Thermal Fatigue in Nuclear Power Plants
Description and Experience
Thermal fatigue is fatigue failure or cracking produced by fluctuating thermal stresses [1, 2]. In general, the conditions for thermal fatigue are present in elevated temperature equipment, where thermal gradients may be present due to temperature changes. Thermal stresses result when a change in dimension of a member as the result of a temperature change is prevented by constraint [1, 2]. This constraint can be caused by mechanical means (such as piping supports) or simply by the adjacent material which may be at another temperature in a thick component.
Two cases of thermal fatigue are also discussed under the topic of corrosion fatigue: boiling water reactor (BWR) and pressurized water reactor (PWR) feedwater nozzle cracking. BWR feedwater nozzle thermal fatigue cracks have formed due to relatively low temperature feedwater bypassing the thermal sleeve/sparger configuration in the nozzle to mix with the higher temperature reactor water and cause thermal cycling at the nozzle corner and bore locations [3, 4]. Stainless steel cladding on the low alloy steel nozzles amplified this effect due to differences in the coefficient of thermal expansion for the two metals .
Low cycle thermal fatigue can be categorized as a series of “thermal shocks”, whereas high cycle (higher frequency) fatigue is sometimes known as “thermal striping” . Fast breeder reactor components are subject to thermal striping in completely mixed streams of sodium at different temperatures impinge on a metal surface, as in a liquid sodium mixing tee [7, 8].
Components such as nuclear pressure vessels, turbine blades and heat exchanger tubing can undergo cyclic thermal fluxes, which give rise to thermal stresses and fatigue . For example, pressurized thermal shock of PWR reactor vessels, caused by the introduction of cold safety injection water into a relatively hot reactor vessel, is a low cycle event which can cause significant cracking in some postulated cases . More commonly, large diameter steel pipe, reinforced by stiffening rings and saddle supports, can be subject to thermal fatigue due to system startup and shutdown .
Material Selection, Treatment and Testing
Since thermal fatigue results from thermal mixing and stratification of fluids and from constraints during system heating or cooling transients, it is usually a design problem rather than a materials issue. However, materials may be chosen or treated in order to ameliorate the thermal fatigue problem.
For example, austenitic stainless steel is quite sensitive to thermal fatigue because of relatively low thermal conductivity and high thermal expansion . Other guidelines to reduce susceptibility to thermal fatigue include the use of fatigue resistant material which is not notch sensitive, as discussed under other fatigue damage mechanisms.
Operation and Design Limitations
Three of the categories in which thermal stresses may be classified are: through-wall thermal gradients and stresses in thick-walled components, top-to-bottom thermal gradients and stresses due to flow stratification in piping, etc., and global thermal stresses induced during heating and cooling of equipment which is constrained by supports, etc.
Through-wall thermal stresses can be caused by rapid heatup and cooldown of thick-walled reactor vessels; limits on heatup/cooldown rates are required . Heavy-walled flanges and valves may also be susceptible to such thermal gradients when subjected to rapid temperature changes. Usually, components must be on the order of at least 1/2″ to 2″ thick for through-wall stresses to be significant, but stiffening rings and saddles on piping can add constraint and cause significant thermal stresses in even thinner pipes and tees [1, 11]. Stainless steel cladding exacerbates thermal fatigue problems due to through-wall thermal gradients, because of the different thermal properties of stainless steel and ferritic steels . Temperature gradients may be reduced by minimizing temperature differences between hot and cold fluids and by mixing them away from metal walls.
Low flow conditions, as well as turbulent flow, should be avoided to mitigate top-to-bottom flow stratification and rapid temperature cycling, for piping and vessels. In general, the rules for mitigating fatigue, such as avoiding aggressive environments, stress concentrations, and surface roughness or degradation, should be followed when possible.
Mitigating Actions and Remedies
The design, operation and materials guidelines stated above should be followed. Mixing baffles may be added, and spargers and thermal sleeves may be redesigned to avoid incomplete or improper mixing. Fatigue resistant materials (eg., low sulfur content) should be specified, and environmental controls may be implemented to limit corrosive attack.
Periodic inspection, especially by surface methods such as liquid penetrant or magnetic particle testing, is recommended at locations where thermal fatigue is suspected. Quantification of cycles and loads is a key input for the fracture mechanics analysis employed to evaluate fixes and to predict remaining life for equipment.
- Dieter, Jr., G. E., Mechanical Metallurqy, McGraw-Hill, New York, c. 1961, pp. 333 – 334.
- Sehitoglu, H. and Morrow, J-D., “Characterization of Thermo-Mechanical Fatigue”, 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. 93 – 110.
- Szabo, B. A., et al, “An Analysis of Ductile Crack Extension in BWR Feedwater Nozzles”, EPRI NP-1311, Jan. 1980.
- Watanabe, H., “BWR Feedwater Nozzle/Sparger Final Report”, General Electric Company, NEDO 21821-A, Feb. 1980.
- 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.
- 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.
- Pradel, P., “The Main Objectives of Thermal Striping Studies in Progress for French LMFBR Thermal Hydraulic and Design Aspects”, Ibid., p. 143 – 146.
- Clayton, A. M. and Irvine, N. M., “Structural Assessment Techniques for Thermal Striping”, Ibid., pp. 147 – 152.
- 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.
- 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.
- 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, and Failure Analysis, Dec. 2 – 6, 1985, Salt Lake City, Utah, ASM, c. 1986, pp. 227 – 236.
- “LWR Structural Materials Degradation Mechanisms – Preliminary Assessment of BWR Internals Life Limiting Concerns”, Structural Integrity Associates Draft Report, EPRI RP2643-5, Feb. 1986.
- Copeland, J. F., “Application of Fatigue Crack Growth Models to Data for BWR Feedwater Nozzle Evaluations”, Structural Integrity Associates Report
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