Measurement of radon concentration in the output water of the 100~t/h ultrapure water system at the Jiangmen Underground Neutrino Observatory

TL;DR

This work addresses the challenge of suppressing $^{222}$Rn in the water surrounding JUNO's Central Detector to minimize background in a 20 kton liquid scintillator system. It introduces a 100 t/h UPW system that combines a five-stage degassing membrane cascade with a microbubble generator to achieve radon removal >99.9%, producing $0.61 \\pm 0.50$ mBq/m^3 in recirculation after detector filling and well below the target of $10$ mBq/m^3. The study characterizes radon contributions along the UPW train, optimizes membrane pressure and purge gas flow, and demonstrates a synergistic effect with microbubbles that yields sub-mBq/m^3 levels. The results establish a scalable, world-leading approach for low-radon UPW production in multi-kilotonne-scale experiments, with clear operational pathways for continuous recirculation and background suppression. Overall, the paper provides a detailed architectural blueprint and empirical validation for achieving stringent radiopurity in large-volume neutrino detectors.

Abstract

The Jiangmen Underground Neutrino Observatory (JUNO), a 20 kton multi-purpose low background liquid scintillator detector, was proposed primarily to determine the neutrino mass ordering. To mitigate radioactivity from surrounding rock and enable cosmic muon tagging, its central detector is immersed in a Water Cherenkov Detector (WCD) containing 40~ktons of ultrapure water instrumented with 2400 20-inch micro-channel plate photomultiplier tubes. Stringent radiopurity requirements mandate a radon concentration below 10 ~mBq/m$^3$ in the WCD. To achieve this, we developed a two-stage (ground and underground) ultrapure water system with 100~t/h production capacity, integrating a five-stage degassing membrane for radon removal. A novel microbubble technique was implemented to optimize the degassing membranes' radon removal efficiency. The synergistic combination of the microbubble technology and the multistage degassing membranes achieved a radon removal efficiency exceeding 99.9\%, reducing the system's output to 0.61 $\pm$ 0.50~mBq/m$^3$ in recirculation mode, surpassing design specifications and establishing world-leading performance standards. This paper details the ultrapure system architecture, quantifies the radon contributions of each device, and presents a comprehensive study on microbubble-augmented membrane degassing for low radon ultra-pure water production in a 100~t/h water system.

Figures (12)

• Figure 1: The flow chart of the JUNO 100 t/h UPW system. The system comprises aboveground and underground components, the produced water from the overground water system will be delivered to the underground water system as source water via a $\sim$1,300-meter Stainless STeel (SST) pipeline. Standard PVC pipes were used pre-EDI, while post-EDI, Sekisui Clean PVC pipes (Japan), specifically engineered for UPW conveyance, were implemented.
• Figure 2: Picture of the second group of degassing membranes used in the JUNO UPW system.
• Figure 3: Schematic diagram of the degassing membrane.
• Figure 4: Picture of the microbubble generator used in the JUNO UPW system.
• Figure 5: The schematic diagram of the microbubble generator.