O.K. Chopra

Using artificial neural networks to predict the fatigue life of carbon and low-alloy steels

Abstract The ASME Boiler and Pressure Vessel Code contains rules for the construction of nuclear power plant components. Figures I-9.1 through I-9.6 of Appendix I to Section III of the Code specify fatigue design curves for structural materials. However, the effects of light water reactor (LWR) coolant environments are not explicitly addressed by the Code design curves. Recent test data indicate significant decreases in the fatigue lives of carbon and low-alloy steels in LWR environments when five conditions are satisfied simultaneously. When applied strain range, temperature, dissolved oxygen in the water, and sulfur content of the steel are above a minimum threshold level, and the loading strain rate is below a threshold value, environmentally assisted fatigue occurs. For this study, a data base of 1036 fatigue tests was used to train an artificial neural network (ANN). Once the optimal ANN was designed, ANN were trained and used to predict fatigue life for specified sets of loading and environmental conditions. By finding patterns and trends in the data, the ANN can find the fatigue life for any set of conditions. Artificial neural networks show great potential for predicting environmentally assisted corrosion. Their main benefits are that the fit of the data is based purely on data and not on preconceptions and that the network can interpolate effects by learning trends and patterns when data are not available.

Corrosion and compatibility considerations of liquid metals for fusion reactor applications

Abstract The current understanding of corrosion and environmental effects on the integrity and mechanical properties of structural materials used with liquid metals in fusion reactors is reviewed. Corrosion processes in liquid lithium systems are examined and their influence on material degradation is discussed. Compatibility considerations that might arise from use of molten lead, bismuth, lead-bismuth, or lead-lithium are reviewed relative to the possible use of these liquids in fusion reactors.

Influence of temperature and lithium purity on corrosion of ferrous alloys in a flowing lithium environment

Abstract Corrosion data have been obtained on ferritic HT-9 and Fe-9Cr-lMo steel and austenitic Type 316 stainless steel in a flowing lithium environment at temperatures between 372 and 538°C. The corrosion behavior is evaluated by measurements of weight loss as a function of time and temperature. A metallographic characterization of materials exposed to a flowing lithium environment is presented.

Compatibility of ferrous alloys in a forced circulation Pb-17Li system

Abstract Corrosion data have been obtained on ferritic HT-9 and Fe-9Cr-1Mo steel and austenitic Type 316 stainless steel in a flowing Pb-17 at.% Li environment at temperatures between 371 and 482°C. The corrosion behavior is evaluated by measurement of weight loss as a function of time and temperature. Metallographic examination of the materials exposed to the flowing Pb-17Li environment is presented.

Low-cycle fatigue of piping and pressure vessel steels in LWR environments

Abstract The ASME Boiler and Pressure Vessel Code provides rules for the construction of nuclear power plant components. Figure I-90 of Appendix I to Section III of the Code specifies fatigue design curves for structural materials. Although effects of reactor coolant environments are not explicitly addressed by the design curves, test data suggest that the Code fatigue curves may not always be adequate in coolant environments. This paper reports the results of recent fatigue tests that examine the effects of steel type, strain rate, dissolved oxygen level, strain range, loading waveform, and surface morphology on the fatigue life of carbon and low-alloy steels in light water reactor environments.

Compatibility of ferritic steels in forced circulation lithium and Pb17Li systems

Abstract Corrosion data are presented for various ferritic steels in flowing lithium and Pb17Li environments at temperatures between 371 and 538°C. The corrosion behavior is evaluated by measuring weight loss as a function of time and temperature. The influence of chemical interactions between nitrogen and alloy elements on the corrosion behavior in lithium is discussed.

Corrosion behavior of vanadium alloys in flowing lithium

Corrosion data are presented for several vanadium alloys exposed to flowing lithium at 427, 482, and 538/sup 0/C. The corrosion behavior is evaluated by weight change measurements. Metallographic results and data on the nonmetallic element transfer in lithium-exposed specimens are also presented. The influence of alloy composition and exposure conditions on the corrosion behavior of vanadium alloys is discussed. 6 refs., 9 figs., 2 tabs.

Compatibility of materials for use in liquid-metal blankets of fusion reactors

A review of corrosion and environmental effects on the mechanical properties of austenitic and ferritic steels for use with liquid metals in fusion reactors is presented. The mechanisms and kinetics of the corrosion processes in liquid lithium and Pb-17Li systems are examined and their influence on degradation of structural material is discussed. Requirements for additional data are identified.

Statistical models for estimating fatigue strain-life behavior of pressure boundary materials in light water reactor environments

The existing fatigue strain versus life (S-N) data for materials used in nuclear power plant components have been compiled and categorized according to material, loading and environmental conditions. Statistical models have been developed for estimating the effects of the various service conditions on the fatigue life of these materials. The results have been used to estimate the probability of initiating a fatigue crack. Data in the literature were reviewed to evaluate the effects of the size, geometry and surface finish of a component on its fatigue life. Fatigue S-N curves for components have been determined by adjusting the probability distribution curves of smooth test specimens for the effect of mean stress and then applying design margins to account for the uncertainties that arise because of component size, geometry and surface finish. The significance of the effect of the environment on the current code design curve and on the proposed interim design curves published in NUREG/CR-5999 is discussed. Estimations of the probability of fatigue cracking in sample components from boiling water reactors and pressurized water reactors are presented.

Carbon and nitrogen transfer in Fe-9 Cr-Mo ferritic steels exposed to a sodium environment

Kinetics of carburization/decarburization of five commercial and two high-purity Fe-9 Cr-1 to 2.5 Mo feiritic steels have been studied in a sodium environment at temperatures between 773 and 973 K. Carbon concentration-distance profiles were obtained as a function of sodium-exposure time, temperature, and carbon in sodium and the carburization/ decarburization rate constants were evaluated. The results show that the Fe-9 Cr-Mo steels are more resistant to carbon transfer than the low-alloy Fe-214 Cr-1 Mo steel. The conditions of temperature and carbon concentration in sodium for carburization or decarburization of Fe—9 Cr—Mo steels are quite similar to those for stainless steels. However, the extent of carbon transfer in Fe-9 Cr—Mo steels is lower than that of the stainless steels. The composition and carbide structure of the steel had a significant effect on the carburization/decarburization behavior. Fe-9 Cr-Mo steels that decarburize to very low carbon concentrations either contain M2X phase or have M6C as the only stable carbide.