The Solubility and Diffusivity of Helium in Mercury with Respect to Applications at the Spallation Neutron Source

Models for solubility of noble gases in liquid metals are reviewed in detail and evaluated for the combination of mercury and helium for applications at the Spallation Nuetron Source (SNS) at Oak Ridge National Laboratory (ORNL). Gas solubility in mercury is acknowledged to be very low; therefore, mercury has been used in ASTM standard methods as a blocking media for gas solubility studies in organic fluids and water. Models from physical chemistry predict a Henry coefficient for helium in mercury near 3.9x10¹⁵ Pa-molHg/molHe, but the models have large uncertainties and are not verified with data. An experiment is designed that bounds the solubility of helium in mercury to values below 1.0x10^-8 molHe/molHg at 101.3 kPa, which is below values previously measurable. The engineering application that motivated this study was the desire to inject 10 to 15 micron-radius helium bubbles in the mercury target of the SNS to reduce pressure spikes that accompany the beam energy deposition. While the experiment bounds the solubility to values low enough to support system engineering for the SNS application, it does not allow confirmation of the theoretical solubility with low uncertainty. However, methods to measure the solubility value may be derived from the techniques employed in this study.