Service-life analysis of nuclear reactor elements under high-frequency random loading

Abstract

Service life extension for working equipment and nominal service life extension (to 60 or more years) for nuclear reactors under construction make it even more urgent to evaluate the effect of the high-frequency component of loading (vibrations) on the long-time serviceability of structural elements. The main methodological problem of the present evaluations is to reduce the random process of vibrational loading to a harmonic form using the criterion of equivalence of fatigue damage to structural material. On the basis of experimental studies of 08Kh18N10T austenitic steel samples, an algorithm is proposed for systematizing the random process of vibrational loading to a harmonic form taking account of the slope of the fatigue curve in the high-cycle loading range.Vibrational loads, which arise unavoidably in structural elements in coolant flow as well as rotating mechanisms, can limit the nominal service life but cannot be fully taken into account at the design stage. The ASME code contains only a recommendation to minimize vibrations during setup and adjustment operations. The norms [1] recommend that at the design stage the natural frequencies of a structural element be displaced from the frequencies of forced oscillations by 30% for the first three frequencies in increasing order and by 10% for the subsequent frequencies. Since this recommendation cannot always be followed and the problem of structural vibrations in coolant flow does not always have an exact solution, the norms [1] regulate the methodological approach to taking account of the experimental vibrational loading when evaluating the fatigue strength of structures. The normative method based on the use the amplitude-frequency characteristics of a narrow- (Fig. 1a) or wide-band ( Fig. 1b) spectrum of the vibrational process [2-5] is still used today, though modern computational means for schematizing random processes make it possible to use direct methods of analyzing damage done to metal by high-frequency loading cycles.Practical experience in evaluating the effect of vibrations has been gained largely because the instrumental means for analyzing vibrational processes make it possible to obtain directly the amplitude-frequency characteristic which is used to confirm fatigue strength. However, the method used overstates the service life of structures subject to vibrations, and for the wide-band spectrum or white noise it requires conservative solutions for calculating fatigue strength (for example, the maximum amplitude of the vibrational load-maximum frequency) under conditions where the vibration process is superimposed on low-frequency cycling of the working stresses.For reactor core elements operating in the coolant flow and exposed to neutron radiation, the vibratory loading can determine their service life. Because the main structural material used in core elements is corrosion-resistant austenitic steel fatigue tests were performed on 08Kh18N10T steel under loading with two frequencies (Figs. 2, 3). The spectral parameters of high-frequency ra...