Environmental radon control in the 700 m underground laboratory at JUNO

Abstract The Jiangmen Underground Neutrino Observatory is constructing the world’s largest liquid scintillator detector, with a 20 kt target mass and approximately 700 m of overburden. The total underground space of civil construction is around 300,000 m $$^3$$ 3 , with the main hall comprising about 120,000 m $$^3$$ 3 , making it the largest experimental hall in the world. Maintaining a low radon concentration in the underground air is crucial for both human health and the accuracy of experiments involving rare decay detection, such as neutrino and dark matter experiments. To ensure human health and the integrity of neutrino physics experiments, the nominal radon concentration in the main hall must be kept below 200 Bq/m $$^3$$ 3 with a maximum value below 400 Bq/m $$^3$$ 3 . Introduction of fresh air from above ground can significantly lower radon concentration. A benchmark experiment conducted in the refuge room near the main hall revealed that the radon emanating from underground water is a significant source of radon in the underground air. The total underground ventilation rate is approximately 160,000 m $$^3$$ 3 /h of fresh air with about 30 Bq/m $$^3$$ 3 $$^{222}$$ 222 Rn from the bottom of the vertical tunnel after the installation of powerful fans. Of this, 55,000 m $$^3$$ 3 /h is used for ventilation in the main hall. As a result of these measures, the radon concentration inside the main hall has decreased from 1600 Bq/m $$^3$$ 3 to below 200 Bq/m $$^3$$ 3 under stable working conditions, with exceptions during rare adverse weather events or fan failures. The employed strategies to control radon concentration in the underground air are described in this paper.