Theoretical Study of Noble Gas Bubble Behavior in Mercury

Spallation Neutron Source (SNS) uses heavy liquid metal (mercury) as the target material for high power proton beam bombardment to produce neutrons for scientific research. Though the liquid target is not subject to material degradation due to radiation damage, the stainless steel pressure boundary confining the liquid metal flow is damaged by radiation and cavitation erosion induced by the thermal shock waves caused by the deposition of the incoming high-power proton beam. This puts a limit on the lifetime of the target holder.

To mitigate the cavitation-induced erosion damage to the target holder, it is aimed to introduce microbubbles to the target mercury with expected nominal size of 30μm diameter and volume fraction of 0.5%, which can substantially lower the pressure amplitude resulting from the proton beam deposition due to the added compressibility.

The noble gas bubble behavior in mercury is studied in this thesis. The acoustics of the two-phase mixture under the perturbation due to beam deposition, specifically acoustic streaming, is simulated in a bubbly two-phase flow for the first time in the literature. The numerical simulation shows the magnitude of obtained streaming velocity is much smaller than the pumped mercury flow in the target and will not cause distortion to flow patterns and heat transfer in the target.

Single bubble dynamics, which includes noble gas solubility evaluation in mercury and the bubble radius evolution under the effect of mass diffusion across the bubble wall, is also simulated. Two different profiles of bubble size distribution are studied. The solubility evaluation provides a theoretical basis for the inert gas solubility measurement experiments. The mass diffusion induced bubble behavior simulation based on the solubility results indicates that xenon bubbles creates a more viable and stable bubble population in mercury than helium bubbles, which means xenon is a possible better candidate to add compressibility to pure mercury in the SNS target.