An Overview of Radon Emanation Measurement System for South African Communities

Abstract

The aim of the study is to evaluate radon emanation levels in South African communities and to implement possible strategies to reduce radon levels in order to minimize potential health hazards. The major contributing factor to high levels of radon is the history of mining. To precisely measure emanation levels of radon indoors and outdoors, calibrated radon detectors are necessary. In this study, areas of high radon emanation levels are spotted, and based on the radon emission point or entry points in buildings, applicable and possible mitigation strategies are discussed for implementation.

1. Introduction

Radon is the most contributing source of ionizing radiation to human beings, in the atmosphere. It is a colorless, invisible, undetectable gas to human senses that causes lung cancer especially in areas where it is abundant. Areas of high levels of radon include underground uranium mines and locations close to mine tailings of which its buildings are non-ventilated. Lubin et al. [1] state that about 40% of deaths in mine workers are linked to the radon emanation in underground mines, in United States, radon is the second most contributing source to deaths due to lung cancer [2]. The International Commission on Radiological Protection recommends that the radon concentration level in dwellings should not exceed 300 Bq/m³ [3]. In South Africa, radon from areas of possible high radon emanation is a concern due to the history of mining, disposal of uranium tailing, use of mine waste for building materials [4]. Areas of high radon emanation include:

• Large Tailing Storage Facilities in Gauteng Province. Mine Tailings mismanagement run off to the public.

• Granite hill in Saldanha Bay area and houses built on granite bedrock in Paarl, Western Cape.

• Houses built by uranium ore stockpiles in Karoo, Western Cape.

Mitigation strategies to reduce radon to acceptable levels to the public and mine workers are of necessity for implementation. Strategies such as reducing radon levels in underground mines by ventilation systems, reducing radon emanation in tailing dams by pouring 2 m of clay and topsoil to trap radon, and identifying radon entry points for sealing in houses or mines. To evaluate the effectiveness of the mitigation system to reduce radon to ensure safe public health, a properly calibrated radon detection device is necessary to ensure accurate exposure levels of radon in workplaces and houses [5]. With a calibrated radon detector, results of measurement before and after application of a mitigation system will absolutely indicate the rate of reduction of radon emanation. Therefore, buildings can be fixed if high radon emanations are detected, shortening the statistics of lung cancer deaths due to excessive exposure to radon.

The most common method of calibrating the radon detectors is exposing them to a steady flow of radon concentration from a standard radon source in an airtight radon chamber, under controlled environmental parameters [6]. A facility to calibrate radon detectors in a designed radon chamber at the Centre of Applied Radiation Science and Technology is in development phase.

In the following subsections, radon radiological properties, its transport behavior in the environment, and mitigation system will be introduced.

1.1 Radon radiological properties

²²²Rn is a noble radioactive isotope of atomic number 86, and it originates from the decay chain of ²³⁸ U as one of its decay product, ²²⁶Ra transforms into ²²²Rn by alpha particle emission as seen in Figure 1.

Figure 1.

The emission of radon from radium in underground rocks or soil to water [7].

Radon alpha particles travel a distance of 4–7 cm with an energy of 5.5 MeV. Alpha particles’ energy from radon daughter nuclei is more than that of radon, the parent. Polonium (²¹⁸Po) with half-life of 3.05 minutes has energy of 6.0 MeV, while polonium (²¹⁴Po) has 7.7 MeV. Hence, radon daughter nuclei are dangerous and pose health risk in an enclosed space such as offices and houses.

1.2 Transport of radon in the environment

Radon is found in rocks and soils, and its abundance in the environments depends on moisture, porosity, and the activity of uranium and radium in soils and rocks. The concentration of radon in soil differs due to the concentration of its parent radionuclide and the ability of the soil to emanate radon.

Factors that affect the mobility of radon in the soil include porosity, moisture content of soil, and permeability. Hosoda [8] found that the moisture content of soil in the range of 0–8% in a rectangular volume of 2840 cm³ increases radon emanation but, moisture content that is above 8% decreases radon emanation. Permeability of soil such as gravel and sand allows the transport of radon gas from several depths in the ground as seen in Figure 2. Impermeable soil such as clay and silt has low porosity, and therefore, the transport of radon is small. Decreased levels of radon in clays are due to the amount of water content in them, but in the dry clays with cracks, radon migration is more in non-cracked clay [10, 11].

Figure 2.

Permeability of soil from underground to surface soil [9].

Rocks with varying degrees of radium activity content are the source of radon in groundwater. The measure of ²²²Rn activity differs from surface water of lakes or rivers, as radon in underground water is not mobile as in the surface of the surface of the earth. Methods of obtaining water from underneath the earth in wells or boreholes disperse high levels of radon to the atmosphere [12]. Figure 3 illustrates the dispersion of radon from rocks to water.

Figure 3.

Migration of radon in underground radium rocks to water [7].

1.3 Mitigation systems

Radon mitigation system is a practical approach to minimize high levels of radon emanation in identified areas through determining radon entry points and the nature of the foundation of buildings. In mines, to deal with radon entry points, radon sealants and bulkheads are used to block or control radon emanation from rocks, while in dusty atmospheres of the mine dust, controls such as air filters are of use [13] . In communities closer to radon-prone areas, natural ventilation system is important followed by radon suction system to drive radon away from the house or building.

1.4 Aim

This work presents an overview on radon emanation measurements and the possibility of applying mitigation system to reduce the public exposure to radon, which will in turn improve the public health and allow for better land use.

2. Radon measurement

Radon is measured by a wide range of techniques developed over years and improved in sensitivity, portability, and performance parameters [14]. The radon measurement technique mostly depends on the concentration of radon-222 and the characteristics used to classify them as follows:

• whether the technique measures radon-222 and radon progeny

• detection of radioactive decay mode—whether alpha, beta, or gamma

• time resolution

An alpha particle detected can be used to measure radon-222 decay products by using scintillation counters. Conversely, in the measurement of radon-222, gamma rays emitted from radon-222 daughters can be used to determine or measure radon-222, for example, bismuth-214 and lead-214 [15].

Time resolution sampling analysis and measurement are classified into three techniques, namely grab sampling, continuous technique, and integrating technique. Integrating technique utilizes passive detectors to measure the integrated radon-222 concentration annually or monthly mostly in buildings. Continuous technique provides the simultaneous act of sampling and determination of radon-222 in samples of soil or water in minutes, hours, and days with a device such as scintillation radon monitors and smart radon duo. Grab sampling technique includes the collection of samples of groundwater air samples, in a short time whereby devices such as RAD7 are used to measure radon-222 in samples collected. Awhida [16] states that radon measurement technique differs to the point that one technique cannot meet the requirement such as radon survey type, environmental parameter measurement, and cost of the apparatus.

2.1 Determination of radon-222 by its progeny

Baskaran [17] states that the decay of ²²²Rn leads to the formation of numerous radon-222 progenies before ²¹⁰Pb:

²²²Rn (3.82)$\to$²¹⁸Po(3.10 min) $\to$²¹⁴Pb(26.8 min) $\to$²¹⁴Bi(19.7 min) $\to$²¹⁴Po(40.2 min) $\to$²¹⁰Pb(22.3 yr) $\to$²¹⁰Bi(5.0d) $\to$²¹⁰Po (138.4d).

All the progenies of ²²²Rn are metals, and the longest-lived progeny is ²¹⁴Pb. Polonium(Pb) and bismuth (Bi) are particle reactive and attach to atmospheric aerosol particles. ²²²Rn concentration can be determined from the activity concentration of its progenies. Using Geiger Muller counter, ²²²Rn activity concentration can be determined from the alpha particles of progenies such as ²¹⁸Po (6.002 MeV), ²¹⁴Po (7.687 MeV) or 214Bi (β_max = 3.272 MeV), and ²¹⁴Pb (β_max = 1.022 MeV) beta decay particles.

2.1.1 ²²²Rn determination by solid-surface barrier detector

One of the ways of determining ²²²Rn activity it is through alpha spectrometry by counting alpha emitting progenies of ²²²Rn.

A known quantity of radon air is drawn by two filters in a cylindrical system, whereby one filter keeps the radon progeny and particular matter, the other filter receives radon. On the second filter, ingrowth of radon takes place, the second filter faces the surface barrier detector, and therefore, alpha spectra are produced.

In a simple method, positively charged progenies are collected onto a metal surface that is placed inside chamber’s negative potential. For example, the positively charged 218Po particles gathered on a metal surface are counted by a surface barrier detector.

The advantage of using a surface-barrier detector is high resolution, low background, ability to distinguish the signals of ²²²Rn progenies and ²²²Rn due to the fact that they have varying alpha energies [17].

2.1.2 ²²²Rn determination using beta counter

Assuming that there is secular equilibrium between ²²²Rn and its progeny, the beta particles of ²¹⁴Pb and ¹²⁴Bi on a filter can be used for determining the radon concentration by measuring their beta activity. Beta counter such as Geiger Muller counter is used to count the radon progeny on filter paper. Also, plastic scintillators installed on photomultipliers tubes can be used to determine activity.

2.1.3 Direct progeny monitoring technique

One of the ways to measure a time integrated radon progeny for determination of radon is with direct radon progeny sensor, which is made up of solid-state nuclear track detector fitted with absorber of sufficient thickness. The direct radon progeny absorber comprises aluminized Mylar and cellulose nitrate appropriate thickness of 37 cm, whereby 7.67 MeV alpha particles emitted from ²¹⁴Po are detected [18].

2.1.4 Radon chamber

A radon chamber is a container that houses a radon atmosphere for calibration of radon detectors and conduction of studies in the field of radon metrology. It consists of tools for relative humidity, pressure, and temperature, which must be at a constant range for calibration of radon detectors. Table 1 summarizes the type, size, and accommodation capacity of radon chambers; furthermore, the radon generating source must be traceable to an international or national standard.

Type	Size	Accommodation capacity	Cost
Accumulation	Normally small in size (0.2–1.3	Accommodates limited number of radon detectors	Cost less to build
Flow-Through, Walk in chamber	Normally big in size	Accommodates a lot of radon detectors	Expensive to build

Table 1.

Radon chamber.

3. Studies of indoor radon emanation levels in South Africa and possible mitigation system

Over the years, mining activities and mine dumps without proper regulation or disposal have led to the rise in background activity due to the presence of naturally occurring radioactive materials (NORM). Naturally occurring radioactive materials are in varying activities in soil, rocks, and underground water. In mining activities, when NORMs are processed, their activity becomes increased and is known as TENORM. NORMs contain radionuclides from the decay of uranium-238, thorium-232, and potassium-40, which result in the formation of radium and radon, posing a health hazard to exposed workers or nearby communities.

Many of the gold mines in South Africa process NORMs, and there is lot of mine waste, of which some is closer to communities in the Gauteng province [19]. There are abandoned mines of which some communities or instance Tudor-shaft houses build on top of mine tailings. An intervention of government and regulatory body in terms of regulating mines especially waste and dust control is necessary for the lessening of health threat by radioactive materials from contaminated sites. Maximizing distance between the mine and communities and restricting access to abandoned mine dumps or tailing dams help in preventing NORMs from reaching dwellings.

3.1 Radon emanation mitigation system

Radon mitigation system is a set of steps designed with a goal to achieve the reduction of radon in buildings in order to minimize the risk of lung cancer as seen in Figure 4. USEPA [2] recommends that radon levels of 148 Bq/m³ (4 pc/l) or above in buildings must be reduced by radon mitigation system, also radon levels between 74 Bq/m³ (2 pc/l) and 148 Bq/m³ (4 pc/l) are to be considered for radon reduction, as no level of radon is safe.

Figure 4.

Radon mitigation process [20].

For the application of radon mitigation systems on houses, how the house is constructed in terms of its foundation, as seen in Figure 5, determines the mitigation system. There are mainly three types of house foundation, namely:

• slab on grade (concrete poured at ground level),

• basement and

• crawlspace (a shallow unfinished space under the first floor)

Figure 5.

Foundation types [21].

USEPA [2] recommends radon reduction methods that prevent radon from entering the house rather than a method that deals with radon when it has entered the house. A Radon Specialist runs diagnostic test to check possible pathways where radon emanates by shooting chemical smoke into cracks, holes, or drains and observing possible emission points. Then mitigation methods are followed.

3.1.1 Basement or a slab-on-grade foundation

For house foundations that are basement or a slab-on-grade foundation type, a radon mitigation system applicable is commonly one of four types of soil suction: active or passive sub-slab suction, drain-tile suction, block-wall suction, or sump-hole suction [2]. The sub-slab suction also known as sub-slab depressurization is a commonly used system whereby holes are drilled on the foundation of the house to beneath the foundation (to crushed rocks or soil), for insertion of suction pipes. Number of suction pipes installed depends on how radon air can be extracted from beneath the house concrete or slab with the use of the radon vent fan connected to the suction pipes to the air outside. Drain tiles or perforated pipes are commonly used in some homes for directing water away from the foundation. Therefore, suction on these pipes or tiles can effectively reduce radon levels.

Block wall suction is a radon mitigation system that is good for houses with basement where their walls are made of hollow blocks. The block wall suction system consists of a fan and ductwork, radon air is drawn from the hollow blocks of the basement and vented outside through exhaust fan. The block wall suction system is advantageous over other mitigation system as it prevents radon from reentering the building [22].

3.1.2 Crawlspace

Sub-membrane suction system is a mitigation system ideal for houses without basement. A high-density polyethylene plastic sheet is used as a radon reduction barrier by covering the exposed dirt or soil on the floor and also the walls as seen in Figure 6. Then, suction pipes are installed through the plastic sheet to depressurize the soil and draw the radon gas outside the house through installed radon fan and suction pipe [23].

Figure 6.

Example of sub-membrane suction system [23].

Other types of radon mitigation systems applicable to any house foundations include sealing, natural ventilation, house or room pressurization [2].

The sealing of cracks and openings method to is the primary part of most radon mitigation systems. Identification of areas where there is radon entry is mostly not easily detectable as cracks happen overtime. Sealing of cracks or voids minimizes the flow of radon into the house.

House or room pressurization system keeps radon air trapped in the basement by the blowing of the air into the basement. The limitations of the technique affecting its effectiveness are the house construction, appliances in the house or house occupants’ lifestyle.

Natural ventilation happens in all houses. Ventilation for radon reduction can be improved by opening doors, windows, and vents on lower floors. When windows and doors are closed, radon concentration returns to its previous value in about 12 hours [2].

3.2 High indoor radon levels in South Africa and applicable radon mitigation system

3.2.1 Gauteng Province

In South Africa, there are studies of indoor radon measurements, some of which indicate levels of radon higher than 148 Bq/m³ recommended by EPA [24]. In a study done by Radebe [25] for the design of radon chamber, alphaguards that were calibrated measured radon levels above 1000 Bq/m³ from Tudor shaft soil samples.

Tudor shaft, an informal settlement in Krugersdorp, is known to be affected by gold mine shaft, and tailing dam is one of the areas of note that radon mitigation methods must be implemented. The inhabitants of Tudor shaft built their shacks on top of mine dump with radioactive uranium soil as seen in Figure 7. Vegetables are grown on top of soil potentially carry high levels of radioactivity and environment due to polluted air from mine tailings. The community is at risk of radioactive health risk arising from mine dumps. Typical entry points in shacks in Tudor shaft comprise cracks on the floor or ground, holes on mats used to cover the soil, and gaps found on the edges of the interior of the shack between the mat and the shack or poles. It is also noted that when doors and windows are closed, radon accumulates and returns to an average value, while during the day, it reduces indoors.

Figure 7.

(a) Example of shack interior for Tudor shaft. (b) Children playing on soil that has potentially higher levels of radiation. (c) Shacks built on soil that potentially have higher levels of radiation [26, 27].

A radon mitigation system applicable to Tudor-shaft inhabitants for shacks can be a passive sub-slab depressurization system. A passive sub-slab system relies on stack effect, which is a term that defines radon reduction by the reliance on air pressure differentials to extract radon from underneath the foundation to the outdoor air via vent pipes. Mats or floor covers must be checked to any opening and removed for the installation of a high-density polyethylene plastic to retain the radon gas. Then the radon vent pipe can be installed for moving the radon retained by the plastic to the outdoor air as seen in Figure 8. Furthermore, when a floor cover or mat is placed on top of the high-density polyethylene plastic, openings or voids must be sealed with caulk or epoxy sealant. Passive sub-slab system together with sealing and natural ventilation application can reduce the radon in informal settlement such as Tudor shaft especially for shacks without electricity means. The government has done a good job in relocating some families affected by mine dumps.

Figure 8.

Passive sub-slab system [28].

A best solution would be to relocate the community to a better place. A temporary solution would be to apply radon mitigation methods to minimize the risk of lung cancer, also community awareness about reducing emanation of radon via natural ventilation and sealing of openings or cracks is important.

3.2.2 Western cape

Houses in Paarl, Western Cape province, with a type of crawlspace foundation used for storage by occupants recorded radon levels off up to 800 Bq/m³ as seen in Figure 9 [30]. Also houses with wooden floor recorded higher radon levels than concrete floor. At the foothill off Paarl Mountain, higher levels of radon levels were found and some of the houses are nearby the foothill. An active sub-slab depressurization system and sub-membrane suction system for crawl space can be applied for houses at Paarl.

Figure 9.

Radon in Paarl crawlspace type of a foundation [29].

Furthermore, methods of foundation crack repair and staples are essential for the reduction of radon in houses, although mitigation systems are the best. Foundation crack repair inhibits the entrance of radon indoors. By utilization of products such as concrete staples or epoxy, which works like a glue, and holes or cracks that are in the concrete or wooden foundation are repaired and reinforced [31], which thus stops the primary entrance of radon in houses.

The houses with walls having stability problems tend to lean or bow; therefore, carbon staples can be used to repair and reinforce the cracked or bowed wall to achieve radon emanation reduction. Epoxy, which comprises epoxy resin and hardener, is essential for closing the cracks in foundation where there is radon entry [31] .

4. Conclusion and recommendation

There are areas of high levels of radon emanation in South Africa. and there are mitigation techniques to be applied. Therefore, there is a need of trained radon mitigation specialist to precisely reduce radon in radon-prone areas to lower levels. This will prevent rates of lung cancer from going high and thus support good public health. In addition, radon calibration facilities play a major role in determining accurate measurement of radon by radon detectors. Implementation of radon awareness campaigns that cover topics of radon mitigation is necessary with collaboration with the national regulatory body to affected communities.