Experimental Study on Cavitation of a Liquid Lithium Jet for International Fusion Materials Irradiation Facility

Abstract

A liquid Li jet flowing at 15 m/s under a high vacuum of 10−3 Pa is intended to serve as a beam target (Li target) in the planned International Fusion Materials Irradiation Facility (IFMIF). The engineering validation and engineering design activities (EVEDA) for the IFMIF are being implemented under the broader approach (BA) agreement. As a major activity of the Li target facility, the EVEDA Li test loop (ELTL) was constructed by the Japan Atomic Energy Agency. A stable Li target under the IFMIF conditions (Li temperature: 523.15 K, velocity: 15 m/s, and vacuum pressure: 10−3 Pa) was demonstrated using ELTL. This study focuses on a cavitationlike acoustic noise detected in a downstream conduit where the Li target flowed under vacuum conditions. This noise was investigated using acoustic-emission (AE) sensors installed at eight locations via acoustic wave guides. The sound intensity of the acoustic noise was examined against the cavitation number of the Li target. In addition, two types of frequency analysis, namely, fast Fourier transform (FFT) and continuous wavelet transform (CWT), were performed to characterize the acoustic noise. Owing to the acoustic noise's intermittency, high frequency, and the dependence on cavitation number, we conclude that this acoustic noise is generated when cavitation bubbles collapse and/or the structural material of the pipe is cracked because of the collapse of cavitation bubbles (cavitation pitting). The location of the cavitation was fundamental for presuming the mechanism. In this study, the propagation of acoustic waves among AE sensors placed at three locations was used to localize the cavitation and a method to determine the location of cavitation was formulated. As a result, we found that cavitation occurred only in a narrow area where the Li target impinged on the downstream conduit; therefore, we concluded that this cavitation was induced by the impingement. The design of the downstream conduit of the IFMIF Li target facility should be tackled in future based on information obtained in this study.