Analysis of Radon Measurement Methods
and Current Application Status in China
Wei Zhang
Associate researcher, Key Laboratory of Safety and High-Efficiency Coal Mining,
Ministry of Education, Anhui University of Science & Technology, Huainan, Anhui,
232001, China; IoT Perception Mine Research Center, The National and Local
Joint Engineering Laboratory of Internet Technology on Mine, China University of
Mining & Technology, Xuzhou, Jiangsu, 221008, China; School of Environment
Science and Spatial Informatics, China University of Mining & Technology, Xuzhou,
Jiangsu, 221116, China
Corresponding author, e-mail: [email protected]
Peng Li
Master student, School of Mines, China University of Mining & Technology,
Xuzhou, Jiangsu 221116, China
e-mail: [email protected]
Dongsheng Zhang
Professor, State Key Laboratory of Coal Resources and Safe Mining, China
University of Mining & Technology, Xuzhou, Jiangsu, 221116, China
e-mail: [email protected]
Zhi Yang
Master student, School of Mines, China University of Mining & Technology,
Xuzhou, Jiangsu 221116, China
e-mail: [email protected]
ABSTRACT
This paper introduced the physical and chemical properties of radon from the distribution of
radionuclide deposition in coal bearing formations. Based on the analysis of several common
measuring methods (instantaneous measurement, cumulative measurement, and continuous
measurement) and China's current radon measurement standards, it was determined that a
practical method must be applied according to different measuring situations, purposes, and
objects and combined with relevant measurement standards. Furthermore, the effects of time,
cost, and other factors should also be considered. We also summarized the current status of
China's research and application of radon measurement from the perspective of environmental
pollution and health protection, geological engineering, and preliminary exploration of coal
mine safety. The new detection technology involves various technologies with technical
problems that need to be solved, especially involving external factors that influence the
process of radioactive decay. Therefore, the author suggests that the radon measurement
methods should be continuously improved in the aspect of accuracy, especially after research
has made important breakthroughs of radon migration theory in sedimentary coal bearing
formations. Moreover, it is possible to further widen the application field and scope of radon
measurement.
KEYWORDS: sedimentary coal bearing formations; radionuclide; radon measurement
method; application status; radon migration
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INTRODUCTION
In 1900, radioactive radon (chemical symbol Rn) was discovered in radium-bearing minerals
by the German physicist F. E. Dorn. In 1902, researchers in Marie Curie’s group proposed the
idea that certain solid substances release radioactive inert gas and should be studied as a new
topic of physical and chemical analysis [1]. It was their theory that methods could be used to insert
materials into radium isotopes to be analyzed, and that radionuclides go through alpha decay after
releasing radon; this idea led to the initial prototype of a radon measurement method. With the
continuous development of modern science and technology, radon measurement methods are
increasingly being used to solve specific field engineering problems [2], and have been
acknowledged by technical personnel as being successfully applied in related fields. It should be
noted that most applications are only tentative explorations such as measuring the concentration
of radon on the earth’s surface; there is no in-depth understanding of the geological foundation
and radon migration mechanism. Thus, it is difficult for one measurement method to be widely
applied in various fields.
Different geological conditions produce different radon anomalies, which is an important
basis for the interpretation of subsurface inconsistencies. The formation of radon migration
mechanism is a complex problem, and many international scholars have studied the topic. A
number of hypotheses have been proposed including diffusion and convection, fluids in
micropores, transportation by micro-bubbles, stress and strain, relay transmission, deeppenetrating geochemistry, composite cluster theory, and the Inherent action of itself [3-10].
Although radon anomalies cannot be explained by a single mechanism, it’s clear that deep
underground radon can migrate to the surface, which provides the basis for measuring radon
levels. This paper briefly introduced the physical and chemical properties of radon from the
perspective of distribution of radionuclide deposition in coal bearing formations. Then, several
common radon measurement methods were analyzed, and the current radon measurement
standards in China were evaluated. Finally, from the aspects of environmental pollution and
health protection, engineering application in geological field, and preliminary exploration of coal
mine safety, an overview of the research on the application of radon measurement in China in
recent years was provided.
DISTRIBUTION CHARACTERISTICS OF
RADIONUCLIDES IN SEDIMENTARY COAL BEARING
FORMATIONS
Coal bearing formations belong to the stratigraphic concept, which refers to the formation of
a series of coal or coal seams formed in a certain geological period. In addition to coal, Coal
bearing formations are usually accompanied by other mineral resources. China's main Coal
bearing formations consist of the Carboniferous Permian coal forming period (e.g. Juye coalfield),
Late Permian (e.g. Liupanshui coalfield), Triassic (e.g. Pingxiang coalfield), Jurassic (e.g.
Shenfu-Dongsheng coalfield), Cretaceous (e.g. Tiefa coalfield), and Third century (e.g. Longkou
coalfield). Because of the existence of a certain amount of radionuclides in the coal strata, the
coal bearing rocks have a certain radioactivity [11]. For example, the coal bearing formations in
North China are mainly terrestrial sediments, mainly composed of clastic rocks of various sizes
(including clay rocks). Clay rock is one of the most common sedimentary rocks, making up about
Vol. 22 [2017], Bund. 04
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45%-55% of sedimentary rocks [12]. The main mineral composition of clay rock is kaolinite (about
80%-95%), which is formed by the weathering of granitic rocks with high radionuclide content.
The content of radionuclides in an underground coal seam depends on the elemental
composition of the original matter of the coal formation, the geochemical action of the coal
forming period, and the coal forming process. Research [13] has shown that the adsorption of
radioactive substances by humic acid in the initial stage of formation is an important aspect of
radioactivity enrichment. Uranium has a strong organic affinity, and is widely distributed in low
metamorphic coals (especially lignite). With the increase of the capacity of coal metamorphism,
uranium adsorption is gradually weakened, and diffusion and convection result in the release of
uranium to the coal seam roof, which leads to floor enrichment. Furthermore, continental
sedimentary coal gangue is mudstone, thus coal gangue sometimes also contains high levels of
radioactivity. In addition, the radionuclides in the surface layer (soil) are generally derived from
the bedrock, which are related to the composition of the coal bearing formations. When the
thickness of the overburden layer is shallow, the radioactive anomaly in the coal bearing
formations is continuous from the bedrock to the surface. Therefore, the continental sedimentary
environment provides a good geological basis for the application of radioactive measurement.
PROPERTIES OF RADON
A group of radioactive nuclides composed of a “living” mother nuclide and a decaying one
is called a radioactive series. At present, there are three natural radioactive series in nature
(Uranium series, Thorium series, and Actinides series) [14], and the atomic number in these series
is between 81 and 92. A radioactive gas (radon) is produced in the course of the decay of the
three natural radioactive series.
Physical properties
Radon is a colourless, odourless, tasteless, and rare radioactive gas, and is not detectable
with human senses. At standard temperature and pressure, it is a monatomic gas with a density of
9.73 kg/m3, which is about 8 times denser than the atmosphere at sea level (1. 217 kg/m3). Radon
has the highest density of rare gases, and it is also one of the densest gases at room temperature.
Although radon presents a colourless state at standard temperature and pressure, it will glow as a
result of radioactivity at temperatures below -71. 15° C and changes from yellow to orange with
decreasing temperature. Radon is slightly more soluble in water compared to other rare gases, and
its solubility in organic compounds is much higher than that in water [15].
Chemical properties
The atomic number of radon is 86, and its most stable isotope is 222Rn, which is the decay
product of 226Ra, while 226Ra is the decay product of 238U. Radon’s radioactivity makes it difficult
to study its chemistry. Because the radon atom has 8 outer valence electrons, it is inert to most
common chemical reactions such as combustion reactions. This electron configuration forms a
stable low energy configuration, while the outer electrons tightly bind in atoms. According to the
trend in the periodic table, the electronegativity of radon is lower than xenon, meaning it has
higher chemical activity than xenon. There are a few known compounds from radon, which
belong to fluoride or oxide [16].
Vol. 22 [2017], Bund. 04
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RADON MEASUREMENT METHODS AND RADON
MEASUREMENT STANDARDS IN CHINA
Radon measurement methods
The instruments and methods currently used mainly depend on the type of radiation
measured, the duration of the measurement, and whether the radon decay daughter is measured.
The primary measurement methods include instantaneous measurement (Double filter membrane
method, Scintillation method, and Balloon method), cumulative measurement (Activated carbon
method, Track etching method, and Thermo-luminescence detection method), and continuous
measurement (Diffusion method and Electrostatic spectrometry) [17]. The instrument choice is
determined according to the following three categories: (1) Classification according to different
measurement occasions, such as air radon measurement, soil radon measurement, and water
radon measurement, which each include a variety of methods. (2) Classification according to
different purposes such as to survey uranium deposits, explore oil, predict earthquakes, determine
environmental pollution, etc. (3) Classification according to the determination of the object such
as for the direct measurement of radon itself and indirect measurement of its first decay daughter
radioactivity. In the measurement of radon and its progeny, choosing the suitable method is the
key to achieving reasonable results. At present, there are many radon and its progeny
measurement methods, some of which can quickly and accurately obtain radon concentration,
while some obtain the average value over a period of time. Therefore, the choice of measurement
methods should not only meet the needs of scientific research, but also give full consideration to
other factors such as time, cost, and conditions.
According to the above principles, one measurement method was selected from each of the
three categories (e.g. instantaneous measurement, cumulative measurement, and continuous
measurement). The corresponding pros and cons, as well as application circumstances, were
discussed.
(1) Instantaneous measurement: The double filter membrane method was chosen from this
category. The measurement method of FT648-type emanometer (CNNC Nuclear Instrument
Factory, Beijing, China) is a typical double filter membrane method. In the process of pumping,
the inlet filter membrane filters out the existing radon daughters in the air, and the pure radon
generates a new daughter (mainly 218Po) in the process of the double filter tube. The radioactivity
in the outlet membrane is measured, and the radon concentration in the air can be calculated
according to the decay law of the radon daughter. The advantage of this method is that it can be
used to measure the concentration of the daughter (inlet filter) and can also measure the primary
radon concentration (outlet filter) with great lower limit (about 3. 7 Bq/m3). This method is a
quick and convenient operation. Drawbacks of this method are that it is necessary to ensure that
the outlet membrane is not contaminated by radon outside the two membranes and it is largely
affected by relative humidity.
(2) Cumulative measurement: The activated carbon method was chosen from this category.
The activated carbon method generally refers to the passive adsorption method and is based on
the characteristics of strong adsorption capacity for inert gas, which could potentially measure
radon content. An activated carbon sampler is usually made of plastic or metal, with a membrane
or bronze powder-constructed filter at the inlet. After radon is diffused into the activated carbon
bed and the radon decay progeny is deposited on the activated carbon bed, a gamma ray
Vol. 22 [2017], Bund. 04
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spectrometer can be used to measure the spectral peak, which can in turn calculates the adsorbed
radon amount. Next, this value is converted into an average radon concentration using scale
calculation. Advantages of this method include convenience of sampling, and it is reusable and
suitable for large-scale radon surveying. The disadvantage is that the adsorption by activated
carbon is a cumulative process, meaning the average radon concentration is affected by different
sampling. Therefore, this method can only be used for short term measurements (2-7 days).
(3) Continuous measurement: The electrostatic diffusion method was chosen in this category.
Typical measuring instruments include the 1027-type continuous emanometer (Sun Nuclear
Corporation, Melbourne, USA) and the RAD7-type emanometer (Durridge Company
Incorporated, Billerica, USA). The principle of this method is that radon concentrations are
different inside and outside of the detector. The radon enters into the cavity through diffusion
from the outside to the inside, which reaches equilibrium within 15 minutes. The radon diffused
into the chamber decays into radon progeny (mainly 218Po positive ions); subsequently, 218Po
further decays to produce α particle, which can be collected in the central electrode. Then,
through the electronic circuit, pulse shaping is obtained, which in turn yields the radon
concentration in the air based on relative scales. The advantage of this method is that the
detection limit is low, meaning it can be used for the measurement of indoor radon concentration
and it is also suitable for continuous monitoring of the dynamic changes of radon concentrations.
However, disadvantages include the high cost of corresponding measuring instruments and the
complicated operation process.
Radon measurement standards in China
Table 1 lists the current standards on radon measurement methods in China [18]. The methods
described here are mostly based on the principle of measuring α particles produced by the
radioactive decay of radon, which is converted into radon concentration. However, gamma rays
or parent radium content are also used to calculate radon concentration. Therefore, each of these
methods has advantages and disadvantages. In the measurement process, the appropriate method
should be selected according to the specific circumstances.
Table 1: Radon measurement standards in China
standard number
GBZ/T182-2006
GBZ/T155-2002
GB/T16143-1995
GB/T14582-1993
name of standard
method
specifications for monitoring of
indoor radon and its decay
products
scintillation flask method for
measuring radon concentration in
the air
charcoal canister method for
measuring 222Rn exhalation rate
from building surface
RaA method
track etching method
activated carbon method
standard methods for radon
measurement in environmental
air
scintillation method
activated carbon method
track etching method
double filtering membrane method
activated carbon method
Balloon method
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RESEARCH STATUS OF CHINA'S APPLICATION OF
RADON MEASUREMENT
Due to its special physical and chemical properties, domestic and foreign scholars gradually
began to use radon to solve practical engineering problems. For example, researchers in Germany
and the United States used 222Rn as a tracer to estimate groundwater discharged into lakes,
reservoirs, and the ocean through the acquisition and measurement of the concentration of 222Rn
profiles at different water depths, which could be potentially used for tracking pollutants. A joint
group from United States, Japan, and Thailand found that compared to 222Rn, 220Rn has a shorter
half-life, which can determine the location of underground water pollution emissions more
accurately than 222Rn. These results greatly expanded the application of radon measurements, and
gradually formed the initial stage of current radon measurement technology. Scholars in China
became interested in the application of radon measurement more recently than countries
mentioned previously, but have made great strides in recent years.
Environmental pollution and health protection
Radon is a radioactive gas, and when its concentration exceeds a certain value will cause
serious pollution to indoor environments, which increases the incidence of lung cancer and
leukemia. "Code for Indoor Environmental Pollution Control of Civil Building Engineering (GB
50325-2010)" clearly stipulates the following: class I civil engineering limit is less than or equal
to 200 Bq/m3, class II civil engineering limit of less than 400 Bq/m3. Therefore, residents should
take measures to reduce indoor radon concentrations, such as drilling water injection to the house
foundation, and increasing the ventilation. In addition, when the radon concentration is less than a
certain dose level (i.e. within the body's maximum allowable dose range) radon can kill bacteria,
which plays a role in the protection of human health [19]. For example, "Geologic exploration
specification of natural mineral water (GB/T 13727-92)" clearly stipulates that the medical level
of mineral water (radon) quality standards for dissolved radon concentration is around 3. 7e4
Bq/m3.
Engineering application in geological field
1.
Exploration of unknown underground mineral resources
One study introduced an abnormal status of radon concentration in the Liaoning
Lianshanguan – Qijiapuzi area, showing high peak status primarily along the contact zone of rock
[20]
. According to the soil radon measurement results, researchers successfully delineated five
abnormal radon zones, and in which they identified three abnormal uranium deposit holes, which
provided effective information for the exploration of deep uranium resources in the area. Another
study discovered that radon often distributes along oil and gas fields around micro cracks and oil
(gas) water boundaries with upward migration [21]. Therefore, radon measurements in the Sichuan
basin, Moxi, and Mayangchang gas field showed a zonal radon anomaly consistent with the gas
water boundary. Moreover, radon measurements in the Xinjiang Zhunzhong basin area confirmed
the oil (gas) water boundary and oil and gas field area, which supported the above conclusion,
achieving good effect for oil and gas discovery.
2.
Searching for geothermal and bedrock groundwater
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1259
Previous studies used water radon and soil radon measurement methods to provide
information about regional radon concentration anomalies, which revealed groundwater in
Jiamusi City, Heilongjiang [22]. Combined with other unusual external information, researchers
determined a likely prospecting area of underground hot water in the city. Another group used α
cup method of radon measurement [23]. The field detection results of Chengdu Longtang Temple
area found many meaningful radon anomalies according to hydrological data; the detection
results were confirmed when a three-aquifer structure was discovered. This solved the problem of
water sources in that area.
3.
Detection of concealed structures such as faults
One study in Henan Huangcun performed a soil radon measurement along the strike
direction, and the results revealed a soil radon concentration anomaly above the fracture site,
where it corresponded to the strike position [24]. The study further identified the main fracture
surface and fracture location. Another study demonstrated a radon concentration difference inside
and outside a karst collapse column, which showed abnormal characteristics of radon
concentration on the surface above [25]. Then, the approximate location of the karst collapse
column and range was determined by the peak concentration of radon. The results were verified
in the Shanxi Yangquan Coal Mine and Lu'an Shigejie Coal Mine site.
4. Prediction of landslide and other geological disasters
A previous study measured the soil radon concentration from four landslide types and its
surrounding areas in Gansu Province [26]. The analyzed results showed the relationship between
characteristics of radon concentration distribution from four types of landslide areas (horizontal
and vertical) and their prediction coefficient (critical time point and specific location). Another
research characterized a precursory pattern of groundwater radon anomaly characteristics
corresponding to strong earthquakes in North China [27]. Two types of abnormalities were
proposed: the ordinary term anomaly (rise type) and short term anomaly (turning type). The
images of different seismic precursory anomalies were similar, which provided a theoretical basis
for the prediction of earthquake disasters in North China.
Preliminary exploration of coal mine safety
1.
Location of spontaneous combustion of coal seam
Spontaneous combustion in an underground coal seam seriously affects the safety of coal
mine production, and it is necessary to understand the underground coal spontaneous combustion
position before the ignition point can be identified. Scholars from Taiyuan University of
Technology first carried out basic research of detecting underground fire by using radon in China.
For example, reference [28] proposed the first domestic controllable condition of medium low
radioactive radon bench created to carry out experiments for revealing the relationship between
the radon concentration in coal gangue and environmental temperature. They found that the
ground radon measurement technique effectively detected the location of underground fire areas
in complicated terrains and ranges. In recent years, a number of field engineering practices have
shown that the use of surface radon measurement methods can provide technical support for mine
fire prevention. For example, one research introduced a radon measurement method that
determined the fire area and high temperature zone before the governance of the Shanxi Taiyuan
Wanbailin District Dongfeng Coal Mine combustion [29]. This provided direct evidence for fire
Vol. 22 [2017], Bund. 04
1260
control and monitoring. After the fire was under control, secondary radon detection showed that
the fire was successfully controlled, and which confirmed the accuracy of the radon detection in
the coal mine gob area.
2.
Detection of concealed old goaf in coal mine
The hidden old goaf areas in coal mines pose great potential danger to production and
ground construction. Domestic researchers have carried out practical engineering detection of
buried coal mine goafs with radon measurement methods. For example, reference [30] described
the radon concentration differences between the top of coal mine goafs and non goaf areas. And
through the ground, α cup measurement of radon concentration in Zhangcun Coal Mine, Shanxi,
verified the feasibility of such an approach. Another study applied radon measurement in small
coal mines of unknown goaf in Yuxian, Shanxi [31]. The method rapidly identified the coal mine
goaf area and its influence range, which effectively shortened the detection duration and reduced
operation costs.
3.
Coal rock dynamic disaster prediction and early warning
Coal and gas outbursts, rock bursts, and other disasters are typical coal-rock dynamic
disasters. To this end, relevant scholars used radon measurement for non-contact prediction of
coal-rock dynamic disasters. For example, reference [32] established a mathematical model for
continuous radon measurement, discussed the relationship between the radon concentration and
gas volume fraction in coal mine ventilation roadways, and obtained statistical curves of changes
on radon and gas concentrations with time. Then, the theory of gray system fitting analysis was
used on the grey correlation degree, showing a good correlation between the two. Another paper
proposed a model of radioactive radon anomaly with coal and gas outbursts based on a common
prediction method [33]. Based on this, a radioactive radon concentration anomaly index for coal
and gas outbursts was established, and its feasibility was validated by experimentation.
CONCLUSION
(1) Sedimentary coal bearing formations contain certain radioactivity, which depends on the
coal raw material elements and the earth's physical and chemical effects during the coal forming
period. Radioactive nuclides on the earth’s surface are generally derived from underground
bedrock, which is related to the composition of coal bearing strata. The continental sedimentary
environment provides a good geological basis for the application of radioactivity measurement.
(2) The current radon measurement methods can be classified into three categories including
instantaneous, cumulative, and continuous. However, in the specific measurement process,
different measurement occasions, purposes, and object should be taken into consideration and
combined with the relevant measurement standard. Then, a reasonable measurement method
should be selected. At the same time, the effects of time, cost, and other factors should also be
considered.
(3) Due to various disciplines involved in the radon measurement methods, such as nuclear
radiation detection, nuclear analysis technology, and statistical analysis, many technical problems
must be solved. Especially given the fact that many external factors can influence the process of
radioactive decay; for example, rock types, permeability, and porosity can all affect radioactivity.
Vol. 22 [2017], Bund. 04
1261
Therefore, in the follow-up study, continuous improvement on the accuracy of radon
measurement should be sought.
(4) As a new detection technology, radon measurement methods have been potentially
applied for engineering practice in related fields, e.g., environmental pollution and health
protection, engineering of geological field, and preliminary exploration of coal mine safety.
Along with the development of in-depth radon understanding, especially after the progress of
radon migration theory in sedimentary coal bearing formations, radon measurement could be
implemented in many more fields and scopes.
ACKNOWLEDGEMENT
The research is financially supported by the National Natural Science Foundation of China
(No. 51404254), the National Basic Research Program of China (No. 2015CB251600), 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) and the Jiangsu Qing Lan Project (No. 2016-15). Special thanks are given to
Mapletrans Company in Wuhan, China, for its professional English editing service.
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© 2017 ejge
Vol. 22 [2017], Bund. 04
Editor’s note.
This paper may be referred to, in other articles, as:
Wei Zhang, Peng Li, Dongsheng Zhang, Zhi Yang: “Analysis of Radon
Measurement Methods and Current App-lication Status in China” Electronic
Journal of Geotechnical Engineering, 2017 (22. 04), pp 1253-1264. Available
at ejge.com.
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