. 1986, pp. 75 – 87.
9. O’Donnell, W. J., et al., “Low Cycle Thermal Fatigue and Fracture of Reinforced Piping”, Analyzing Failures – The Problems and Solutions, Presented at the International Conference and Exposition on Fatigue Corrosion Cracking, Fracture Mechanics

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