Environmental radon control in the 700-m underground laboratory at JUNO

This paper details the strategies and results of optimizing fresh air ventilation to successfully reduce radon concentration in the main hall of the 700-m deep JUNO underground laboratory from 1600 Bq/m³ to the required 100 Bq/m³, identifying underground water as a primary radon source.

The Gist

Here is an explanation of the paper, translated into simple language with some creative analogies.

The Big Picture: A Giant Bathtub in a Cave

Imagine the JUNO experiment as the world's largest, most sensitive "bathtub" (a liquid scintillator detector) built deep underground in a cave in China. This cave is about 700 meters deep (roughly the height of two Empire State Buildings stacked on top of each other).

The scientists are trying to catch tiny, ghostly particles called neutrinos. To do this, their "bathtub" needs to be perfectly clean. If there is even a tiny bit of dust or "noise" in the air, it will ruin the experiment.

The biggest "noise" culprit is Radon.

The Villain: Radon

Think of Radon as an invisible, radioactive gas that seeps out of the ground like a slow, toxic fog.

• Why it's bad for humans: If you breathe it in, it can cause lung cancer.
• Why it's bad for the experiment: It creates "static" on the scientists' equipment, making it impossible to see the tiny signals they are looking for.

In the open air, Radon is like a light mist (about 10 units of "pollution"). But deep underground, without ventilation, it can become a thick, choking smog (up to 1,600 units!). The scientists needed to get this down to a safe level of about 100 units.

The Investigation: Finding the Leak

When the scientists first started working in the cave, the air was full of Radon. They needed to find out where it was coming from. They set up a small "test room" (a refuge room) to play detective.

They discovered two main sources of the Radon fog:

1. The Rock Walls: The granite rock itself leaks Radon, like a sponge slowly dripping water.
2. The Underground Water: This was the surprise! There is a massive amount of water flowing through the cave (450 bathtubs worth of water every hour!). This water is soaked with Radon. When the water hits the air, it "burps" out huge clouds of Radon gas.

The Analogy: Imagine trying to keep a room clean while someone is constantly spraying a mist of glitter (Radon) from a hose (the water). You can't just wipe the walls; you have to deal with the hose.

The Solution: The Giant Air Purifier

To fix this, the team had to design a massive ventilation system. Think of it as building a giant, high-powered vacuum cleaner and fan system for the entire cave.

1. The Tunnel Strategy (The Wind Tunnel):
They installed 12 giant fans in the tunnels leading to the lab.

• The Problem: They noticed the air quality changed between day and night.
• The Discovery: It turns out the weather above ground acts like a giant bellows. When the sun is hot and the wind is calm above, the air underground gets stagnant. When the wind blows hard above, it pushes fresh air down the shaft.
• The Fix: They adjusted the fans to fight against the natural weather patterns, ensuring air always flowed in the right direction, pushing the bad air out and pulling fresh air in.

2. The Main Hall Strategy (The Bathtub):
The main hall is huge (120,000 cubic meters).

• The Water Problem: At the bottom of the hall, water was gushing out of the rock at a rate of 70 bathtubs per hour. This was the biggest source of Radon.
• The Fix: They placed a massive fan at the very bottom of the hall, right near the water exit. This fan acts like a powerful exhaust hood in a kitchen, sucking the Radon-rich air right out of the room before it can float up and mix with the clean air.
• The Fresh Air: They also built a long pipe running from the surface down to the bottom of the shaft to bring in fresh, clean air from the sky.

The Result: From Smog to Breeze

Before these changes, the air in the main hall was like a dense fog (1,600 units of Radon).
After installing the fans, fixing the airflow, and managing the water, the air cleared up significantly.

• The Result: The Radon level dropped to around 100 units.
• The Analogy: They turned a room that was full of smoke into a room with a gentle, fresh breeze.

Why This Matters

This paper isn't just about one experiment. It teaches us how to build clean environments in deep mines and caves.

• For Science: It allows JUNO to see the neutrinos clearly.
• For Health: It shows how to keep miners and workers safe from radioactive gas.
• For Engineering: It proves that even in a giant, wet cave, you can control the air if you understand the "weather" of the underground and manage your water sources.

In short: The scientists found that water was the main source of the radioactive gas. By building a giant, smart ventilation system that acts like a vacuum cleaner for the water and a fan for the air, they successfully cleared the air so the experiment could begin.

Technical Summary

Here is a detailed technical summary of the paper "Environmental radon control in the 700-m underground laboratory at JUNO."

1. Problem Statement

The Jiangmen Underground Neutrino Observatory (JUNO) is constructing the world's largest liquid scintillator detector (20 kton target mass) located 700 meters underground in China. For rare-event physics experiments (neutrino oscillation, dark matter), minimizing background radiation is critical. Radon-222 ($^{222}\text{Rn}$), a radioactive noble gas, is a primary source of background noise due to its short-lived decay products.

• Challenge: The main experimental hall (volume $\approx 120,000 \text{ m}^3$) initially exhibited a radon concentration of 1600 Bq/m³, far exceeding the required limit of ~100 Bq/m³ to prevent radon daughter deposition on the detector.
• Constraints: Unlike other major underground labs (e.g., SNOLAB, LNGS), JUNO faces unique challenges:
  • Higher uranium content in the surrounding granite ($\approx 120 \text{ Bq/kg}$).
  • Significant groundwater discharge ($\approx 450 \text{ m}^3/\text{h}$ total displacement).
  • Limited ventilation capacity compared to the massive volume (max fresh air inlet $\approx 40,000 \text{ m}^3/\text{h}$ for the main hall, roughly 1/3 of SNOLAB's rate relative to volume).
  • Complex airflow dynamics influenced by seasonal weather changes.

2. Methodology

The authors employed a multi-stage approach combining benchmark experiments, source identification, and iterative ventilation optimization.

A. Benchmark Experiment in the Refuge Room

To identify and quantify radon sources, a controlled experiment was conducted in a "refuge room" ($175 \text{ m}^3$) near the main hall, which mimicked the hall's conditions (rock walls, concrete floor, water outflow).

• Modeling: The team used a differential equation modeling radon concentration ($C_{air}$) based on emanation rates ($E$), ventilation rate ($\Phi$), diffusion ($L$), and decay ($\lambda$).
• Source Quantification:
  • Rock: Measured using a RAD7 instrument.
  • Water: Measured using a RAD AQUA accessory to extract radon from water samples.
  • Intervention: The team systematically blocked/unblocked air leaks and diverted water outflows to isolate the contribution of each source.

B. Ventilation System Optimization

Based on source identification, the ventilation strategy was redesigned in two phases:

1. Tunnel Ventilation: Deployed 12 fans (156 kW total) to control airflow direction and speed in the access tunnels, preventing radon-rich air from construction tunnels from entering the main hall.
2. Main Hall Ventilation:
   • Intake: Initially, fresh air was drawn from the bottom of the vertical shaft (non-ideal). Later, infrastructure was completed to draw air from the surface.
   • Exhaust: Installed a 22 kW fan at the bottom of the hall to actively exhaust radon-rich air accumulating near the water pool.
   • Pressure Management: Converted a circulation cabinet to provide additional fresh air to maintain positive pressure, preventing radon-rich air from adjacent drainage galleries from leaking in.

C. Long-term Monitoring

Continuous monitoring of radon concentration, wind speed, temperature, and pressure was conducted at various locations (main hall, tunnels, vertical shaft) to correlate environmental factors with radon levels.

3. Key Contributions & Findings

Identification of Water as a Dominant Source

Contrary to the assumption that rock emanation is the primary source, the study revealed that radon emanating from underground water is a critical, often dominant, source.

• Quantification: In the refuge room, radon from water outflow ($16.6 \pm 0.3 \text{ kBq/h}$) was found to be **~30 times higher** than radon from rock surfaces ($0.52 \pm 0.06 \text{ kBq/h}$).
• Mechanism: Radon accumulates in groundwater (concentration $\approx 75 \text{ kBq/m}^3$) and rapidly emanates into the air upon discharge.
• Impact: In the main hall, the bottom water discharge ($70 \text{ m}^3/\text{h}$) was identified as the primary driver of high radon levels, contributing significantly more than the rock walls.

Ventilation Dynamics and Seasonal Effects

• Diurnal Oscillation: A clear day-night oscillation in radon concentration was observed (higher during the day, lower at night), which is the inverse of surface patterns.
• Correlation: This oscillation is driven by wind speed rather than temperature. Higher surface temperatures reduce underground wind speed (due to atmospheric stability), decreasing ventilation efficiency and increasing radon concentration.
• Seasonal Variation: Summer months (lower wind speeds, $\approx 1.5 \text{ m/s}$) result in higher radon levels compared to winter ($\approx 3.0 \text{ m/s}$).

Optimization Results

• Tunnel Optimization: Redirecting airflow in the No.1 construction tunnel reduced radon in the tunnel area to $<100 \text{ Bq/m}^3$.
• Main Hall Reduction: Through the combination of active exhaust at the bottom, positive pressure maintenance, and improved fresh air intake, the radon concentration in the main hall was reduced from 1600 Bq/m³ to ~100 Bq/m³.
• Stability: Since October 2022, the concentration has remained stable around the target, despite seasonal fluctuations, provided the surface ventilation infrastructure is operational.

4. Results Summary

Parameter	Initial State	Optimized State (Post-2022)
Main Hall Radon	1600 Bq/m³	~100 Bq/m³ (Target met)
Tunnel Radon	600–1100 Bq/m³	<100 Bq/m³ (in optimized zones)
Fresh Air Rate	Limited/Inefficient	~52,000 m³/h (Main Hall + Aux)
Primary Source	Assumed Rock	Underground Water
Key Mitigation	Passive Ventilation	Active Bottom Exhaust + Positive Pressure

5. Significance

• Experimental Success: Achieving a radon level of ~100 Bq/m³ is crucial for the JUNO experiment's physics goals, ensuring that background noise from radon daughters does not obscure the neutrino signal.
• Methodological Insight: The paper establishes that in deep underground laboratories with significant groundwater flow, water emanation must be treated as a primary radon source, often outweighing rock emanation.
• Engineering Guidelines: The study provides a blueprint for large-scale underground facilities:
  • Water Management: Drainage systems should ideally be closed loops or actively ventilated to the outside to prevent radon accumulation.
  • Airflow Control: Ventilation must be designed to prevent "dead zones" and ensure wind direction moves radon away from the detector, not toward it.
  • Weather Adaptation: Ventilation systems must account for seasonal weather variations (wind speed/temperature) that affect natural ventilation efficiency.
• Global Applicability: These findings are transferable to other underground physics experiments (e.g., dark matter searches) and mining operations where radon control is a safety and operational necessity.