Syuichi Ishikura

R & D on mercury target pitting issue

Abstract A technical issue in mercury spallation target development is pitting, which appears on the target vessel in conjunction with the pressure wave. Pitting has been found in off-beam line test by split Hopkinson pressure bar (SHPB) test as well as in the on-beam test of mercury target at WNR of LANSCE. In SHPB tests pressure in mercury was reduced from 80 to 40, 20 and 10 MPa. Specimens made of type 316 stainless steel were inspected before and after the impacting test at ×450 magnification. Results show that over 20 MPa pitting was generated. But at the lowest pressure in mercury, the number of pits was very limited and substantial damage was small. Substantial damage by pitting is characterized by holes where mass is removed from the wall. Depression itself may not be a substantial damage as long as it is not accompanied by holes.

Bubble dynamics in the thermal shock problem of the liquid metal target

The thermal shock stress in a mercury target vessel was analyzed. The target receives the incident proton beam at an energy of 1 MW with a pulse duration of 1 μs. A negative pressure of 61 MPa was generated following the dispersion of the compression field at 52 MPa which was generated by the proton beam injection. It is expected that cavitation may be caused by the negative pressure. In order to evaluate the cavitation behavior and the following material damage mechanism, a simulation study was carried out using the equation of motion based on bubble dynamics for a single bubble, and fundamental parameter analysis was carried out. It is found that a bubble has a volume expansion of more than 1000 times with a change of the pressure at the window of the target vessel. Consequently wave propagation will be affected. Theoretical consideration was given to the wave motion of propagation in a bubbly liquid. The equation of state in a bubbly liquid can be approximated by polynomials. The diameter of a bubble and the bubble volume fraction inherent in mercury can be estimated if the critical pressure, the sound velocity, and resonance frequency are measured by static and dynamic experiments.