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Study on the radon removal for the water system of Jiangmen Underground Neutrino Observatory

  • C. Guo
  • J. C. Liu
  • Y. P. Zhang
  • P. Zhang
  • C. G. Yang
  • Y. B. Huang
  • W. X. Xiong
  • H. Q. Zhang
  • Y. T. Wei
  • Y. Y. Gan
Original Paper
  • 130 Downloads

Abstract

Background

The Jiangmen Underground Neutrino Observatory (JUNO), a 20-kton multipurpose underground liquid scintillator detector, was proposed with the determination of the neutrino mass hierarchy as a primary physics goal. To veto the cosmogenic background and shield the central detector from the environmental radioactivity, a multi-veto system, which consists of a water Cherenkov detector and a top tracker detector, is required. In order to keep the water quality good and remove the radon from water, an ultrapure water system, a radon removal system and radon concentration measurement system have been designed.

Method

The JUNO ultrapure water system was designed on the basis of the water system of the Daya Bay experiment. By installing the degassing membrane devices on the water system of JUNO prototype and Daya Bay experiment which can remove radon from water, the radon removal efficiency has been measured at the conditions of different gas–liquid phase pressures and different gas concentrations in water using the radon measurement system.

Result

Loading carbon dioxide into the water and increasing the inlet water pressure could help to improve the radon removal efficiency of the degassing membrane devices, and the radon concentration in water can be reduced to \(\sim 0.1 \hbox { Bq/m}^3\).

Conclusion

A reliable ultrapure water production and circulation system has been designed to keep the water quality good and to reduce the radon concentration in water for JUNO water Cherenkov detector. The radon concentration in water can satisfy the requirement of JUNO by using the Liqui-Cel degassing membrane devices.

Keywords

Ultra-pure water Radon Degassing membranes 

Introduction

The Jiangmen Underground Neutrino Observatory (JUNO) [1, 2] is a multipurpose neutrino experiment designed to determine neutrino mass hierarchy and precisely measure oscillation parameters by detecting reactor neutrinos from the Yangjiang and Taishan Nuclear Power Plants, with a 20-thousand-ton liquid scintillator (LS) detector of unprecedented 3% energy resolution (at 1 MeV) at 700-m-deep underground. To suppress radioactivity and cosmogenic background, the outer of the central detector is filled with ultra-pure water as passive shielding for radioactivity from surrounding rock and is equipped with about 2400 microchannel plate photomultiplier tubes (MCP-PMTs, 20 inchs) to form a water Cherenkov veto detector to tag muons. The conceptual design of the detector is shown in Fig. 1.
Fig. 1

Conceptual design of the water Cherenkov detector

Fig. 2

Conceptual scheme of the water system

Three basic requirements have been put forward for the ultra-pure water system:
  1. (A)

    There will be different kinds of materials submerged in the water, including stainless steel, Tyvek, PMT glass and cables. The complexity of the underground environment makes it difficult to seal the pool completely and almost impossible to keep the water quality good for a long time. Therefore, it is necessary to build a reliable ultra-pure water production, purification and circulation system [1].

     
  2. (B)

    The temperature stability of the central detector is critical for the entire experiment; thus, one of the most important functions of the water system is to keep the overall detector temperature stable [1].

     
  3. (C)

    According to the Monte Carlo simulation of JUNO experiment, the radon concentration in the water should be less than 0.2 Bq/m\(^3\) [2]. However, radon can be emanated from the surface of various radium-containing substances, including the wall of the water pool, the PMT glass and the stainless steel. Thus, the water system should have the function of removing the radon in water.

     
The JUNO water system was designed on the basis of the water system of the Daya Bay experiment [3], and its conceptual scheme, which consists of the ultra-pure water production system and the circulation system, is shown in Fig. 2. The overground part of the water system which includes the ultra-filter and the first-stage reverse osmosis (RO) device can purify the tap water into pure water with a resistivity of \(\sim \) 1 M\(\Omega \) cm by removing sediment, rust, colloidal matter, suspended species and inorganic salt in water. The underground part which includes the secondary-stage RO device, total organic carbon (TOC) removal, electrodeionization (EDI) and resin could produce ultra-pure water and maintain the water resistivity \(\sim \) 18 M\(\Omega \) cm by further removing ions from water. An ultraviolet (UV) device is used to sterilize. A chiller, which is used as exchanger, could regulate the temperature of the water. The degassing membrane devices, which are widely used in ultra-pure water circulation systems to remove gases in water, are used to remove carbon dioxide (CO\(_{2}\)), oxygen (O\(_{2}\)) as well as radon here. In order to measure the radon removal efficiency and radon removal limit of the degassing membranes, the relevant measurements were carried out based on the water systems of JUNO prototype and Daya Bay experiment, respectively.

Experiment setup

A prototype detector has been built to test the key technical issues as well as to test the performance of the water system of JUNO [1]. In order to study the radon removal efficiency of the degassing membrane device, a three-stage degassing membrane device and a high-sensitive radon measurement system have been installed on the water circulation system. The schematic diagram of the water circulation system is shown in Fig. 3. By controlling the operation of the degassing membrane and different valve switches, the radon concentration in source water and in degassing water can be measured, respectively.
Fig. 3

Conceptual scheme of the water system for JUNO prototype

Fig. 4

Photograph of the degasser membrane device

Fig. 5

Schematic diagram of the membrane

Degassing membrane device

Figure 4 shows the real picture of the degassing membranes, and they have been installed in the JUNO prototype. The main part of the degasser is the Liqui-Cel membrane contactor, and the schematic diagram of the membrane is shown in Fig. 5 [4]. A membrane contactor can contain thousands of microporous polypropylene hollow fibers knitted into an array that is wound around a center tube. The hollow fibers are arranged with uniform spacing, allowing greater flow capacity and utilization of the total membrane surface area. Because the hollow fiber membrane is hydrophobic, liquids will not penetrate the membrane pores. When high-pressure liquid flows over the shell side (outside) of the hollow fibers, a vacuum, strip gas, or both in combination is applied to the lumen side (inside) of the fibers to create the driving force for dissolved gas in the liquid to pass through the membrane pores. The gas is carried away by the vacuum pump or sweep gas to achieve the purpose of gas removing from the liquid. Depending on the pore size of the fiber membrane, there are two types of hollow fiber membranes, namely the X-40 and X-50 [5]. X-40 is suitable for removing O\(_2\) from water, while X-50 is for CO\(_2\) removing. In the water circulation system, two-stage X-50 and one-stage X-40 degassing membranes were installed and serial-connected.

In the experiment, the radon removal efficiency and radon removal limit of the degassing membrane device were studied at the conditions of different gas–liquid phase pressures and different gas concentrations in water.
Fig. 6

Conceptual diagram of the radon measurement system

The radon measurement system

A high-sensitivity radon concentration measurement system has been developed to measure the radon concentration in water. The conceptual diagram is shown in Fig. 6, and the detailed information about the detector can be found in Ref. [6]. The atomizer and radon detector are the key components of the system.
Fig. 7

Left: schematic diagram of the atomizer. Right: schematic diagram of the radon detector

  1. (A)
    The atomizer which is shown in the left panel of Fig. 7 is a water vapor balancing device, and it can be used to transfer the radon from water into vapor by spraying. When the water and vapor are at equilibrium state, the radon concentration in the two media (water/vapor) is correlated by the Ostwald coefficient and the ratio of radon concentrations in water to vapor follows Eq. 1 [7]:
    $$\begin{aligned} \quad R = 0.105 + 0.405\hbox {e}^{-\,0.0502{T}}, \end{aligned}$$
    (1)
    where R stands for the ratio and T is the temperature in unit of centigrade. For JUNO prototype, the temperature of the water has been kept at around 20 centigrade; thus, R is around 0.25. Therefore, the radon concentration in water can be derived from the gas measurement result.
     
  2. (B)

    The radon detector which is shown in the right panel of Fig. 7 consists of a cylindrical electro-polished stainless steel vessel, a cylindrical high-purity oxygen-free copper vessel and a Si-PIN photodiode. The principle of the radon detector is to collect the daughter nuclei of \(^{222}\)Rn to the surface of the Si-PIN with an electric field and to measure the energy of \(\alpha \)s released by the collected nuclei [8, 9]. The sensitivity of the radon detector is around 9.0 mBq/m\(^{3}\) for a single-day measurement [6].

     
  3. (C)

    The drier is used to keep the relative humidity below 3% because 90% of the radon daughters are positive and the high humidity will decrease the collecting efficiency [10]. The pump is used to circulate the gas in the measurement system.

     
Before measuring the radon concentration in water, the entire measurement system was filled with evaporated nitrogen to eliminate the influence of radon from the air. During the measurement, the measured water flows into the atomizer device at a flow rate of 1 L/min for atomization. After the water vapor is sufficiently mixed with the nitrogen, they are transferred to the measurement chamber through the drier under the action of a circulation pump. During the tests, the diffusing equilibrium time of radon in water and gas as well as the decay equilibrium time of radon and it daughters had been taken into consideration.
When the water and gas are at equilibrium state, the radon concentration in measured water can be derived from the gas measurement result using Eq. 1.
Fig. 8

Measurement results of radon concentration in water at different conditions. Case 1: 0.15 MPa inlet water pressure; case 2: 0.15 MPa inlet water pressure with carbon dioxide loaded; case 3: 0.35 MPa inlet water pressure

Measurement results

The radon removal efficiency has been tested at different conditions, and the results are shown in Fig. 8. In case 1, the degassing membranes are tested with the JUNO prototype water system at 0.15 MPa inlet water pressure and \(\sim \) 0.5 m\(^{3}\)/h flow rate; the results show that the degassing efficiency for one stage of the degassing membrane is around 65%. While after several times of degassing, the radon concentration is still around 0.23 Bq/m\(^{3}\), which implies that the radon removal limit of the degassing membranes cannot satisfy the JUNO requirement at this condition.

To improve the radon removal efficiency, two kinds of different methods have been put forward, namely loading carbon dioxide into the inlet water of the degassing membrane device and increasing the inlet water pressure.

In order to load the carbon dioxide into the water, a small vessel was used instead to the JUNO prototype water tank. In case 2, without loading carbon dioxide into the inlet water, the radon removal limit of the degassing membrane is \(\sim \) 0.24 Bq/m\(^{3}\) which is consistent with the result in case 1 under similar conditions after several times of degassing. When the carbon dioxide loading flow rate is \(\sim \) 0.3 L/min, the radon concentration can be reduced to \(\sim \) 0.1 Bq/m\(^{3}\), which can satisfy the requirement of JUNO. When the degasser is off, the difference of radon concentration between case 1 and case 2 is mainly due to the fact that the water in the JUNO prototype water system was in a sealed environment for a long time, while the water in the small vessel was filled for a short time.

Considering that the ultra-pure water circulation and purification system will be installed in the underground laboratory with high radon concentration environment in the future, it is necessary to measure the radon removal efficiency of the degassing membrane device in the high radon concentration environment. For this reason, the degassing membrane device and the radon measurement system were transported to the Daya Bay experimental site and were connected together with the water system for measurement. The water flow rate is \(\sim \) 0.5 m\(^{3}\)/h, and the inlet water pressure of the degasser was adjusted to 0.35 MPa. The results show that the environmental radon concentration will not affect the radon removal efficiency and higher inlet water pressure could help to increase the radon removal efficiency, and in case 3 the radon concentration in degassing water can meet the requirement of JUNO.

Since the water flow rate of the JUNO water system will be \(\sim \) 80 t/h [1], the next step is still necessary to measure the radon removal efficiency of the degassing membrane device at large flow rates to ensure the feasibility of using degassing membrane devices to remove radon in the water for the future JUNO water system.

Summary

A reliable ultra-pure water production and circulation system has been designed to keep the water quality good and to reduce the radon concentration in water for JUNO water Cherenkov detector. The Liqui-Cel degassing membranes are used to remove the radon in water, and a high-sensitive radon detector has been developed for radon concentration measurement. Both of these equipments have been installed in the water system of the JUNO prototype. According to the measurement results, loading carbon dioxide into the water and increasing the inlet water pressure could help to improve the radon removal efficiency of the degassing membranes and the radon concentration in water can be reduced to \(\sim \) 0.1 Bq/m\(^{3}\), which can satisfy the requirement of JUNO.

Notes

Acknowledgements

Many thanks to Shoukang Qiu and Quan Tang of University of South China for their help during the experiment. This work was supported by the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDA10010300).

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Copyright information

© Institute of High Energy Physics, Chinese Academy of Sciences; Nuclear Electronics and Nuclear Detection Society and Springer Nature Singapore Pte Ltd. 2018

Authors and Affiliations

  1. 1.Key Laboratory of Particle Astrophysics, Institute of High Energy PhysicsChinese Academy of ScienceBeijingChina
  2. 2.School of PhysicsUniversity of Chinese Academy of ScienceBeijingChina

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