Thermally-driven scintillator flow in the SNO+ neutrino detector

TL;DR

This study uses flow simulations to explain unexpected scintillator motion in the SNO+ detector, showing heat transfer-induced buoyant flow and internal gravity waves can transport contaminants, with stratification mitigating mixing effects.

Contribution

The paper demonstrates how thermal buoyancy and internal gravity waves influence fluid motion and contaminant transport in the SNO+ detector, providing insights into flow dynamics and mitigation strategies.

Findings

Heat transfer induces buoyant flow causing contaminant transport.

Internal gravity waves occur under thermal stratification.

Thermal stratification can reduce mixing from heat fluxes.

Abstract

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.