Determining the frequency dependence of ultrasonic attenuation in Zr-2.5%Nb

The cold - worked Zr-2.5%Nb alloy is used for the pressure tubes of CANDU nuclear reactors. During the service life in reactor, diffusion of hydrogen and/or deuterium in the pressure tubes wall will occur. Below a certain temperature, a stable hydride will form, as a brittle phase and could lead to catastrophic failure. For this reason, it is very important to be able to investigate the hydrogen effect on the elastic properties of zirconium alloys. In the present paper, a non-destructive testing technique known as ultrasonic spectral analysis is used. By means of this technique it was determined the frequency dependence of ultrasonic attenuation from the amplitude spectra of any two consecutive backer emission-reception (pulse-echo technique). In materials examination, a narrow response in time is required for a good definition of the pulse> This is reflected in the frequency response which has a relatively broad band. This algorithm works in the frequency domain by determining the frequency dependence of the attenuation. The measurement method was applied to Zr-2.5%Nb samples cut from the same pressure tube and hydrided at different hydrogen concentrations. It was found that the ultrasonic attenuation increases with ultrasonic frequency increase. At same hydrogen concentration different behaviour of ultrasonic attenuation can be observed , between fast hydrided and slow-rate hydrided samples. In the case of fast hydrided samples by metal-hydrogen plasma interaction process the attenuation is lower than in slow-rate hydrided samples. A possible explanation of this different behaviour could be the influence of the high level internal stresses developed by the fast hydrogen precipitation. Therefore, the longitudinal wave attenuation in zirconium alloys decreases with mechanical stresses increasing. We plan to investigate in the future the hydrogen effect on the elastic properties of zirconium alloys. (authors)