Distillation and Stripping purification plants for JUNO liquid scintillator

TL;DR

This paper reviews the design, operation, and initial performance results of large-scale distillation and gas stripping plants used to purify liquid scintillator for the JUNO neutrino experiment, ensuring it meets strict optical and radiochemical standards.

Contribution

It provides a detailed description of the layout, principles, and technical aspects of the purification plants specifically built for JUNO, including initial performance evaluations.

Findings

Distillation improves optical clarity and reduces heavy radio-impurities.

Gas stripping effectively removes gaseous radioactive contaminants.

Plants have been tuned and tested during commissioning with promising preliminary results.

Abstract

The optical and radiochemical purification of the scintillating liquid, which will fill the central detector of the JUNO experiment, plays a crucial role in achieving its scientific goals. Given its gigantic mass and dimensions and an unprecedented target value of about 3% @ 1 MeV in energy resolution, JUNO has set severe requirements on the parameters of its scintillator, such as attenuation length (Lat>20 m at 430 nm), transparency, light yield, and content of radioactive contaminants (238U,232Th<10-15 g/g). To accomplish these needs, the scintillator will be processed using several purification methods, including distillation in partial vacuum and gas stripping, which are performed in two large scale plants installed at the JUNO site. In this paper, layout, operating principles, and technical aspects which have driven the design and construction of the distil- lation and gas stripping plants are reviewed. The distillation is effective in enhancing the optical properties and removing heavy radio-impurities (238U,232Th, 40K), while the stripping process exploits pure water steam and high-purity nitrogen to extract gaseous contaminants (222Rn, 39Ar, 85Kr, O2) from the scintillator. The plant operating parameters have been tuned during the recent com- missioning phase at the JUNO site and several QA/QC measurements and tests have been performed to evaluate the performances of the plants. Some preliminary results on the efficiency of these purification processes will be shown.