Radon Detection in Underground Engineering: Methods, Standards, and Applications

Radon detection in underground engineering is a critical radiological protection practice for tunnels, metro stations, underground parking structures, civil defense works, and deep-buried facilities, where radioactive radon-222 gas can accumulate to concentrations that pose significant occupational and public health risks. This article provides a comprehensive technical overview of radon formation and exposure pathways, sampling strategies, measurement methods specified in ISO 11665, GB/T 14582, GBZ 116, and related standards, as well as acceptance criteria, quality assurance requirements, and remediation measures for underground environments.

Technical Overview

Radon-222 is a colorless, odorless, chemically inert radioactive noble gas produced in the uranium-238 decay chain through the disintegration of radium-226, which occurs naturally in soil, bedrock, groundwater, and mineral-based construction materials. Once formed, radon atoms migrate through rock pores, fissures, and building envelopes into confined air volumes. When radon decays, it generates a series of short-lived progeny isotopes, including polonium-218, lead-214, bismuth-214, and polonium-214, several of which emit highly ionizing alpha particles. When these progeny are inhaled and deposited on the bronchial epithelium, they deliver localized alpha radiation doses to lung tissue.

According to assessments by the World Health Organization (WHO) and the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR), radon is the second leading cause of lung cancer after tobacco smoking and the dominant source of natural background radiation exposure for most populations. Underground engineering structures, including railway and highway tunnels, metro stations, underground parking garages, civil air defense shelters, utility tunnels, and underground powerhouses, are inherently prone to radon accumulation for three reasons: direct contact between the enclosed space and radon-emitting rock or soil masses, radon-laden groundwater seepage and degassing, and restricted natural air exchange that dilution ventilation must overcome.

Radon detection in underground works is therefore mandated or recommended by multiple frameworks, including GB 18871 (basic standards for protection against ionizing radiation), GBZ 116 (control standard for radon and its progeny in underground structures), GB 50325 (code for indoor environmental pollution control of civil building engineering), GB/T 14582 (standard measurement methods for radon in ambient air), the ISO 11665 series (measurement of radioactivity in the environment, air, radon-222), IEC 61577 (radiation protection instrumentation for radon measuring instruments), and the guidance of ICRP Publication 126 on radiological protection against radon exposure. The objectives of a detection program typically include pre-construction site evaluation, occupational exposure control during tunneling, completion acceptance testing, routine operational surveillance, and verification of mitigation measures.

Testing Samples

Underground Air Samples

The primary sample medium is the ambient air of the underground space itself. Sampling locations include tunnel bores under excavation, completed tunnel sections, station halls and platform levels of metro systems, underground parking decks, below-grade commercial floors, civil air defense works, utility tunnels, mine workings, and underground machine halls. Samples are collected within the human breathing zone, typically 0.8 to 1.5 meters above the floor and away from doors, ventilation diffusers, and wall surfaces, so that the result reflects actual inhalation exposure.

Soil Gas and Geological Samples

Prior to construction, soil gas radon concentration surveys are performed across the building footprint to classify the site and determine whether anti-radon structural measures, membrane barriers, or enhanced underfloor ventilation are required. In deep tunnel engineering, geological characterization of the surrounding rock mass, including radium-226 content and emanation coefficient, supports the prediction of radon inflow rates and the design of tunnel ventilation systems.

Construction and Decoration Materials

Concrete, cement, sand and gravel aggregates, bricks, stone panels, gypsum products, and granite finishes used in underground fit-out are tested for radium-226, thorium-232, and potassium-40 activity concentrations in accordance with GB 6566, and for surface radon exhalation rate. Materials with elevated radium content can become a sustained internal radon source even where geological inflow is modest.

Groundwater and Seepage Water

Water infiltrating into tunnels and basements frequently carries dissolved radon that degasses into the indoor environment. Where significant seepage or inrush occurs, water samples may be analyzed for radon activity concentration to quantify this contribution pathway and to justify drainage and waterproofing interventions.

Testing Items

The scope of a radon detection program for underground engineering generally encompasses the following measurands:

• Radon-222 activity concentration in air, expressed in becquerels per cubic meter (Bq/m³), for both short-term screening and long-term annual average determination.
• Radon decay product concentration, expressed as the equilibrium equivalent concentration (EEC) or the potential alpha energy concentration (PAEC) in microjoules per cubic meter or working level (WL).
• Radon equilibrium factor (F), the ratio between progeny and radon gas concentrations, which is essential for converting measured gas concentrations into dose estimates; typical underground values range from 0.2 to 0.6 depending on ventilation and aerosol conditions.
• Surface radon exhalation rate of tunnel linings, wall surfaces, and floor slabs, expressed in becquerels per square meter per second, used to locate and quantify material or substrate sources.
• Soil gas radon concentration at the project site, in Bq/m³, for construction planning and anti-radon design classification.
• Radium-226, thorium-232, and potassium-40 activity concentrations in building materials, in becquerels per kilogram, including computation of the internal and external exposure indices.
• Radon activity concentration in groundwater and seepage water, in becquerels per liter.
• Occupational and public effective dose assessment, in millisieverts per year, derived from measured concentrations, occupancy time, equilibrium factor, and the dose conversion conventions of ICRP.

Reference Levels and Acceptance Criteria

Interpretation of results is made against recognized reference levels. WHO recommends a national reference level of 100 Bq/m³ for dwellings, with 300 Bq/m³ as the maximum acceptable where the lower value is not feasible. ICRP recommends reference levels within the 100 to 300 Bq/m³ band for homes and up to 300 Bq/m³ for workplaces. In China, GBZ 116 applies an action level of 400 Bq/m³ annual average radon concentration for existing underground structures in occupational use, with a stricter design target of 200 Bq/m³ for underground works under construction; residential or long-occupancy public use of underground space is subject to more stringent limits. GB/T 18883 sets an annual average indoor radon action level of 300 Bq/m³, and GB 50325 specifies radon concentration limits of 150 to 400 Bq/m³ for civil building acceptance depending on the building classification. As a rule of thumb under the ICRP dose conversion convention, a worker spending 2,000 hours per year in an atmosphere of 300 Bq/m³ at an equilibrium factor of 0.4 receives an annual effective dose of approximately 2 mSv.

Testing Methods

Continuous Real-Time Measurement

Continuous radon monitors based on electrostatic collection of positively charged polonium-218 onto a silicon semiconductor detector, pulse ionization chambers, or pumped scintillation cells provide time-resolved concentration profiles. These instruments are indispensable during tunnel excavation, where radon levels fluctuate with blasting, groundwater inflow, and ventilation changes, and for verifying the performance of mechanical ventilation in operating underground facilities. Requirements for such instruments are defined in the IEC 61577 series and the ISO 11665 series.

Scintillation Cell (Lucas Cell) Method

The grab-sampling scintillation cell method described in GB/T 16147 and ISO 11665 involves pumping air into a zinc-sulfide-coated cell and counting alpha-induced scintillations after secular equilibrium of the short-lived progeny is established. The method offers good sensitivity for spot checks and cross-verification of continuous instruments.

Activated Charcoal Canister Adsorption

Passive charcoal canisters exposed for two to seven days adsorb radon, after which the accumulated gamma-emitting progeny lead-214 and bismuth-214 are quantified by gamma spectrometry or a calibrated sodium iodide counter. The technique is economical for large-scale screening surveys but is sensitive to humidity and is not suitable for long-term averaging.

Solid-State Nuclear Track Etch Detectors

Electret Ion Chamber Method

Electret ion chambers integrate the ionization produced by radon and its progeny as a measurable voltage discharge of a charged Teflon electret. They are compact, reusable, and well suited to intermediate-duration monitoring, although corrections for environmental gamma background and temperature are required.

Radon Progeny and Exhalation Rate Measurement

Progeny concentrations are determined by drawing air through a membrane filter and performing gross alpha counting or alpha spectroscopy on the deposited activity, yielding EEC, PAEC, and the equilibrium factor. Surface exhalation rate is measured by accumulating radon under a sealed enclosure or accumulation chamber mounted on the lining surface and monitoring the concentration growth curve. Material radium content is quantified by high-resolution gamma spectrometry with hyper-pure germanium detectors.

Sampling Conditions and Quality Assurance

Reliable results depend on controlled sampling conditions and rigorous quality assurance. For acceptance testing of enclosed civil spaces, natural ventilation openings are closed for at least 24 hours before sampling, while mechanically ventilated spaces are tested under representative operating conditions. Sampling point density is scaled to floor area, commonly one point for areas under 50 square meters, two points for 50 to 100 square meters, and three to five points for larger zones. All measurements should be performed by accredited third-party laboratories operating in conformity with ISO/IEC 17025, using instruments calibrated against primary or secondary radon chambers traceable to national metrology institutes, supported by field blanks, duplicate deployments, and documented measurement uncertainty budgets. Seasonal variation is addressed by long-term integrated measurement or by scheduled periodic campaigns covering different seasons.

Applications

Radon detection services for underground engineering serve a wide range of project phases and facility types:

• Pre-construction site investigation and soil gas radon classification for buildings with below-grade levels, basements, and civil air defense components.
• Occupational radiological protection during tunnel construction, ensuring that ventilation capacity, work rotation, and respiratory protection keep worker doses within the limits of GB 18871 and ICRP recommendations.
• Completion acceptance testing of metro stations, road and railway tunnels, underground parking structures, and commercial basements as required by GB 50325 and project specifications.
• Routine operational monitoring of occupied underground facilities, including metro systems, civil defense works converted to commercial use, underground data centers, archives, and warehousing.
• Underground hydropower stations, pumped storage caverns, and utility tunnels where deep burial, high rock stress, and limited airflow promote accumulation.
• Mining and Naturally Occurring Radioactive Material (NORM) management programs for both uranium and non-uranium mines.
• Verification of mitigation measures such as enhanced mechanical dilution ventilation, positive pressurization, anti-radon membranes and coatings, drainage control, and air sealing of the building envelope.

FAQ

When should radon testing be performed in an underground project?

Testing is appropriate at four stages: during pre-construction site assessment for soil gas radon; during tunneling or excavation for occupational protection; at completion acceptance before handover; and periodically during operation, particularly after modifications to the ventilation system, changes in occupancy patterns, or observed groundwater inflow.

How long does a radon measurement take?

Continuous monitoring can yield indicative values within 24 to 48 hours, charcoal canister screening requires two to seven days of exposure, and integrated track etch or electret measurements for annual average assessment typically span one to twelve months. Regulatory acceptance testing generally requires the standardized closure period before sampling begins.

What happens if measured radon exceeds the reference level?

Exceedance triggers a structured response: confirmation with repeat or long-term measurement, source identification through exhalation rate and material testing, and engineering mitigation. Increasing dilution ventilation is usually the primary measure, supplemented by air sealing, anti-radon barriers and coatings, drainage of radon-bearing water, and, in severe cases, restriction of occupancy until levels are controlled. Follow-up testing verifies the effectiveness of the intervention.

Why is radon higher underground than in surface buildings?

Underground spaces are surrounded on all sides by radon-emitting rock, soil, and backfill; groundwater seepage adds a dissolved radon pathway; and natural air exchange is minimal. Unless mechanical ventilation is correctly sized, radon released from the enclosing geology accumulates faster than it is diluted.

Is radon testing legally required for underground works?

In many jurisdictions, yes. Under the Chinese regulatory framework, radon is a mandatory parameter in the indoor environment acceptance of civil building engineering under GB 50325, GBZ 116 establishes control requirements for radon in underground structures used for work or public activity, and GB 18871 imposes general obligations to keep occupational exposures as low as reasonably achievable. Equivalent obligations exist in international guidance from ICRP, WHO, and the IAEA, and compliance testing should be entrusted to accredited third-party laboratories.

Can a single short-term measurement demonstrate compliance?

Short-term screening is useful for identifying acute problems during construction or ventilation faults, but regulatory compliance for occupied underground spaces is best demonstrated with long-term integrated measurement covering representative occupancy periods, because radon levels vary significantly with season, ventilation operation, weather-driven pressure differences, and groundwater conditions.