Cavitation damage in double-walled mercury target vessel

A liquid mercury target is operated as part of a spallation neutron source at the Japan Proton Accelerator Research Complex (J-PARC). The mercury target vessel is made of 316L stainless steel, (3 mm thick) and its beam window is damaged by cavitation due to pressure waves in mercury. To mitigate the pressure waves and the cavitation damage, a double-walled structure with a narrow channel of 2 mm was added to the beam window along with gas microbubbles injection technology. After operation of up to 670 MWh for 1670 h, the beam window of the used target vessel was cut out using an annular cutter to investigate the effect of the double-walled structure on mitigating cavitation damage. Band-like damage distribution due to cavitation was observed on the outer wall that faced the narrow channel, where the maximum pit depth was estimated to be 25 μm. Furthermore, to clarify the mechanisms possibly contributing to the band-like damage distribution, numerical simulations were conducted in terms of flow velocity, gap width, and pressure waves in the narrow channel. The results show that the distribution of accumulated saturation time of negative pressure period obtained from FEM simulation correlated well with the experimentally observed band-like damage distribution.