Effect of proton beam profile on stress in JSNS target vessel

Abstract

A cross-flow type (CFT) mercury target with flow guide blades has been developed for JSNS that suppresses the generation of a stagnant flow region near the beam window where the peak power density is generated due to the spallation reaction. In addition, a flat type beam window has been applied to the CFT target that suppresses dynamic stress caused by pressure waves, that have been estimated with an elastic model for mercury. Recent experimental results obtained by using a proton beam incident to mercury targets suggested that using a cut-off pressure model for mercury would be appropriate for predicting a dynamic stress behavior in a target vessel. Dynamic stress analyses were carried out with the cutoff pressure model in which the negative pressure less than −0.15 MPa could not be sustained. The dynamic stress generated in the flat beam window became much larger than that in a semi-cylindrical type window as a result of using the cut-off pressure model. Regardless, even the stress generated in the semi-cylindrical type beam window exceeded the allowable stress of SS316L under the peak power density of 668 MW/m 3. In order to decrease the dynamic stress in the semi-cylindrical beam window, the incident proton beam was defocused to reduce the peak power density down to 218 MW/m 3. Although the dynamic stress could be suppressed to less than the allowable stress, the high power density generated on the end of the flow guide blades due to defocus of the proton beam caused high thermal stress exceeding the allowable stress. Several shapes of blade ends were studied analytically to decrease the thermal stress, that did not affect the mercury flow distribution. A simple thin-end blade showed low thermal stress below the allowable stress.