Damage inspection of the first mercury target vessel of JSNS

The liquid mercury target system for the Japan Spallation Neutron Source (JSNS) began operating in May 2008. A mercury target vessel composed of type 316L stainless steel suffers radiation damage in the proton and neutron environment. In addition to this damage, the inner wall of the target vessel in contact with mercury is damaged by cavitation-induced erosion, which is caused by rarefied pressure waves. The target vessel was replaced with a new target in November 2011, because the pneumatic bellow type flange was damaged during the Great East Japan Earthquake, that occurred on March 11, 2011. Before replacing the target, 50 mm diameter disk specimens were cut from the beam window of the target vessel in order to investigate the cavitation damage inside the target vessel and to evaluate the change in the mechanical properties due to radiation damage. A video camera was used to inspect the inside of the target vessel and no flow induced erosion damage was detected on the flow guide by visual inspection. Cavitation damage from the proton pulse was concentrated at the center and approximately 15 mm from the center of the beam window. In addition, the depth of the cavitation damage was quantitatively measured using a laser profilometer and a metallurgical surface replica. Based on the detailed measurements, it was concluded that the eroded damage depth of the beam window was 250 μm after 475 MW h of operation.