Thermally-driven scintillator flow in the SNO+ neutrino detector

The SNO+ neutrino detector is an acrylic sphere of radius 6 m filled with liquid scintillator, immersed in a water-filled underground cavern, with a thin vertical neck (radius 0.75 m) extending upwards about 7 m from the sphere to a purified nitrogen cover gas. To explain a period of unexpected motion of the scintillator, time-dependent flow simulations have been performed using OpenFoam. It appears that the motion, inferred from subsequent 24 h-averaged patterns of transient $^{222}\mathrm{Rn}$ contamination introduced during earlier recirculation of scintillator, can be explained as owing to heat transfer through the detector wall that induced buoyant flow in a thin wall boundary layer. This mechanism can result in transport of contaminant, should it be introduced, down the neck to the sphere on a time scale of several hours. If the scintillator happens to be thermally stratified, the same forcing produces internal gravity waves in the spherical flow domain, at the Brunt-V\"{a}is\"{a}l\"{a} frequency. Nevertheless, oscillatory motion being by its nature non-diffusive, simulations confirm that imposing strong thermal stratification over the depth of the neck can mitigate mixing due to transient heat fluxes.