Volume 14 No. 3, 2017
Journal of Residuals Science & Technology (JRST)
doi: 10.14355/jrst.2017.1403.054
Micro-release Mechanism of Radon in
Underground Strata and Factors Affecting Its
Emanation
Wei Zhang*1,2,3, Dongsheng Zhang*4, Wenmin Hu1, Peng Li5, Zhi Yang5
IoT Perception Mine Research Center, National and Local Joint Engineering Laboratory of Internet Application
1
Technology on Mine, China University of Mining & Technology, Xuzhou 221008, China
State Key Laboratory of Coal Mine Disaster Dynamics and Control, Chongqing University, Chongqing 400044,
2
China
Key Laboratory of Safety and High-efficiency Coal Mining, Ministry of Education, Anhui University of Science &
3
Technology, Huainan 232001, China
State Key Laboratory of Coal Resources and safe Mining, China University of Mining & Technology, Xuzhou
4
221116, China
School of Mines, China University of Mining & Technology, Xuzhou 221116, China
5
[email protected]; *[email protected]; Tel: +86-15862187549, Fax: +86-516-83591725
*1
Abstract
This paper begins with a discussion of the source and characteristics of radon, introduces the essence of damages caused by
radon and its daughters, and China’s current radon-prevention limit standards. Theoretical analysis of the release of radon from
a microscopic perspective highlights three main processes: emanation, migration, and exhalation. On this basis, the intrinsic and
extrinsic factors affecting the emanation of radon are presented. Specifically, these factors include stratum lithology, radium
nuclide content, pore size, porosity, particle size, permeability, emanation coefficient, air pressure, temperature, humidity, and
wind velocity. Next, the progress of research into the external and internal causes of radon migration is reviewed. The external
causes mainly include action of diffusion and convection, action of fluids in micropores, action of transportation by microbubbles, action of relay transmission, action of stress-strain, and action of deep-penetration geochemistry, while the internal
causes mainly comprise the composite cluster theory and inherent action of itself. Finally, by reviewing the current research
progress, we present prospects for research in the following five areas: prevention and control of the damages caused by radon
and its daughters, the quantification of radon’s micro-release process, fractal characteristics of radon emanation, radon
migration based on the combination of multiple mechanisms, and radon detection techniques in engineering application.
Keywords
Underground Strata; Environmental Radioactivity; Micro-release Mechanism of Radon; Radon Emanation; Affecting Factors; Migration
Mechanism
Introduction
RADON was first identified in radium-bearing minerals in 1900 by F. E. Dorn, a German physicist. Radon, a
chemical element (86Rn) that generally exists as a gas, is the heaviest known inert radioactive gas that people are
exposed to (other known inert gases include 2He, 10Ne, 18Ar, 36Kr, 54Xe). Radon is an indirect decay daughter of
uranium (92U), a natural radioactive element, and it can be found in underground coal seams, rocks, soil, and
water [1]. If humans inhale air with a high radon concentration, radon and its decay products are deposited on the
respiratory tract, causing radiation damage and maybe even inducing lung cancer [2]. Because of its radioactivity,
radon can be detected even at quite low concentrations. Meanwhile, it is an inert gas that can transport and
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accumulate in micro-fractures or micropores, providing scientific foundations for radon detection on the ground
surface. Additionally, radon is widely applied in many fields as a tracer gas, and its use has contributed to
achievements in residential pollution control [3], human health protection [4], exploration of underground mineral
resources [5], searching for geothermal and bedrock groundwater [6], detecting concealed structures such as faults
[7], forecasting geological disasters such as landslides [8], locating spontaneous combustion regions in
underground coal seams [9], detecting concealed goafs of coal mines [10], and predicting some dynamic coal-rock
disasters such as rockburst [11].
Based on the geophysical and geochemical characteristics of radon, our research groups have focused on applying
the radioactivity measurement method to underground coal mining field. In the past, we conducted pioneering
studies on radon detection of fractures in overlying strata [12]. We hope to establish a new set of simple, easy,
rapid, and reliable method for the advanced prediction of radon detection on surface, thus providing a theoretical
basis and technical support for scientific coal mining in ecologically fragile mining areas in western China.
However, some fundamental and essential issues remain in relation to accurately and reliably solving the
engineering problems in coal mines by means of radon detection. To solve these problems, this study discusses
sources of radon and its characteristics, introduces the essence of damages caused by radon and its daughters, the
existing standards for radon prevention limits in China, and theoretically analyzes the micro-release mechanism of
radon in underground strata. On that basis, this study focuses on the intrinsic and extrinsic factors affecting radon
emanation and reviews the research progress into the external and internal causes of radon migration. Finally, this
paper describes the research prospects from the following five perspectives: prevention and control of the damages
caused by radon and its daughters, the quantification of micro-release process of radon, fractal characteristics of
radon emanation, radon migration based on the combined mechanisms, and radon detection techniques in
engineering application.
Characteristics of Radon and Its Damages
2.1. Sources of Radon
Radium, which has a chemical symbol of Ra and an atomic number of 88, is the direct parent of radon in three
kinds of natural radioactive decay. As many as 27 radon isotopes exist in nature, denoted as 200Rn to 226Rn;
among these, 219Rn (a product of the actinium decay chain, with a half-life period of 3.96 s), 220Rn (a product of
the thorium decay chain, with a half-life period of 55.6 s), and 222Rn (a product of the uranium decay chain, with a
half-life period of 3.82 d) are common. Figure 1 illustrates the whole process of the uranium decay chain. After
multiple α and β decays, radon isotopes eventually produce stable plumbum isotopes (82Pb). Since the content of
219Rn is extremely low in underground strata, and its half-life period is quite short, radon decay process can
almost be disregarded. The concentration of 220Rn is less than 10% of that of 222Rn, which are generated during
deep underground decay, and the half-life period is also quite short; in other words, 220Rn and quickly decays into
other nuclide. Therefore, 220Rn is far less important than 222Rn.
238
234
U
4.5e9 a
Th
24.1 d
234
Pa
1.17 min
6.75 h
234
230
U
2.45e5 a
226
Th
7.7e4 a
Ra
1600 a
222
Rn
3.82 d
218
214
Po
3.1 min
214
Nuclide symbol
A
Z TB
X
X:
A:
Z:
TB:
Nuclide
Mass number
Atomic number
Half-life
Atomic number
Decay mode
U:
92
α-decay
Pa:
Th:
91
90
β-decay
Ac:
89
Ra:
88
Rn:
86
Po:
84
Bi:
83
Pb:
82
Pb
26.8 min
Bi
19.7 min
214
210
Po
1.64e-4 s
Pb
22.3 a
γ photon
210
Bi
5.0 d
210
Po
138.4 d
206
Pb
Stable state
FIGURE 1.THE WHOLE PROCESS OF THE URANIUM DECAY CHAIN
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2.2. Characteristics of Radon
1) Physical Properties
Under standard pressure, the liquefaction temperature and condensation temperature of radon are -65 °C and 113 °C, respectively. Under standard temperature and pressure, the density of radon (9.73 kg/m3) is approximately
7.5 times greater than air density (1.29 kg/m3). As an inert gas, radon shows stronger migration ability than other
gases in the geological environmental system. Not only can radon migrate in gaseous form, but it can also migrate
while dissolved in underground fluids like water, oil, and gas. Radon is freely soluble in water and some organics
like ethyl alcohol and methylbenzene, whose solubility and solution rate decrease gradually as the temperature
rises.
2) Chemical Properties
In the periodic table of elements, radon is one of group-zero elements in the sixth period, whose isotopic atoms
show no charge. A radon molecule consists of a single atom, and the electron configuration outside the atomic
nucleus is 6s26p6 (2, 8, 18, 32, 18, 8) as shown in Figure 2. The outermost layer consists of eight electrons, such that
radon achieves a stable structure. The decay constant of radon is 2.097×10-6 /s, and the entire Ra-Rn radioactive
decay process can achieve an equilibrium state after approximately 30 days. Although radon, as an inert gas,
generally cannot chemically react with other materials, it is extremely prone to adsorption by other substances such
as silicon dioxide, activated carbon, and silica gel. Activated carbon has the strongest capacity for adsorption.
FIGURE 2.ELECTRON CONFIGURATION OF RADON ATOM
2.3. Damages Caused by Radon and Its Daughters
1) Essence of Damages
Radon and all of its daughters are radioactive and exist widely in nature and indoor environments, which endow
them with influence over human health. In recent years, a great deal of radiodosimetry research has been
conducted on this issue, and the results indicate that incidence of lung cancer and leukemia is closely correlated
with high levels of radon exposure [13]. International bodies including the World Health Organization (WHO),
International Commission on Radiological Protection (ICRP), and United States National Academy of Sciences
(USNAS) have recognized the carcinogenic effects of radon exposure, and it is now regarded as the second most
significant factor contributing to lung cancer, after smoking. According to research results from the Chinese Center
for Disease Control and Prevention (CCDCP) [14], the risk of a resident suffering from lung cancer increases as the
indoor radon concentration increases. Figure 3 illustrates the sources of indoor radon. Essentially, radon and its
daughters destroy human bodies through high-energy α particles that generate during the decay process; α
particles have high energy and short range, and after entering a human body they destroy cell structures and
translate normal cells into cancer cells. Inhaled radon and its daughters are mainly deposited in the human
respiratory system, significantly increasing the incidence of lung cancers.
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FIGURE 3. THE SOURCES OF INDOOR RADON
2) Limit Standards
As awareness of radioactive elements grew and contact and exposure increased, Chinese scholars began
investigating and researching radon in the 1980’s, resulting in the radon-prevention limit standards [15] listed in
Table 1. China’s radon concentration control standards are basically consistent with the required control values of
indoor radon and its daughters in ICRP’s No. 65 Publication (1993) and Radiation Protection Statement on Radon
(2009). According to Basic Standards for Protection against Ionizing Radiation and for the Safety of Radiation
Sources (GB 18871-2002), the equilibrium factor for the conversion between the measured concentration and the
equilibrium equivalent concentration equals 0.4.
TABLE 1. RELATED RADON-PREVENTION LIMIT STANDARDS IN CHINA.
Codes
Names
Limits
Remarks
GB/T 16146-2015
Requirements for control of
indoor radon and its progeny
100 or 300 Bq/m3
Annual average concentration
GB/T 17216-2012
Hygienic requirements for peacetime
utilization of civil air defense works
200 or 400 Bq/m3
Equilibrium equivalent
concentration
GB 50325-2010
Code for indoor environmental
pollution control of civil building
engineering
200 or 400 Bq/m3
Measured concentration
GB/T 18883-2002
Indoor air quality standard
400 Bq/m3
Annual average concentration
GBZ 116-2002
Standard for controlling radon and
its progenies in underground space
200 or 400 Bq/m3
Equilibrium equivalent
concentration
Micro-release Mechanism of Radon in Underground Strata
Coal seams, broken rocks, and mine water are the major sources of radon in underground coal mines. Radon atoms
originating in the decay of radium within the strata’s solid grains may not be released into the pore space due to
their very low diffusion coefficients. However, when radon atoms escape into the interstitial space between grains,
they may be released to the surface atmosphere [16]. Figure 4 illustrates the whole micro-release process of radon
atoms from underground strata to the surface atmosphere. Strata or soil represent the U- and Ra-bearing materials
that can release radon; these consist of fine particles with different shapes, sizes, and pores at a micro-scale. The
radon release process includes the following three stages: a) Emanation, the process by which radon atoms move
from the source material grains to the interstitial space between the grains; b) Migration, the process of diffusion
and convection that causes the movement of the emanated radon atoms through the material to the ground surface;
and c) Exhalation, the process by which radon atoms move from the ground surface into the surface atmosphere.
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Surface atmosphere
Ground surface
Ground surface
Exhalation
Migration
Source material grain
Emanation
Radon atom
FIGURE 4. THE MICRO-RELEASE PROCESS OF RADON AATOMS FROM UNDERGROUND STRATA TO THE SURFACE ATMOSPHERE
Factors Affecting Radon Emanation in Geological Environment
An underground stratum is a typical porous medium. Under the synthetic action of multiple factors in its
formation, a stratum is a complex geologic body that contains a great variety of pore structures. The emanation of
radon in geological environments is subject to many factors; it is an intricate and complex process occurring
through generation in a porous medium, migration, and finally emanation from the medium’s surface [17]. The
whole emanation process is not only determined by inherent intrinsic factors such as the porous medium’s
composition, structure, particle size, porosity, permeability, and radon content and distribution, but also affected
by other extrinsic factors like air pressure, temperature, humidity, and wind speed. In summary, the emanation of
radon is a complex process affected by both internal and external factors.
4.1. Intrinsic Factors
Many inherent physical properties of a porous medium significantly affect radon emanation. These factors include
stratum lithology, content of radium nuclide, pore size, porosity, particle size, permeability, and emanation
coefficient.
1) Stratum Lithology
Lithology plays an important role in controlling radon emanation from a stratum [18]. Radon emanation depends
mainly on the stratum’s characteristics rather than on its uranium concentration. Rocks with different lithologies in
an underground stratum differ greatly in their physical properties. For example, relatively more radon emanates
from packsand than from gritstone. It has further been reported that radon emanation is higher from mudstone
than from sandstone because of mudstone’s relatively high specific surface area. Shashkin et al. [19] observed that
radon emanation from coalified wood is generally low due to the adsorption of radon within the coal matrix.
Although the uranium content of black shale is higher than that of granite, higher radon activity concentration has
been reported in granite [20], the concentrations of 222Rn in soil derived from different bedrocks have been listed
in Table 2; this disparity may be due to the concentration of uranium at the grain boundaries and in the microcracks of granite, as well as the adsorption of radon within the matrix of carbonaceous black shale. The emanation
coefficients of rhyolite and granite samples are reported [21] at 0.22 and 0.31, respectively.
1) Content and Distribution of Radium Nuclide
Radium is the direct parent of radon, and the content and distribution of radium in underground strata and soil
determine their inside radon concentration and emanation rate of radon on the ground surface. Related research
results reveal that the content of U or Ra in a porous medium has a linear relationship with the emanation rate of
radon. For example, Sun et al. [22] made field measurements and pointed out that radon’s emanation rate from the
ground surface is positively correlated with the soil’s radon concentration and radium content; however, in a real
environment, radon emanation is susceptible to environmental factors such as moisture. Figure 5 illustrates a linear
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Journal of Residuals Science & Technology (JRST)
Volume 14 No. 3, 2017
relation between emanation rate and radium content. Other research results show that radon concentrations in
porous media such as soil and rocks are not necessarily linearly correlated with radium content [23]. Girault et al.
[24] stated that the non-uniformity of radium content in soil at different depths, as well as significant errors in
measuring radon, may account for the different relationships between radon concentrations and radium contents
in soil.
TABLE 2. CONCENTRATIONS OF 222RN IN SOIL DERIVED FROM DIFFERENT BEDROCKS (UNIT: BQ/L).
Bedrocks
Sample
numbers
Mean
Median
Geomean
Granite
10
30
30
25
Limestone
6
17
14
14
Black shale
12
29
26
26
Phyllite schist
5
12
12
12
Mica schist
5
16
15
16
Banded gneiss
7
44
40
38
Granite gneiss
8
52
36
36
Emanation rate (mBq/m2·s)
140
120
R=0.7480
100
80
y=1.26x-17
60
40
20
0
-20
10
20
30
40 50 60 70 80
Radium content (Bq/kg)
90
100
FIGURE 5.A LINEAR RELATION BETWEEN EMANATION RATE AND RADIUM CONTENT
2) Pore Size
Pore size mainly refers to a pore channel’s shape and size in a porous medium. In fact, pores are extremely
irregular in a medium, but they are generally considered as circles. Therefore, pore size is mainly characterized by
the radius. In 1966, scholars from the former Soviet Union divided pores into four levels while investigating the
causes of coal mine gas leaks [25]; namely, they distinguished micropores (with radii smaller than 10 nm), small
pores (with radii of 10-100 nm), medium pores (with radii of 100-1000 nm), and large pores (with radii over 1000
nm). Pore size determines a gas’s diffusion rate in the medium: the larger the pores, the higher the diffusion rate.
Gases spread with difficulty in media with small pores. Therefore, a wider distribution of large and medium pores
in a porous medium contributes to faster radon diffusion and a larger emanation rate.
3) Porosity
The overall porosity of a porous medium refers to the ratio of the volume of non-solid particles in the medium to
the total volume of the medium; briefly, it describes the prevalence of pores between particles that are occupied by
air or water. Strata and soil represent two typical porous media, and increases in their overall porosity generally
increase the ability of radon to emanate from the medium’s surface [26]. This can be explained as follows: In a lowporosity medium, solid grains are close to each other; therefore, radon atoms in the pore space beneath the surface
find their path to the atmosphere obstructed by grains. If the porosity increases, the emanation rate increases. A
medium’s water saturation is negatively correlated with its effective porosity; moreover, some dead space without
conduction exists in pores, reducing the medium’s effective porosity. Therefore, under low pore pressures, radon
atoms can be easily captured in dead space, such that the emanation rate of radon decreases. Adler et al. [27]
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observed that the porosity of rocks has a more pronounced effect on the radon emanation process. Additionally,
the interconnectivity among pores in a porous medium is an important factor that affects the emanation of radon.
The better the interconnectivity, the easier radon emanation proceeds.
4) Particle Size
Numerous studies have determined that radon emanation is likely to be influenced by particle size in porous
media [28]. According to the research results, grain size varies inversely with radon emanation rate; in other words,
when the grain size increases, the radon emanation rate decreases. The variation of the track densities with the
particle sizes is listed in Table 3. This is explained by the following two facts: On the one hand, reducing the
particle size increases the particle’s surface area and radon’s emanation surface area, thus enhancing the emanation
rate of Radon; on the other hand, the nuclear recoil effect generated during the decay of radon’s parents allows
radon in micropores to escape more easily from the particles and enter the pores, leading to an increase in radon’s
cumulative concentration in porous media, thereby enhancing radon’s emanation rate. Tuccimei et al. [29] reached
similar conclusions while exploring radon’s emanation characteristics in soil. They pointed out that radon
concentration in soil is inversely proportional to soil particle size; radon concentration decreases with an increase
in large sand particles, but increases with an increase in fine sand particles. Radon’s emanation rate significantly
decreases as soil’s average particle size increases.
TABLE 3.VARIATION OF TRACK DENSITIES WITH THE PARTICLE SIZES.
Particle size (μm)
Track density
(tracks/cm2)
63
2475±172
63-150
2138±157
150-300
1213±121
300-600
1300±130
5) Permeability
Permeability is another fundamental physical quantity that measures the capacity of fluid to disperse through a
porous medium. Medium permeability is closely correlated with its particle size, components, porosity, and pore
connectivity. Comparing porous media with the same porosity, the substance with larger pores possesses higher
permeability, because smaller pores impose greater resistance on the fluid’s flow. Given the same air pressure
variations, the seepage effect in a porous medium with higher permeability more significantly affects radon’s
migration behavior, which is beneficial for radon emanation mainly through seepage. For example, Burke et al. [30]
found that the emanation of radon and the permeability of a porous medium show a favorable linear relationship,
such that decreasing permeability directly leads to a reduction in radon emanation.
6) Emanation Coefficient
It is well known that radon atoms are generated from the decay of radium atoms inside solid grains in a medium
like soil or rock. A fraction of the radon atoms escapes from those solid grains into the inner pore space; this
fraction is called the emanation coefficient [31]. The emanation coefficient increases smoothly with increasing water
content until it reaches a certain saturation level in the pore space. If radon atoms can acquire a higher kinetic
energy during the formation process, they can pass through the pore space between the medium’s particles and
become embedded in the opposite particles. Radon’s emanation distance is shorter in water-filled pores than in airfilled pores. Accordingly, if the medium has low moisture content, the existence of water in pores can increase the
probability that radon atoms are retained in pores, thus enhancing the emanation coefficient. Furthermore, radon’s
exhalation rate in a wet porous medium is generally higher than in a dry medium. Radon’s diffusion coefficient is
far smaller in water than in air, and radon atoms’ convection motions caused by air pressure can only occur in pore
spaces without containing water; therefore, if a medium has high moisture content, the presence of water hinders
radon atoms’ diffusion and convection in pores. For that reason, the radon emanation rate on the surface decreases
as moisture content increases. The emanation rates from the dry and moist uranium ores have been presented in
Table 4.
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TABLE 4. EMANATION RATES FROM THE DRY AND MOIST URANIUM ORES.
Emanation
rate from dry
uranium ore
(×10-3Bq/m2·s)
0.53
0.46
0.35
9.07
4.76
3.54
1.71
1.85
16.08
1.75
Emanation
rate from
moist
uranium ore
(×10-3Bq/m2·s)
0.70
0.64
0.52
15.23
6.78
6.80
4.21
9.34
25.71
4.88
Ratio
1.33
1.39
1.52
1.68
1.42
1.92
2.47
5.05
1.60
2.79
4.2. Extrinsic Factors
Extrinsic factors that affect radon emanation include meteorological parameters like air pressure, temperature,
relative humidity, and wind velocity. Many researchers have studied the relationship between these factors and
radon emanation.
1) Air Pressure
Differences and distribution of air pressure play an important role in radon transport from a medium. The air
pressure gradient between the air and a porous medium produces radon’s vertical migration, i.e., seepage.
Increased air pressure promotes radon’s seepage towards the medium. While decreased air pressure forces radon’s
seepage outward. In the presence of a larger air pressure gradient, radon’s seepage velocity is greater, leading to a
higher emanation rate. Clements et al. [32] reported that a pressure change in a material’s pore space from 1% to
2% increases the radon emanation rate from 20% to 60%. However, the effect of air pressure on radon emanation is
not usually observed in high-pressure mine areas.
2) Temperature
A difference between the ground temperature and the air temperature produces differences in density and
pressure between the medium and the air on the medium’s surface, causing gas convection. An increase in air
temperature reduces air density and pressure; therefore, radon migrates towards the medium surface and the
emanation rate increases. An increase in the medium’s temperature also increases radon’s diffusion coefficient and
emanation rate. Thus, we can conclude that higher temperatures are associated with larger radon emanation rates.
Balek et al. [33] investigated the temperature dependence of radon emanation within a temperature range varying
from -80°C to 250°C and observed strong radon adsorption at temperatures below -20°C. It has also been observed
that the alpha-recoil range and diffusion length of radon in grains are almost same when the room temperature
varies between -50°C and 50°C. At temperatures exceeding 100°C, the release of radon from the grains is enhanced
because its diffusion coefficient in solids becomes significantly higher.
3) Humidity
Variations in a porous medium’s humidity lead to variations in the medium’s emanation coefficient and diffusion
coefficient. At high humidity, radon is more severely bound by the medium, and it shows slower diffusion and a
lower emanation rate; in contrast, at low humidity, radon’s emanation rate is higher. Kojima et al. [34] reported
that radon’s emanation rate does not show significant change with light rainfall (within 13 mm), but it decreases
dramatically with heavy rainfall (over 93 mm) and remains low for several days afterward.
4) Wind Velocity
Wind velocity can affect the difference in radon concentration between the internal porous medium and its surface
by varying radon’s migration velocity on the medium’s surface. At a higher wind velocity, radon atoms spread
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more rapidly on the medium’s surface; this increases radon’s emanation velocity in the medium. An increased
wind velocity of 7 m/s enhances the emanation rate by about 15% [35].
Reseach Procgress Regarding Mechanisms of Radon Migration
Since the discovery of radon, many scholars have investigated its mechanisms for migrating from deep
underground to the ground surface. Their conclusions have resulted in grouping migration mechanisms into the
following two categories: external and internal causes.
5.1. External Causes
1) Action of Diffusion and Convection
The first hypotheses about the migration of radon attributed it to diffusion and convection. Flugge et al. [36]
proposed the diffusion theory based on Fick’s law in 1936, and Fleischer et al. [37] proposed the convection theory
in 1979. Both theories attempted to explain radon migration; specifically, the former theory hypothesized that
radon migration is induced by differences in concentration, while the latter hypothesized that radon migration is
induced by pressure gradients. Soonwala et al. [38] proposed an analytic solution for the diffusion and convection
theory for radon migration in overlaying strata and pointed out that the diffusion and convection should be
comprehensively considered when calculating the velocity of upward movement of radon. Liu et al. [39]
established a physical model for radon migration (length is 2.0 m, width is 1.5 m, height is 1.0 m) and
experimentally proved the feasibility of the diffusion and convection theory in describing radon migration in nearsurface soil and air. According to Li’s research results, diffusion dominates radon migration in low-permeability
soil, while convection dominates radon migration in high-permeability soil; in most cases, radon migration results
from the combined action of diffusion and convection [40]. This theory has been widely accepted by most scholars
in China and abroad, such that it generally serves as the basis for accounting for the migration of radon.
2) Action of Fluids in Micropores
Fluids are widely present in underground rock pores; moreover, these fluids are in constant motion. Therefore,
fluid migration drives the migration of dissolved radon, forming the theoretical basis for analyzing radon
migration due to the action of fluids in micropores. Based on Darcy’s law, Tanner et al. [41] proposed micro-porous
fluid action mechanism to explain radon migration. Wilkening et al. [42] pointed out that radon’s short half-life
period prevents it from migrating over long distances in underground rocks, and the long-distance migration of
radon therefore relies on the rapid movement of underground fluids. Malmqvist et al. [43] measured radon
concentrations in soil at different depths using the inverted cup method, and drilled rock cores from the same
profile to measure radon diffusion behaviors. These measurements indicate that radon in bedrock migrates by
being carried by flowing gas or water. Zhang et al. [44] pointed out that capillary-size pores, pre-existing fissures,
and fractured and shattered zones in underground rocks all serve as favorable migration channels for
underground fluids, allowing radon that is dissolved in these fluids to move towards the earth surface.
3) Action of Transportation by Micro-bubbles
Kristiansson et al. [45] proposed the micro-bubbles’ transportation mechanism to account for radon migration.
According to this theory, the rising ground airflow forms micro-bubbles when passing through an underground
aquifer; these bubbles then carry radon from deep underground to the earth’s surface. Somogyi et al. [46] arrived at
a similar conclusion. Varhegyi et al. [47-48] constructed a quantitative model to describe the migration of
underground radon carried by micro-bubbles that are produced by rising airflows; moreover, they investigated the
roles of geo-gas and water in radon migration, derived the theoretical curves of the vertical distribution of radon
concentration in water, and experimentally verified the theoretical results in cylindrical PVC tubes under two
conditions, tube full of water and saturated water with plastic particles, respectively.
4) Action of Relay Transmission
The relay transmission theory was proposed based on the following two considerations: (1) radon isotopes have
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short half-life periods and thus cannot travel over long distances; and (2) Ra nuclides, the parent of radon, have a
long half-life period and can carry out long-distance movements. Through analysis, Folger et al. [49] concluded
that the long-distance migration of radon in water is closely related to its parents (uranium and radium), but the
migration still relies on other external carriers. Bai et al. [50] proposed that the halo degree generated around
underground uranium ore (including a primary halo and a secondary halo) determines radon migration distance
in strata; specifically, the migration distance of radium, the direct parent of radon, plays a decisive role. The same
study constructed a broad, preliminary hypothesis of radon relay-transmission migration mechanism. Wu et al. [51]
proposed the relay transmission action (two-baton or multiple-baton relay) that was used to explain the
exploration of deep mineral reserves, prediction of earthquakes, and research in geodynamics using Rn-based
methods. Specifically, relay transmission may refer to the transmission from radium to radon or the transmission
from radon to radon, or transmissions with the aid of the other substances or natural forces so as to achieve longdistance migration of radon.
5) Action of Stress-strain
Variations in stress and strain can cause the compression and expansion of rock pores, the generation and
expansion of fractures, deformation and secondary effects in rocks, and diastrophism and creep in faults. All of
these phenomena can produce radon migration. Figure 6 illustrates a qualitative pattern of radon concentration
(CRn) changes associated with stress changes (Q). Ma et al. [52] proposed the stress/strain action theory to account
for the migration of radon. According to this theory, high-pressure gas produced underground in the crust drives
the upwelling of radon, giving rise to radon’s precursor anomaly in near-surface water and soil; the anomaly’s
degree depends on the strength of the ascending flow. Wattananikorn et al. [53] hypothesized that the Earth’s
interior is in a stress extrusion state, and internal micropores constitute a non-linear narrow channel; therefore,
radon migrates constantly towards the earth’s surface. Based on the physical relationship between radon
concentration in soil and the rock’s effective stress, Dutta et al. [54] evaluated a method for analyzing the
geodynamical model using radon volume, reaching the conclusion that variations in underground radon
concentration rely on variations in pre-earthquake stress and strain. Che et al. [55] found that underground waterbearing strata are similar to rocks in mechanical properties; they can be deformed under low stresses, leading to
variations in hydrodynamic characteristics and radon in underground water. The same hydrodynamical
mechanisms can account for the anomaly of radon in underground water.
6) Action of Deep-penetration Geochemistry
Action of deep-penetration geochemistry was proposed by Cameron, a Canadian scholar, and Xie, an academician
of Chinese Academy of Sciences, in a conversation during the 16th International Geochemical Exploration
Conference in Jerusalem in 1997 [56]. They also discussed some new methods for effectively exploring the
distribution of concealed deposits several hundred meters below the surface. Wang et al. [57] concluded that three
continuous factors constitute the whole process of deep-penetration geochemistry action: the migration of
underground chemical elements towards the earth’s surface, the occurrence state of chemical elements on the
surface, and the anomalous precursor pattern. Figure 7 illustrates a conceptual model of deep-penetrating
geochemical migration. Li et al. [58] pointed out that the anomalous 222Rn observed on the earth’s surface is not
directly produced from deep uranium ore bodies; instead, under deep-penetration geochemistry action, the direct
parent 226Ra or the indirect parent 238U migrates upward, deposits, and decays to produce 222Rn. This can also
explain the long-distance migration of radon.
Other external factors that can account for radon migration include geothermal action, suction action, and
pumping action [59]. Due to the little contribution from these actions for radon migration, we do not go into detail
about these factors.
5.2. Internal Causes
Compared with the extensive research on external causes, few studies have been conducted on the internal causes
of radon migration, and only some conclusions have been reached under ideal conditions. These studies are mainly
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the work of Chinese scholars.
CRn
Elastic strain
Failure
Failure
C0
-Q
Extension
Q0
+Q
Compression
FIGURE 6. QUALITATIVE PATTERN OF RADON CONCENTRATION (CRN) CHANGES ASSOCIATED WITH STRESS CHANGES (Q).
Curve of element content
Unloading zone
Geo-gas stream
Transmission zone
Geo-gas stream
Geo-gas stream
Surrounding rock
Ore body
FIGURE 7. A CONCEPTUAL MODEL OF DEEP-PENETRATING GEOCHEMICAL MIGRATION.
1) Composite Cluster Theory
Jia’s research team from Chengdu University of Technology in China proposed the composite cluster theory [60-61].
According to this research, although radon and its daughters have high densities, they show significant capacities
for upward migration within ideal air. This is due to the fact that radon and its daughters and parents are mostly
α-radiators that produce α particles during the radioactive decay process; the α particles become helion (4He) after
deceleration and then react with radon and its daughters and parents to form composite clusters as shown in
Figure 8. When the buoyancy of a composite cluster exceeds its gravity, the cluster ascends, serving as an internal
cause of the upward migration of radon and its daughters. In other words, during migration, radon and its
daughters can be regarded as long-lived radioactive elements, and the distance of their ascent in the crust far
exceeds dozens or hundreds of meters. Additionally, the helion, its parents, or its daughters constitute clusters
because of the Van der Waals forces between them, which form an airflow stream that migrates upward. Taking
into account external causes such as underground temperature differences, pressure differences, upward
movement of underground water, and convection, an airflow stream that moves significantly upward would
certainly form.
1) Inherent Action of Itself
Liu’s research team from Taiyuan University of Technology in China first conducted experiments on the inherent
migration laws of radon and its daughters under ideal conditions [62]. Using a α-cup, they measured the
probabilities that radon and its daughters migrate along vertical and horizontal directions; based on the results,
they then conducted contrast experiments along three directions [63]. According to the experimental results, they
concluded that radon’s inherent migration is primarily vertical. The upward migration ability, which strengthens
as the distance increases, far exceeds the capacities for downward and horizontal migration.
In conclusion, there are relatively a lot of research achievements on mechanism of radon migration, but no matter
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which kind of migration theory or action mechanism, it is an indisputable fact that radon could migrate from
underground to the earth surface. What is more, action of diffusion and convection is still considered as a main
migration mechanism, which has laid a relevant theoretical basis to carry out radon detection on surface.
He
He
He
He
He
Rn
He
He
He
FIGURE 8. SCHEMATIC DIAGRAM OF COMPOSITE CLUSTER.
Research Prospects
(1) The damages radon and its daughters cause to human bodies are essentially damage from high-energy α
particles that are generated during the decay process. High-energy α particle can destroy human cell structures
and cause normal cells to become cancerous. However, the damages caused by radon and its daughters have been
poorly investigated by the Chinese academic community. In particular, as the economy has improved, massive
amounts of decorative materials such as marble and natural granite have been used in residential houses,
potentially aggravating indoor radon pollution. Therefore, it is urgent to strengthen investigations into indoor
radon damage in urban buildings, to research radon pollution control, and to formulate and perfect radiation
safety standards for related building materials.
(2) The micro-release process of radon includes the following three stages: a) Emanation, the process of radon
atoms’ moving from the source material grains to the interstitial space between the grains; b) Migration, the
process of diffusion and convection driving the movement of the emanated radon atoms through the material to
the ground surface; and c) Exhalation, the process of radon atoms’ moving from the ground surface into the surface
atmosphere. Future studies of radon’s micro-release process should focus on achieving quantitative calculations
from qualitative descriptions.
(3) The emanation of radon is quite a complex process, and it is subject to the combined action of many intrinsic
and extrinsic factors, including stratum lithology, radium nuclide content, pore size, porosity, particle size,
permeability, emanation coefficient, air pressure, temperature, humidity, and wind velocity. Because of the
complexity of the porous medium’s internal structure, we can try to incorporate fractal theory to explore radon
emanation characteristics in porous media and apply the research results to the governance of radioactive
environments. For example, we can reduce radon emanation rate by varying the porous medium’s fractal structure,
so as to effectively prevent and control radon pollution.
(4) Radon migration mechanisms can be grouped into two categories: external and internal causes. External causes
mainly include action of diffusion and convection, action of fluids in micropores, action of transportation by microbubbles, action of relay transmission, action of stress-strain and action of deep-penetration geochemistry, while
internal causes mainly comprise the composite cluster theory and inherent action of itself. In our opinion, the
migration of radon is a complex ‘black-box’ process that greatly depends on the external environment; in most
cases, the migration can hardly be explained by a kind of action mechanism. Therefore, future studies of the
migration mechanisms of radon should focus on the combined effects of multiple mechanisms.
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(5) Reviewing the research into the migration mechanisms of radon can yield fruitful results. In all of these studies
on radon migration and its mechanisms, it is indisputable that radon can migrate from deep underground to the
ground surface. Moreover, diffusion and convection are still regarded as the primary causes of radon migration.
These research results provide related theoretical foundations for radon detection on surface. On this basis, future
studies could further strengthen radon detection techniques in engineering application, such as monitoring of
underground water movement and radon pollution prevention in coal mining areas.
ACKNOWLEDGMENT
We acknowledge the financial support for this work provided by the National Natural Science Foundation of
China (No. 51404254), the National Basic Research Program of China (No. 2015CB251600), the Scientific Research
Foundation of State Key Laboratory of Coal Mine Disaster Dynamics and Control (No. 2011DA105287-FW201602),
the Research Fund of Key Laboratory of Safety and High-efficiency Coal Mining, Ministry of Education (No.
JYBSYS2015106), the China Postdoctoral Science Foundation (Nos. 2014M560465 and 2015T80604), the Jiangsu
Planned Projects for Postdoctoral Research Funds (No. 1302050B), the Jiangsu Qing Lan Project (No. 2016-15) and
the Natural Science Foundation of Jiangsu Province of China (No. BK20140186). We also wish to thank Mapletrans
Company in Wuhan, China, for its professional English editing service. Special thanks are given to the anonymous
reviewers for their constructive comments and helpful suggestions.
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