Liquid Handling of the JUNO Experiment

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 (6.410^4 m^3) and 200 days for LS (2.310^4 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.

💡 Research Summary

The Jiangmen Underground Neutrino Observatory (JUNO) is a next‑generation, 20‑kiloton liquid‑scintillator (LS) neutrino detector designed to determine the neutrino mass ordering and to measure oscillation parameters (θ₁₂, Δm²₂₁, |Δm²₃₂|) with sub‑percent precision. Achieving these physics goals requires an ultra‑large, ultra‑pure LS volume (≈ 2.3 × 10⁴ m³) contained in a 35.4‑m‑diameter acrylic sphere, submerged in a high‑purity water pool (WP) that also serves as a muon veto. The LS composition (linear alkyl benzene with PPO and bis‑MSB fluors) must have an optical attenuation length > 20 m and radioactive contaminant levels below 10⁻¹⁵ g g⁻¹ for 238U and 232Th, with even stricter limits for 210Pb and radon.

The paper presents the design, construction, and performance of the Filling‑Overflow‑Circulation (FOC) system, the subsystem responsible for safely introducing water and LS into the detector, managing thermal expansion, and providing continuous LS circulation and re‑purification during the experiment’s lifetime. The FOC system fulfills four primary functions: (1) simultaneous water filling of the central detector (CD) and the surrounding WP to purge air and radon, (2) controlled replacement of water by LS in the CD, (3) handling LS overflow caused by temperature‑induced volume changes, and (4) enabling online LS circulation for optical and radiopurity maintenance.

Key engineering features include:

• Materials and Cleanliness – All wetted components (stainless‑steel tanks, pipes, valves) are fabricated from low‑radioactivity steel, electropolished to ≤ 0.4 µm surface roughness, and undergo a three‑stage cleaning protocol (degreasing, acid pickling, passivation). Leak‑tightness is specified at < 10⁻⁸ mbar·L·s⁻¹ for individual parts and < 10⁻⁶ mbar·L·s⁻¹ for assembled connections, preventing radon ingress and moisture contamination. Ultra‑pure nitrogen blankets the entire system to suppress oxygen and radon exposure.

• Storage and Overflow Infrastructure – One primary LS storage tank and two 50 m³ overflow tanks act as buffers during filling and long‑term operation. Their capacity accommodates the LS thermal expansion coefficient (8.8 × 10⁻⁴ °C⁻¹), allowing automatic compensation for temperature fluctuations without manual intervention.

• Piping, Valves, and Pumps – A dedicated network of stainless‑steel pipelines links the surface LS production plant to the underground hall. Dual‑redundant electromagnetic pumps handle LS, while self‑priming centrifugal pumps manage water. All valves are double‑O‑ring sealed, with a nitrogen‑filled inter‑seal space, and meet a leak‑rate requirement of < 10⁻⁶ mbar·L·s⁻¹. The design includes top and bottom chimneys to facilitate draining and venting.

• Automation and Control Architecture – The control system follows the ISA‑88 batch‑control model, comprising four layers: Sensor, Controller, Actuator, and Alarm/Data Management. High‑precision sensors (Endress+Hauser) provide level measurements via differential pressure, laser, and radar technologies with ≤ 0.2 % full‑scale accuracy; flow meters achieve 0.5 % precision. A Siemens S7‑300 PLC executes real‑time PID loops for level and flow, while a TIA Portal HMI offers operator visualization. EPICS integration enables cross‑system data exchange and archiving within the Detector Control System (DCS). A multi‑mode alarm system delivers on‑site audio‑visual alerts, remote notifications, and hard‑wired emergency stop circuits.

• Filling Procedure and Performance – Water filling required 45 days to deliver 6.4 × 10⁴ m³, maintaining the CD‑WP level differential within a 5 cm safety envelope. LS filling spanned 200 days for 2.3 × 10⁴ m³, achieving ±2 cm level control and ±0.5 % flow regulation. Continuous radon monitoring showed 222Rn concentrations below 1 mBq m⁻³ throughout the process. Post‑fill assays confirmed 238U and 232Th concentrations below 10⁻¹⁶ g g⁻¹, well within the experiment’s background budget.

• Long‑Term Operation – The overflow tanks automatically absorb volume changes, while the circulation loop continuously pumps LS through purification modules (distillation, alumina filtration, water extraction, stripping). This maintains optical attenuation length and suppresses re‑introduction of contaminants. The system is engineered for a 20‑year operational lifetime, with redundancy, interlocks, and remote diagnostics to ensure reliability.

In summary, the FOC system successfully delivered the world’s largest LS detector volume with unprecedented precision and radiopurity. Its combination of rigorous material selection, meticulous cleaning, high‑accuracy instrumentation, and robust PLC‑EPICS automation provides a blueprint for future large‑scale low‑background neutrino experiments. The documented performance—level control within centimeters, flow stability within half a percent, and background levels orders of magnitude below design thresholds—demonstrates that the FOC system is a critical enabler of JUNO’s scientific mission.