Liquid Handling of the JUNO Experiment

TL;DR

The paper presents the design, implementation, and commissioning of JUNO's Filling, Overflow, and Circulation (FOC) system, a complex subsystem tasked with safely handling 20 kton-scale water and ultra-pure liquid scintillator (LS) while preserving radiopurity for 20 years of operation. It details a four-task approach—initial water filling, LS exchange, LS overflow management, and online LS circulation—under tight background budgets and mechanical safety constraints. The system integrates ultra-pure water purification, RS radon control, multi-stage cleanliness, and a PLC+EPICS automation stack to achieve precise liquid-level control (within $\pm 2$ cm) and flow regulation (within $\pm 0.5\%$), while keeping $^{222}$Rn below 1 mBq/m$^3$ and $^{238}$U/$^{232}$Th below $10^{-16}$ g/g during filling. The successful filling campaigns (water in 45 days; LS in 200 days) and demonstrated long-term stability establish FOC as a foundational capability for JUNO’s physics program, with prospects for online LS circulation to further enhance LS radiopurity for future upgrades and double-beta decay studies.

Abstract

The Filling, Overflow, and Circulation (FOC) system is a critical subsystem of the Jiangmen Underground Neutrino Observatory (JUNO), responsible for the safe handling of the Liquid Scintillator (LS) and water throughout the detector's commissioning and operational lifetime. This paper details the design and operation of the FOC system, which accomplished the filling of the world's largest LS detector--taking 45 days for water (6.4*10^4 m^3) and 200 days for LS (2.3*10^4 m^3). Throughout water filling, the liquid level difference between the Central Detector and Water Pool was rigorously maintained within safety limits. During LS filling, level control achieved +/-2 cm precision with flow regulation within +/-0.5% of setpoints. An automated control system based on Programmable Logic Controllers and the Experimental Physics and Industrial Control System framework ensured reliable operation. The system preserved LS radiopurity, maintaining 222Rn below 1 mBq/m^3 during filling and achieving 238U/232Th concentrations below 10^-16 g/g. The successful commissioning and operation of the FOC system have established it as an indispensable foundation for the stable long-term operation of the JUNO detector.

Figures (9)

• Figure 1: Schematic of the FOC system layout
• Figure 2: FOC automatic control system
• Figure 3: Limit of the height difference ($\Delta H$) between the CD and WP during water filling FEA
• Figure 4: Three filling stages of water filling
• Figure 5: Level and flow changes during the water filling process