THE SAHLGRENSKA ACADEMY
DEPARTMENT OF RADIATION PHYSICS
TIME-RESOLVED MEASUREMENTS OF
RADON CONCENTRATION IN THE
STOCKHOLM SUBWAY TUNNELS
M.Sc. Thesis
Christoffer Andersén
Essay/Thesis:
Program and/or course:
30 hp
Medical Physics Programme
Level:
Semester/year:
Second Cycle
At/2015
Supervisor:
Mats Isaksson, Magnus Ahnesjö, Kirlna Skeppström
Examiner:
Report no:
Magnus Båth
Abstract
Essay/Thesis:
Program and/or course:
30 hp
Medical Physics Programme
Level:
Second Cycle
Semester/year:
Supervisor:
Examiner:
Report No:
Keyword:
At/2015
Mats Isaksson, Magnus Ahnesjö, Kirlna Skeppström
Magnus Båth
Radon, Tunnel, Subway
Purpose:
The purpose of this study is to examine how radon concentration levels vary in a
subterranean environment (subway) and evaluate potential interconnections between
radon concentration levels and factors like geology, pressure, temperature and
humidity.
Theory:
Radon is a progeny from the decay chain of uranium which is found in various amounts
in different kinds of rock. Radon exhaled from soil and rock can accumulate in confined
spaces like houses or underground tunnels. Currently very little is known about the
radon concentration levels in the Stockholm subway system and therefore it is difficult
to make an assessment of the workers exposure to radon.
Method:
Two pulsed ionization chambers, alphaGUARD, were used to measure the radon
concentration levels, relative humidity, pressure and temperature. A GR-130 BGO
detector was used to measure the gamma radiation and give an estimation of the
potassium, thorium and uranium content. The geology assessments where perform by
BergAB. The measurement sites were located along the blue line in the Stockholm
subway system.
Result:
All the measurement locations were under the Swedish Work Environment Authority’s
exposure limit for radon concentration when working below ground. The mean radon
concentration for the blue subway line was 130 ± 10 Bq/m3. This report focuses on
radon concentration levels for workers in the tunnel and the results are not applicable
to the public. Radon concentration, temperature and humidity seem to be interconnected
but the direct interconnection between could not be seen.
Table of content
1.
Introduction .................................................................................................................................... 1
1.1
2.
Radon ......................................................................................................................................... 2
1.1.1
Radon decay chains............................................................................................................. 2
1.1.2
Health effects of radon ........................................................................................................ 3
1.1.3
Radon source: rocks ............................................................................................................ 4
1.2
Naturally occurring radioactive nuclides ................................................................................... 5
1.3
Literature study: Variations in indoor radon concentration ....................................................... 6
1.4
Radon progenies and the equilibrium factor .............................................................................. 7
1.5
Methods for radon measurements .............................................................................................. 7
1.5.1
Pulsed ionization chamber .................................................................................................. 7
1.5.2
Etched track detector ........................................................................................................... 8
1.5.3
Electret ................................................................................................................................ 8
Materials and methods ................................................................................................................ 10
2.1
Instruments ............................................................................................................................... 10
2.2
Study sites ................................................................................................................................ 10
2.2.1
Geological gamma map and underground subway stations .............................................. 10
2.2.2
Measurement locations ..................................................................................................... 12
2.2.3
The trains daily schedule....................................................................................................... 16
2.3
Methodology ............................................................................................................................ 16
2.4
Uncertainty estimation and analysis......................................................................................... 17
3.
Results ........................................................................................................................................... 18
4.
Discussion and conclusions .......................................................................................................... 22
4.1
Variations of radon................................................................................................................... 22
4.2
Radon concentration and train movements .............................................................................. 22
4.3
The impact of air circulation .................................................................................................... 22
4.4
Peaks and valleys of radon ....................................................................................................... 23
4.5
Different tunnel types............................................................................................................... 24
4.6
Tunnel mean and exposure limits ............................................................................................ 24
4.7
Uranium estimates and radon concentrations .......................................................................... 24
Outlook ................................................................................................................................................. 25
Acknowledgment ................................................................................................................................. 26
Reference list ........................................................................................................................................ 27
Appendix A .......................................................................................................................................... 30
1. Introduction
Radon is a radioactive noble gas that arises mainly from decay of thorium and uranium. Thorium and
uranium are present in various amounts in different kinds of rock from which radon emanates into the
surrounding air. In areas with poor ventilation, radon can accumulate for example in underground
structures or in houses. Radon exposure can cause lung cancer and is the most common cause next after
smoking.
Since the 1950s epidemiological studies have shown a significantly higher incidence of lung cancer
amongst miners in uranium and zinc mines. This increased risk for lung cancer is due to exposure of
airborne radon isotopes and their short-lived progenies also called radon daughters. Radon and its
daughters are present in the atmospheric air and inhalation of these radionuclides contributes to the
exposure from natural radiation sources. Present studies have concluded that miners exposed to radon
have a higher incidence of lung cancer which cannot be explained by other factors (ICRP, 1987), (Darby
et al., 2004).
The radon concentration levels are seen to vary significantly from house to house and also with time
and season. These types of variations are also seen in underground constructions. It is found that the
radon concentration levels are higher in underground constructions where water is present due to the
ability of water to transport radon.
Although research indicates that there are higher levels of radon gas in subterranean structures, there
are not many studies about this. Measurements are made quite frequently over areas where the public
temporarily stay but measurements for some working areas are lacking, often because of unclear
situation of whose responsibility it is to perform measurements. For example, in the tunnels of the
Stockholm subway system owned by the County Council, most of the work carried out in the tunnels is
done by subcontracted entrepreneurs.
This leads to a lack of knowledge about radon concentration levels in the tunnels, which in turn leads
to difficulties in making radon exposure assessments for the workers working in the tunnels. The radon
concentration varies with ventilation, which also affects different factors such as temperature, humidity
and air pressure, which in turn might affect the radon measurements.
The aim of this study is to examine how radon concentration levels vary in a subterranean environment
(subway) and evaluate potential interconnections between radon concentration levels and factors like
geology, pressure, temperature and humidity.
1
1.1 Radon
1.1.1 Radon decay chains
The noble gas radon originates naturally from Uranium-238, Uranium-235 and Thorium-232 where it
forms as a decay product:
U  …  226Ra  222Rn (“Radon”)  …
238
U  …  223Ra  219Rn (“Actinon”)  …
235
Th  …  224Ra  220Rn (“Thoron”)  …
232
If the decay time is short the radon isotope will not be able to escape from its place of origin before
decay. Radon-222 “radon” has a half-life of 3.82 d, “thoron” has a half-life of 55 s and “actinon” has a
half-life of 3.9 s. In natural uranium the ratio 235U/238U is as low as 0.00725 which makes it acceptable
to omit the contribution from actinon when discussing dose to the lung tissue.
When radon decays it goes from the gaseous state into being metallic particles. These particles can
attach to aerosols in the air, which humans can inhale. The inhaled particles stick to the respiratory tract
(Marsh & Bailey, 2013). In Figure 1 and 2 it is shown that these metallic particles also known as short
lived radon daughters decay by alpha decay, sending out an alpha particle and the sensitive tissue in the
lung is exposed to alpha radiation.
Figure 1 – the decay chain of thorium-232 which ends with
lead-208. Radon-220 with half-life 55 s is also known as
thoron. Image: (Wikipedia, 2011).
Figure 2 – the decay chain of uranium-238 which ends with
lead-206. The relative thickness of the arrows indicates the
probability of each decay path (thicker arrow – more probable
path). Radon-222 with half-life 3.8 days is commonly referred to
as radon. Image: (Wikipedia, 2015).
2
The short lived radon daughters have half-lives ranging from 300 ns to 10.6 h for the thorium decay
series and between 164 µs to 26.8 min for the uranium decay series. In addition to an isotope’s
radiological half-life it also has a biological half-life, the time for half of the amount of the substance to
be expelled from the body. The biological half-life can for some isotopes be shorter than the radiological
half-life. This means that the nuclides will be expelled from the body before the majority of the nuclides
can decay, however this is not the case of the short lived radon daughters which decay before they can
be removed from the lung tissue. Lead-210 on the other hand has a long enough radiological half-life to
be removed from the lung tissue before it decays. Thorium and uranium is present in rocks but due to
thoron’s short half-life only a small fraction is able to diffuse from its place of origin before it decays
into a metal particle. Because of radon’s longer half-life it is therefore more likely to inhale the radon
daughters instead of thoron daughters. As seen in Figure 1 Polonium-218, have two different decay
paths. When discussing the short-lived radon daughters it is the path that is most probable that is
considered. The considered isotopes are presented in Table 1. The table gives information of the shortlived radon daughters’ half-life and which radiation type is sent out during the decay (Chu S et al., 1999)
Table 1 - The decay chain of uranium when considering the most important radionuclides in regards to radon and its
short-lived daughters. The short-lived radon daughters are listed in boldface. Source: (Chu S et al., 1999).
Radionuclide
226
Ra
222
Rn
218
Po
214
Pb
214
Bi
214
Po
210
Pb
210
Bi
210
Po
Half-life
1600 y
3.82 d
3.05 min
26.8 min
19.9 min
164 µs
22.3 y
5.0 d
138.4 d
Radiation type
α
α
α
β, γ
β, γ
α
β, γ
β
α
1.1.2 Health effects of radon
The main exposure of radiation comes from inhalation of the short-lived radon daughters. The inhalation
of the daughter-nuclides will lead to an inhomogeneous dose distribution within the respiratory tract.
When the radon gas itself is inhaled it is often also exhaled before depositing energy through radiation.
That is because radon is a noble gas which is not very reactive, it does not chemically bind to the body’s
tissue or dust particles present in air. The radon daughters however are more reactive and attach
themselves to condensation nuclei and dust particles present in air, which, when inhaled, can bind to the
respiratory tract. This is why the radon daughters contribute mainly to the dose and not the radon gas
itself. Because the progenies have short half-lives they deposit most of their dose in the bronchial
epithelium, except for Pb-210, which has a longer radiological half-life and biological half-life of 18
days (Chamberlain, 1991). The human body does not make a difference between radioactive and nonradioactive lead so except for a few decays the lead will get out of the body before it deposits all of its
energy.
It is stated that the α-particles can cause damage to cells; if the cellular damage is not effectively
repaired the cell will try to prevent itself from surviving or reproducing. It can also result in two other
outcomes: a deterministic and a stochastic effect. The first outcome: If a few cells are lost it will not
affect the organ that the cells build up but if the loss of cells are larger then it will affect the function of
the organ. It is therefore argued that this effect has a threshold and above the threshold the severity of
the damage is increased with dose. This type of effect is called a deterministic effect. The second
outcome: If the cell is viable after the irradiation it can have mutations in its DNA. This type of effect
will not be expressed immediately unlike the deterministic effect that can occur only moments after the
irradiation. This effect has no threshold and the severity will not increase with the dose, however the
probability of the effect occurring will increase with the dose. This type of effect is called a stochastic
effect. Examples of deterministic effects are cataract and erythema. An example of stochastic effects is
3
cancer (Hall & Giaccia, 2012). Because the epidermis consists of dead skin tissue, the α-particles will
not penetrate to living tissue when radiating from outside the body. The tissue inside the body on the
other hand does not consist of dead tissue and it is therefore more likely that α-particles will cause
damage there. The radon daughters can make their way into the lung via aerosols and then the lung
tissue is exposed to α-radiation.
Epidemiological studies have proven that there is a correlation between respiratory cancer and radon
exposure for workers in mines and underground constructions. Three larger studies from Colorado, USA
(Lundin F E Jr et al., 1971); (National Academy of Sciences, National Research Council, 1972);
(Whittemore & McMillan, 1983), Bohemia, CSSR (ICRP, 1987) and Ontario, Canada (Chovil, 1981);
(Muller J et al., 1983) indicate that there is a strong correlation between respiratory cancer and cumulated
radon daughter exposure. The studies from USA indicated that factors other than radiation were not
responsible for the excess malignancies (Lundin F E Jr et al., 1971). Radon daughters and asbestos
appear to be most strongly carcinogenic in association with cigarette smoking (National Academy of
Sciences, National Research Council, 1972); (Whittemore & McMillan, 1983). In the Canadian study it
was found that most of the “excess deaths” from non-neoplastic lung disease were assigned to silicosis
and chronic interstitial pneumonia (Muller J et al., 1983).
Studies in non-uranium mines in Sweden have also been made, which have found a significant excess
of lung cancer (Snihs, 1973); (Radford & St. Clair Renard, 1984). Swedish studies also support the
theory of a synergistic effect for radon daughters and smoking leading to respiratory cancer (Damber &
Larsson, 1982). A study from USA looked at radon in water and found an increase in lung cancer but
could not exclude that other factors were responsible for the increase (Hess C T et al., 1983). It can thus
be concluded that radon exposure will give stochastic effects. The International Commission on
Radiological Protection (ICRP) concludes that a linear exposure-risk relationship can be assumed below
500 WLM (1770 mJ h/m3, ~ 800 MBq h/m3 for 170 h) for lung cancer from inhaled radon daughters.
WLM, working level month, is the cumulative exposure resulting from breathing in an atmosphere with
a concentration of 1 working level during a working month of 170 hours. A working level is any
combination of the short-lived progeny in one liter of air that will result in the emission of 1.3×105 MeV
potential alpha energy. Estimation of the attributable lifetime risk on the basis of a relative risk concept
seems to be more appropriate than an absolute risk model. The ICRP also suggests that the risk is
independent of sex and age at exposure (ICRP, 1987).
Pooled analyses from Europe, North America and China suggest a relative risk of developing lung
cancer of at least 16 % per 100 Bq/m3 of residential radon exposure. The study also estimated that the
risk for smokers versus non-smokers is 25 times higher (ICRP, 2010). This is supported by Darby et al.
(2004).
1.1.3 Radon source: rocks
The parent nuclides, thorium and uranium, are naturally found in rock, soil and building materials. The
most common source of radon is the soil which also causes the highest indoor concentrations of radon.
When a parent nuclide decays to a radon nuclide it also becomes airborne, which makes the rocks, soil
and building materials exhale the radon into the air. This means that if trying to localize a radon source,
one could start by looking where the uranium concentration is high. For igneous rocks it is found that
ultrabasic rocks have very low content of uranium (0.014 ppm) and granites and pegmatites have a
higher content (2-15 ppm). In metamorphic rocks the content of uranium is generally varying between
0.2-11 ppm. In sedimentary rocks the concentration increases with content of clay, phosphorus and
organic matter. Usually sandstones, shales and limestones contain low amounts of uranium (Edsfeldt,
2001). Table 2 is a concentration table from the Swedish Radiation Safety Authority. It shows the
common concentrations of uranium, thorium and potassium in various rock types. These three elements
contribute most to the natural terrestrial radiation and are mostly appearing together with rocks that are
formed when magma rich in silica penetrates from the earth’s interior, e.g. granite and syenite. The
granite magma also enriches uranium, thorium and potassium to parts that solidifies last which fills the
cracks in the bedrock forming aisles of aplite and pegmatite. Especially pegmatites therefore often have
significantly higher levels of uranium, thorium and potassium compared to granite. The lowest levels in
rocks are found in basic rocks such as gabbro, norit and basalt. Shale and sedimentary gneisses usually
4
have medium levels. Secondary occurrences of uranium can be formed by uranium transported with
flowing groundwater precipitated when water comes in contact with a reducing environment. Many of
the world’s largest deposits and most uranium rich rocks are formed by groundwater flowing through
the clayey layer. Such ores can have very concentrations of uranium, up to 30 % (Andersson P et al.,
2007).
Table 2 - The table shows the concentration of uranium, thorium and potassium in different rock types in Sweden.
Source: (Andersson P et al., 2007).
World average
Intrusive basic rocks
Granites
Granites, uranium-rich
Gneiss, sedimentary origin
Limestone
Sandstone/Quarzites
Shale
Black shale
Alum shale
Sedimentary phosphates
High containing uranium ore
U [ppm]
3
0.1-3
2-6
8-40
2-10
0.2-3
0.5-5
1-10
20-80
50-300
100-400
10000-300000
Th [ppm]
8
1-10
5-20
10-100
5-20
0.1-3
1-10
1-15
2-15
8-15
K [%]
2.4
0.1-3
2-5
4-6
2-5
0.1-0.5
1-6
1-6
1-6
1-6
Sweden has a bedrock consisting of old crystalline and metamorphic rocks, common rocks are gneiss,
granite, granodiorite, sandstone and marble which are rocks from all categories (Geological Survey of
Sweden). In the Swedish capital of Stockholm, Geological Survey of Sweden has collected data and
published a bedrock map. In the city of Stockholm there are mainly four different groups of rocks which
are seen in Table 3 it is also seen which type the rocks are (Geological Survey of Sweden).
Table 3 - The table lists the rocks of Stockholm, Sweden. The table also presents which type of rock every specific
rock is. Source: (Geological Survey of Sweden).
Rock
Pegmatite
Granitoid
Syenite
Gabbro
Dioritoid
Diabase
Ultrabasic rocks
Meta-greywacke
Mica schist
Granite and/or sulphide schist
Para gneiss
Migmatite
Quartzite
Amphibolite
Origin
Igneous
Igneous
Igneous
Igneous
Igneous
Igneous
Igneous
Sedimentary
Metamorphic
Metamorphic
Metamorphic
Mixture of metamorphic and igneous rock
Metamorphic
Metamorphic
1.2 Naturally occurring radioactive nuclides
The three elements that contribute the most to the natural terrestrial radiation are potassium, thorium
and uranium (Philips G W et al., 2005). Naturally occurring potassium consists of three isotopes: 39K
5
(93.3 %), 40K (0.0117 %) and 41K (6.7 %). 40K is radioactive and will decay to stable argon (40Ar) by
electron capture or positron emission, or it will decay to stable calcium (40Ca) by beta decay. 40K has a
half-life of 1.250×109 years. The β-decay to 40Ca is the most common, but it is the decay to 40Ar that
contributes to the external background radiation dose with a γ- photon with energy of 1461 keV. β-rays
have medium penetrating power and also medium ionizing power (Aronson & Lee, 1986). The β-rays
are similar to the α-rays causing more biologic damage inside the body than outside compared to the γrays but the β-rays will cause less severe damage than α-particles. Uranium and thorium also produce
more nuclides from their chains than potassium. This makes thorium and uranium the main concern
when it comes to radioactive nuclides that contribute to the natural terrestrial radiation (the potassium
will contribute, but not from its β-particles but from its γ-rays outside of the body, this applies to 40K
from rocks and not 40K inside the body). Thorium has seven naturally occurring isotopes and none of
them are stable. The most stable of the isotopes is 232Th which has a half-life of 14.05×109 years. In
Figure 1 the decay chain of thorium-232 is shown. It starts with 232Th and ends with lead (208Pb)
(Wickleder M S et al., 2006). Alpha particles have denser ionizing tracks than gamma photons and
electrons and is the most hazardous radiation type concerning internal radiation of the body. Radon is
the radiation source that contributes most to the background radiation, more than 50 % of the natural
sources and more than 40 % of the total background (UNSCEAR, 2008). Natural uranium consists of
238
U and 235U but mainly 238U (99.3 %) with a half-life of 4.468×109 years. In Figure 2 the decay chain
of 238U is shown. It decays through α- and β- decay and ends with the stable lead isotope, 206Pb.
1.3 Literature study: Variations in indoor radon concentration
Indoor radon concentration levels vary with living conditions, the environment and the season.
According to the World Health Organization the radon concentration outdoors is at average somewhere
between 5 to 15 Bq/m3 (WHO). When radon is exhaled from the surface of the earth it is exhaled into
the atmosphere where it is rapidly dispersed and diluted by vertical convection and turbulence which
often makes the radon concentration levels low in the open atmosphere. There are some places around
the world with elevated levels, but compared to concentrations indoors and below ground these levels
are relative low and close to WHO’s average. Radon concentration levels indoor, above ground, will
depend on various matters: the building material, ventilation inside the building, the foundation of the
building and the pressure under the building are examples of contributors to the variation of the indoor
radon concentration levels. Buildings can be found that have a radon concentration level up to 10,000
Bq/m3 but it is more an exception than a rule. In Swedish dwellings the mean radon level was estimated
to be 108 Bq/m3 in 1990 (Andersson P et al., 2007) and revised in 2008 with an estimated mean of 90
Bq/m3 (Axelsson G et al., 2015). The Swedish Radiation Safety Authority has estimated that out of the
annual 3000 cases of lung cancer, around 450 of the cases were caused by inhalation of radon and its
short-lived daughters in dwellings in conjuncture with smoking and around 50 cases without conjunction
with smoking. (Andersson P et al., 2007). There have been studies conducted on radon concentrations
in rock shelters and tunnels with different focuses. The radon concentration level depends on the
geological material in the cave also in addition to the underlying rocks, which was shown in Table 2.
The air flow and ventilation influence the radon concentration levels mainly depending on changes in
the outside temperature and the ratio between the temperature inside and outside the cave as long as the
rock shelter does not have a mechanical ventilation system (Thinova & Rovenska, 2011); (Perrier et al,.
2007). In mines, the radon concentration varies with the location and is not constant for the whole mine.
Higher concentration levels are often found in the middle and at the end of galleries. It is suggested to
be because of the transport of radon-enriched air from the entrance of the mine to the air exhaust (Santos
et al., 2014). A study from China showed that the radon concentration levels will vary during the day
and it will vary differently depending on the kind of underground construction. Tunnels and saps tend
to have the same varying pattern but basements tend to differ from tunnels and saps (Li X et al., 2006).
The same study showed on a seasonal variation with higher radon concentration levels for saps, tunnels
and basements during the summer and lower levels during the winter. The summer-winter ratio for
tunnels which had the highest ratio was 2.41. A comparison between closed and open tunnels was made
pointing at that the ratio was 2.82 for closed tunnels and 1.27 for open tunnels. This phenomenon is also
6
found in a study from Nepal (Perrier et al,. 2007). The radon concentration level was about the same in
the winter for both closed and open tunnels but the closed tunnels had much greater levels during the
summer (Li X et al., 2006). Another way for the radon to get into the air besides diffusing out of the
rock is by water. It is observed that radon can be transported by water (Richon, o.a., 2005). In a study
from Hong Kong radon levels in tunnels with high groundwater ingress was compared to radon levels
in other tunnel projects. The study found that the tunnels with high groundwater ingress had higher
levels of radon (Li & Chan, 2004). A study from the Czech Republic showed that the aerosol particle
concentration can be 100 times lower inside the cave compared with outside (Thinova & Rovenska,
2011). This would affect the radon daughter levels and the attached aerosols ratio.
The influence of thoron to the dose is estimated to be less than 1 % (Thinova & Rovenska, 2011). This
may differ in Sweden because of the rocks with higher content of thorium-232 (Isaksson, 2002). The
levels of γ-radiation coming from 238U, 232Th and 40K in underground environments are dependent on
the type of geology. Due to radon being able to accumulate in certain areas in an underground
environment, the γ-measurement is not related to average radon concentration. Gamma spectrometry
should therefore not be used when determining radon levels but can be used to evaluate the radon
exhalation capability of the rock (Bochiolo M et al., 2012).
1.4 Radon progenies and the equilibrium factor
It is the progenies of radon that cause the most damage to the lung tissue but it is the radon gas
concentration levels that is usually measured and regulated. The knowledge of the radon concentration
only gives an upper limit of the radiation risk due to radon and its short-lived progeny. Radon and the
short-lived progenies are often not in equilibrium; having the same activity concentration, in the air. The
radon daughter concentrations are very complex and to measure radon daughters can give misleading
results because the daughters are not always in the same equilibrium state. It is therefore, from a
measurement point of view, better to measure the radon concentration instead. An equilibrium factor is
assumed between radon and its short-lived progenies. The equilibrium factor depends on parameters
such as ventilation rate, aerosols concentration, volume to surface ratio of the rooms and total amount
of radon progeny deposition (Nikezić & Baixeras, 1996). The equilibrium factor usually varies between
0.4 – 0.6 which would mean that 40-60 % of the progenies that theoretically should be there, if
equilibrium between radon and the progenies were, is there.
1.5 Methods for radon measurements
Alpha decay and the inferred radon concentration can be measured with different measuring techniques,
all with both advantages and disadvantages. When measuring radon it is important that radon daughters
are filtered away, otherwise the alpha decay from the progenies will be taken as the product of radon
decay. When the radon gas has entered the measuring volume, it is the alpha decay that is measured.
The progenies will also contribute to the alpha decay and the detector accounts for this, therefore it is
important to wait for a radiological equilibrium before starting measuring if possible. When measuring
the radon gas for a longer term of time the techniques can be divided into continuous and integrating
measurements. A continuous measurement provides the data in real time as a mean of the radon
concentration, for example every hour. An integrated measurement provides a total concentration for
the measurement period. The advantages with a continuous measuring device are the instantaneous
readout and the information of short-term variations in radon. The disadvantages are that the instruments
are often very expensive and generally not suitable for long-term use in a single location. The integrating
measuring device is the most practical when calculating a long-term average of the radon concentration.
The disadvantage is that the integrating measuring techniques take a long time to perform. In the
following sections a few measuring techniques are discussed.
7
1.5.1 Pulsed ionization chamber
A typical instrument consists of an ionization chamber, an air circulation system and a microprocessor.
When measuring, air is pumped or diffused into the ionization chamber, the air contains radon. The air
goes through a filter, which removes radon decay products (radon daughters). The alpha decay that
occurs in the chamber release electrical charges, the charges are collected on the chamber’s electrodes
with an electrical field. The electrical pulses are then amplified and analyzed by a microprocessor. The
instrument can be programmed to integrate alpha radiation over a selectable period of time e.g. one hour,
and then storing the resulting value as an intermediate result. It is therefore possible to determine radon
variation with time during the measurement period and also get
Figure 3 - the alphaGUARD ionization chamber which is here working by diffusion. The inlay is of stainless
steel and works as an anode with a potential of +750 V. The electrode with the potential of 0 V is located at the
center of the chamber along the longitudinal axis (Saphymo GmbH, 2009).
an average for the entire period. In Figure 3 a schematic picture of an alphaGUARD, which is an
example of a pulsed ionization chamber, is shown. The pulsed ionization chamber is an active
(continuous) measuring instrument. It is used with advantage when time variations are studied. The
pulsed ionization chamber is powered by an internal or external power supply, which is to be considered
when choosing the measuring time. Some pulsed ionization chambers are very heavy which can
complicate transportation.
1.5.2 Etched track detector
A detector material (often a form of plastic) is placed in a sealed box with a filter so that radon gas can
diffuse into the box while radon daughters cannot. When radon decays and omits alpha radiation the
alpha particles hit the detector material and cause damage that is made visible by etching. The number
of tracks per unit area is proportional to the exposure time and radon concentration. The etched track
detector is an integrating measuring method, which is preferred when calculating a long-term average
of the radon concentration. The detectors are cheap but only have a one-time usage.
8
1.5.3 Electret
An electret instrument includes a measurement chamber to which air from the room diffuses through a
filter that removes radon daughters. The chamber walls are electrically conductive. The chamber
contains an electret, an electrostatically charged sheet of Teflon. The electret is positively charged on
the surface that is facing the chamber. The opposite side is negatively charged and is connected to the
walls of the chamber. Alpha particles from the decay of radon and daughters ionize the air in the
chamber. Electrons freed at the ionization moves in the electric field towards the surface of the electret
while the positive ions move toward the walls. The electrostatic charge becomes reduced and the
potential can be measure by a voltmeter. If a chamber with screw cap is used the electret is an integrating
method that can for example be used to measure only during daytime. The electret method has to be
corrected for gamma radiation.
9
2. Materials and methods
2.1 Instruments
In this project an Exploranium GR-130BGO detector (Exploranium GS Ltd, Ontario, Canada), a
gamma-ray spectrometer which uses a bismuth germanium oxide crystal, was used to measure the dose
rate and potassium, thorium and uranium content in the surrounding rock.
To measure radon gas concentration two alphaGUARDs PQ2000 (SAPHYMO GmbH, Frankfurt am
Main, Germany) were used in order to measure at two different locations simultaneously. The detector
is a pulse ionization chamber (alpha spectroscopy). The alpha guard lets the radon gas diffuse through
a large/surface glass fiber filter into the ionization chamber. Only Rn-222 and Rn-220 will pass through
the filter while the radon progenies are prevented to enter the ionization chamber. However the detector
only registers events from the Rn-222 daughters inside the ionization chamber. The filter also prevents
the ionization chamber to be contaminated by dust particles. The alphaGUARD detector also measures
the relative humidity, atmospheric pressure and air temperature. To prevent dust and dirt to enter into
the detectors electronics a protection bag made from Tyvek was used. Tyvek is made from a paper fleece
like functional textile consisting of high-density polyethylene fibers. The manufacturer recommends
using the bag when measuring in an environment with elevated dust and dirt content in the ambient air.
The diffusion time of radon into the detector is only marginally delayed by the bag’s tissue. The
alphaGUARD detectors were calibrated at the Swedish Radiation Safety Authority. The calibration of
the alphaGUARD detectors is traceable to Physikalish-Techniche Bundesanstalt (PTB) in Germany.
The calibration factors were 1.014 ± 0.068 and 1.038 ± 0.070, a calibration factor of 1 means that there
is no difference between the instrument response and estimated true radon concentration.
2.2 Study sites
2.2.1 Geological gamma map and underground subway stations
To assess which locations in the subway system were of the most interest from a radon perspective the
subway stations were plotted on maps of the gamma radiation and bedrock distribution in the Stockholm
region. The Geological Survey of Sweden has produced a gamma radiation map over the Stockholm
region (Figure 4) as well as a map of the bedrock surface. In Figure 5 the Stockholm subway stations
has been marked on the bedrock surface map. By comparing the two maps the blue subway line was
identified to run through areas were a higher concentration of uranium could be expected. Since high
levels of uranium may indicate high radon levels the blue line was chosen. In addition the blue subway
line is the line with the most underground stations. The blue line was built in 1970s and the depth of the
tunnels is between 20 and 30 m. Figure 6 shows the names of the subway stations for the blue line and
that it is divided after Västra skogen. All measurement locations are situated on the way to Akalla except
one, which is situated on the way to Hjulsta. The Rissne depot, located apart from the rest of the subway
system, is accessed by a tunnel from Hallonbergen on the way to Akalla.
10
Figure 4 - a map over the city-area of Stockholm with the approximate uranium levels made out from gamma
measurements made by the Geological Survey of Sweden. The unit of the approximation is ppm uranium (Geological
Survey of Sweden).
Figure 5 - a map over the bedrock in the city-area of Stockholm made by the Geological Survey of Sweden. The
stations of the three lines are shown as rings; green rings for the green subway line and so on. (Geological Survey of
Sweden).
11
Figure 6 - the blue subway line and its stations from 2006. The line divides itself after Västra skogen where it becomes
two lines. The black dots in the figure show the position of the measurement points. Image: (Moorlag, 2016).
2.2.2 Measurement locations
The positions are given as a location between two subway stations and with a kilometer and hundred
meter marker (Table 4). For example the first position is given as – Solna C - Västra skogen, 5+400 –
the detector was placed between the stations Solna C and Västra skogen but more precisely at 5 km and
400 m from the offset point of the blue subway line. The offset point for the blue line is at the station
Kungsträdgården. The determination of rock types was made by BergAB. However most of the tunnels
are reinforced with concrete making it difficult to examine the rock type at the exact measurement
position. To determine the rock types exposed rocks as close as possible to the measurement location
was studied. The measurements were conducted in different characteristic types of tunnel locations. The
locations were given as suggestions from BergAB.
12
Table 4 - information about the study sites such as exact position, tunnel depth, rock types near measurement location
and type of measurement location.
Study site
Västra skogen stadshagen
Västra skogen
Solna C – Västra
skogen
Solna C –
Näckrosen
Näckrosen –
Hallonbergen
Relative
position
[km+m]
4+600
5+150
5+400
7+000
8+100
Depth
Rock types
Approximately
8m
>23 m
Approximately
20 m
Approximately
10 m
Approximately
5m
Medium-grained granite
Hallonbergen –
Kymlinge
8+950
Approximately
11 m
Duvbo – Rissne
9+420
Hallonbergen –
Kymlinge
9+980
Approximately
14 m
Approximately
13 m
Hallonbergen –
Kymlinge
Hallonbergen –
Kymlinge (middle
of tunnel)
Hallonbergen –
Kymlinge (end of
tunnel
The Rissne depot
10+300
Type of
measurement
location
Transverse tunnel
Grained granite
Gneiss with element of granite
aisles and grained granite
Gneiss and Gneiss with
elements of pegmatite
medium-grained reddish-gray
granite and coarse grained white
gray granite
medium-grained white gray
granite with elements of
pegmatit aisles
white gray medium-grained
granite to coarse-grained granite
coarse grained white gray
granite and coarse reddish gray
granite
medium-grained reddish-gray
granite
Coarse-Grained reddish gray
granite
Transverse tunnel
Pump station
Regular tunnel
Transverse tunnel
Transverse
tunnel1
Pump station
Regular tunnel
Regular tunnel2
10+300
Approximately
18 m
-
10+300
-
Coarse-Grained reddish gray
granite
Transport tunnel
Track 9
Approximately
14 m
white gray-gray fine / medium
grained granite / gneiss with
pegmatite aisles
Depot
Transport tunnel
Figure 7 shows a pump station where water is pumped. A pump station always has a pumping house
where the detector was placed. Figure 8 shows a measurement location in a regular tunnel which is
where the workers usually work. Figure 9 shows a transverse tunnel and these tunnels are used to get
from one set of tracks to another. The width of the transverse tunnels can vary and the tunnel shown as
an example is one of the more narrow kind. Figure 10 shows the Rissne depot. All tunnels except the
transport tunnels are reinforced with concrete.
1
This is a pressure equalizing shaft by definition but the site looks like a transverse tunnel with two exceptions:
there are two walls instead of one, and the walls are not very wide compared to those in a transverse tunnel.
2
Other side of the tracks of a transport tunnel.
13
Figure 7 - The white package in the figure is the AlphaGUARD detector. The location is the pump station 5+400. To
the left of the detector is the pump house. During measurements the detector was placed in a protection bag
recommended by the manifacturer.
14
Figure 8 - The tunnel at 7+000 which is in a regular tunnel, the detector placement was to the right of the tracks in the
figure.
Figure 9 - The measurement position 4+600, in the figure the tunnel where one of the tracks goes is shown. Here the
detector is put on the white bag but during the measurements it was inside the bag.
15
Figure 10 - The Rissne depot and where the detector was placed.
2.2.3 The trains’ daily schedule
The typical activity in the subway tunnels during the week: Monday to Thursday, the traveling for
passengers begins around 05.00 which mean that trains leave the depot just before. Last train arrives at
its final destination around 01.00 and then drives for the depot. Monday to Friday the trains depart every
ten minutes from 06.30 and 21.00, every 30 minutes from 00.00 to 01.00. For other hours the trains
depart every 15 minutes. Friday to Saturday and Saturday to Sunday the trains are active all night.
During weekend nights the trains depart every 30 minutes. Rush hours are between 06.00-09.00 and
15.00-19.00 during the working days.
2.3 Methodology
For each measurement location one of the two detectors was used. The detectors were carried to the
measurement locations and placed in a suitable place near the suggested km+m mark. The measurements
were carried out at approximately one week at each location except for the measurements of the transport
tunnel, which lasted for three days. Data was given from the detector as an average value for every hour
except for the transport tunnel, Rissne depot and 9+420 whose data were given as average values for
every ten minutes. During the analysis new means for every hour were constructed by the ten minute
means. Every second week the detectors were brought to the measurement lab – in order to recharge the
internal batteries and collect the measurement data. At each measurement location two other types of
measurements were performed with an Exploranium GR-130 with a BGO crystal; a gamma dose rate
measurement and an estimation of the potassium-, thorium- and uranium-content. The dose rate
measurement was made during 3 minutes. When measuring the composition estimation a 10 second
measurement was carried out to see the count rate and then the measuring time was decided. The
measuring time was either 200 or 300 seconds. When using the GR-130 for gamma dose rate
measurement, the detector and the active measurement point was pointing away from the nearest wall.
16
2.4 Uncertainty estimation and analysis
The software ‘DataEXPERT’ was used to collect data from the detectors and was exported to excel for
analysis. The uncertainty for every measurement value is estimated by Equation (1) where b is the
uncertainty in the calibration of the detector and c is an estimation of uncertainties such as not correcting
for pressure, temperature during the measurements and other factors that will affect the uncertainty.
𝜎 = √𝑏 2 + 𝑐 2
(1)
When doing this kind of uncertainty estimation a third variable, a, is often included that is the square of
the counts counted by the detector. The counts for every measurement value were not provided by the
detector and can therefore not be used but for a measurement period of one hour it is reasonable to
consider the number of counts to be high and thus the importance of the a-term is less. The uncertainty
in the calibration factor, b, for the detectors was ±6.74 % and ±6.71 %. The uncertainty, c, is
approximated, out of experience from working with alphaGUARD detectors, to be 2 % for not correcting
for pressure and temperature and 1 % for other uncertainties. This is giving a total uncertainty, c, of 3
%. The total uncertainty for the detectors is therefore 7.38 % or 7.35 % for every measurement
depending on which detector was used. For every measurement location a mean value of the radon
concentration was calculated and the uncertainty for this mean was estimated.
When estimating the error of the mean value for the measurement location 𝜎 and the standard
deviation should be added together quadratically and then taken the square root of. However radon will
vary with different parameters, this makes the standard deviation not suitable for an estimation of the
uncertainty of the mean. To calculate a mean for the blue subway line tunnel, a mean of all measurement
values was calculated. Its error was estimated by adding the mean 𝜎 and its standard deviation in square
then taking the square root of it.
The radon concentration was plotted against the humidity, pressure and temperature. When analyzing
the data the first measurement value of each measurement series was omitted because the air has not
diffused through the protective bag and into the detector yet. The radon concentrations were compared
with measurements of the humidity, pressure and temperature. The mean of humidity, pressure and
temperature was calculated for every measurement location and the minimum and maximum value was
also found. A comparison between radon levels and estimations of uranium-content was also done. The
GR-130 assay tool is calibrated for a 2π-geometry but since tunnels are a 4π-geometry conversion factor
of 0.6 was multiplied with the result (Cecilia Jenelik, 2015).
17
3. Results
Figures presented in the results are those that are of the typical outcomes of the measurement of the
measurement locations. Figures presented in Appendix A supports the typical results presented in the
results section. Results as means of humidity, temperature and air pressure can be seen in Appendix A.
In Table 5 the mean values of radon concentrations for the measurement locations are listed.
Table 5 - the mean values of the radon concentrations for the measurement locations. The table also gives the
measurements maximums, minimums and the standard deviation.
Measurement location
Västra skogen –
Stadshagen, 4+600
Västra skogen, 5+150
Solna C – Västra skogen,
5+400
Solna C – Näckrosen,
7+000
Näckrosen –
Hallonbergen, 8+100
Hallonbergen –
Kymlinge, 8+950
Duvbo – Rissne, 9+420
Hallonbergen –
Kymlinge, 9+980
Hallonbergen –
Kymlinge, 10+300
The Rissne depot
Hallonbergen –
Kymlinge, 10+300,
middle of transport tunnel
Hallonbergen –
Kymlinge, 10+300, end
of transport tunnel
Radon
concentration
[Bq/m3]
112
Maximum
value
[Bq/m3]
174
Minimum Standard Uncertainty,
value
deviation ±𝜎 [Bq/m3]
3
[Bq/m ]
[Bq/m3]
48
24
8
116
77
169
137
48
26
22
21
9
6
165
229
69
26
12
163
341
63
48
12
103
154
45
23
8
117
187
215
343
72
117
30
39
9
14
182
489
53
72
13
118
132
393
237
38
93
64
29
9
10
107
242
66
35
8
The mean concentration as a representation of the instantaneous radon value in the blue subway line
tunnel was calculated to 134 ± 55 Bq/m3. The fractions of potassium, thorium and uranium are listed in
Table 6 for the measurement locations. The tables also show the dose rate at the study site.
Table 6 - potassium, thorium and uranium fractions at study sites measured with Gr-130.
4+600
5+150
5+400
7+000
8+100
8+950
9+420
9+980
10+300
Dose rate
[nSv/min]
2.66
2.8
3.62
2.94
2.4
2.68
2.38
3.08
3.56
K [%]
Th [ppm]
U [ppm]
3.9
4.2
3.5
2.8
3.1
3.6
3.4
4.0
4.1
25.9
28.6
25.2
21.1
21.8
25.0
21.3
29.9
39.8
5.9
5.9
16.1
12.8
7.0
5.6
5.4
8.0
9.1
18
10+300 (middle)
10+300 (back)
Depot
4.76
4.94
2.26
5.1
5.0
2.9
55.7
56.1
15.5
13.4
15.3
6.1
In Figure 11 an example from the plotting is shown, in this example the humidity and radon
concentration is plotted against the time of the day. The time starts either at 00.00 or 12.00 at the date
given in the title of the figure. A pattern between humidity and radon concentrations is inferred from the
figure. Figure 11 has data from Västra skogen – Stadshagen, 4+600.
Figure 11 - the radon concentrations and humidity is shown over time for the location of 4+600.
In Figure 12 another pattern is inferred between the humidity and radon concentrations. Figure 12 has
data from Hallonbergen – Kymlinge, 10+300.
Figure 12 - the radon concentrations and humidity is shown over time for the location 10+300 in the middle of the
transport tunnel.
Figure 13 infers a pattern between the radon concentration and the temperature. Figure 13 has data from
Solna – Västra skogen, 5+400.
19
Figure 13 - the radon concentrations and temperature is shown over time for the location 5+400.
In Figure 14 a second pattern is inferred between the radon concentrations and temperature. Figure 14
has data from Näckrosen – Hallonbergen, 8+100.
Figure 14 - the radon concentrations and temperature is shown over time for the location 8+100.
A third pattern is inferred in Figure 15 for the radon concentrations and temperature. Figure 15 has data
from Hallonbergen – Kymlinge, 9+980.
Figure 15 - the radon concentrations and temperature is shown over time for the location 9+980.
20
In Figure 16 a pattern is inferred between the radon concentration and the air pressure. Figure X has
data from Hallonbergen – Kymlinge, 10+300.
Figure 16 - the radon concentrations and air pressure is shown over time for the location 10+300.
In Appendix A all figures that were analyzed are presented and tables of the mean values, minimum
maximum and also standard deviation per week for the air pressure, humidity, radon concentration and
temperature are found.
Figure 17 - the radon concentrations and uranium content is shown for the measurement locations.
21
4. Discussion and conclusions
Here follows a discussion about the results, which looks at how the radon concentration varies with time
and also talks about factors that might influence these variations. There is also a discussion of whether
the mean radon concentration level founded in this report will give an exposure which is above or below
the exposure limit given by the Swedish Work Environment Authority. This report focuses on radon
concentration levels for workers and the results are not applicable to the public. The public do not have
access to the tunnel and the relevant measurement locations for the public would be the platforms at the
stations and the trains, not the tunnel.
4.1 Variations of radon
It can be seen that the radon concentrations varies over the day with time. In Figures 11, 193-21 and 23
a decrease in the radon concentration is happening around 03.00-04.00 while in Figures 12, 22 and 2527 an increase is seen during the same time period. The blue subway line is divided into two lines (Figure
6), the locations showing alterations under this time interval all lies on the same track. On the other side
of the division one measurement location (Figure 24) is showing radon concentration incensements
during daytime instead. The Rissne depot (Figure 28) also shows an increase during daytime.
4.2 Radon concentration and train movements
If trains stop travelling around 01.30-02.00 (Monday-Thursday) and the diffusion through the protective
bag and into the detector is 1-2 hours then it would mean that if the radon concentrations are correlated
with movement in the subway tunnels a change in radon concentration is expected to occur around
02.30-04.00. This phenomenon is seen in all locations except 9+420 and the Rissne depot. The activity
in the depot differs from the activity in the subway tunnels; often the depot is empty or has a very low
activity of movements during the day. This makes the Rissne depot expected to have a different radon
concentration pattern compared to the tunnels where trains drive all day. 9+420 however was expected
to show this pattern but do not.
4.3 The impact of air circulation
The movements in the tunnel could be correlated to the radon concentrations, in a study (Perrier et al.,
2007) the radon concentration changed with the air circulation which is indirect related to movements
in the tunnels assuming that trains increase the air circulation when passing by. This would mean that
the more frequently trains passes by the more the air circulates. Another factor influencing the air
circulation is the ventilation system. The ventilation system that is used in the Stockholm subway system
is passive with holes in the ceiling leading up to the ground surface (Figure 18). The passive ventilation
is influenced by changes in temperature; when the temperature in the tunnel and in the outdoor air has
a larger difference a pressure difference is developed. The air strives for balance between them and
therefore increases air circulation. Train movements in the tunnels stop during the night and radon
concentrations are seen to change in the detector around 03.00-04.00 for 10 of the 12 measurement
locations. At the same time interval a temperature decreases can be seen in Figures 13-15, 29, 31, 33,
34 and 37 but an increase is observed in Figure 35. This means that both temperature and radon
concentration changes around 03.00-04.00 which can be seen in Figures 13-15, 29, 31, 34 and 35.
3
Figure 19 and later figure are found in Appendix.
22
Figure 18 - a typical ventilation hole in the subway system besides the tunnel openings. The ventilation hole is a
passive ventilation system; no air is pumped into the tunnels.
Another parameter that has decreases around 03.00-04.00 is the relative humidity. In Figures 11, 19-21
and 23 a decrease can be seen. The relative humidity and temperature are connected to each other. The
relative humidity is defined as the balance where water vapour condensates just as fast as liquid water
evaporates, when equilibrium of the two of them is established the air is saturated, 100 % relative
humidity. If the temperature raises the evaporation speeds up and thereby shifts the balance further
toward water vapour, so the higher temperature the more moisture the air must contain before it is
saturated. So it is expected for humidity and temperature to have a similar pattern but when studying
Tables 5 and 7 to investigate if the mean humidity and mean radon concentration is correlated, no such
conclusion could be made. No obvious correlation between the air pressure and radon concentration
could be seen in the analysis.
4.4 Peaks and valleys of radon concentration
In some measurement locations radon concentrations have high value peaks and others have low value
valleys. One possible explanation is that the rocks at the measurement location have different rates of
radon emission where some rocks emanate more radon than others. When trains pass the locations the
radon concentration levels evens out, therefore only small fluctuations are seen during the day. When
the trains stop for a couple of hours during the night a difference in radon concentration between
measurement locations could be expected. Where the rocks emanate more radon the radon
concentrations are increased while, where the rocks emanate less radon the radon concentrations are
decreased. As is seen in Figures 11, 12 and 19-28 the radon concentration has either increased or
decreased during the time of interest, which then could support the explanation that the trains distribute
the radon concentration more evenly in the tunnels.
23
4.5 Different tunnel types
The measurement locations were chosen to represent different environments to investigate whether that
affected the mean values. If comparing Tables 4 and 5 no indication that the type of measurement
location influences the mean radon concentration could be seen. It should be pointed out as pump
stations were supposed to represent where water was present, the water never left the pump house and
the measurements took place outside the pump house. The pump station in 5+400 was situated in a
transverse tunnel and the pump station in 9+420 was situated in a regular tunnel. It is seen that regular
tunnels have the highest mean radon concentrations out of the studied types. No correlation between the
depth of the tunnel and the mean radon concentration could be seen. No correlation between the present
rock types and radon concentration could be seen. The radon concentration levels were no different
whether the tunnels were reinforced with concrete or not but the gamma measurement showed increased
levels of uranium, and especially thorium, present.
4.6 Tunnel mean and exposure limits
One of the purposes for this project was to investigate radon concentration levels in the subway tunnels.
In Table 5 it is seen that the highest mean value for the radon concentrations is 187 Bq/m3, which is well
below the exposure limit of 0.72 MBq∙h/m3 (based on 1800 working hours in a radon concentration of
400 Bq/m3) given by the Swedish Work Environment Authority for working places below ground. When
the project started the exposure limit was 0.72 MBq∙h /m3 (based on 1800 working hours in a radon
concentration of 400 Bq/m3) The exposure limit for office work is 0.36 MBq∙h /m3 (based on 1800
working hours in a radon concentration of 200 Bq/m3) which also is higher than the total mean value
and the workers do not work 1800 hours as the exposure limit takes to account for indoor work above
ground. The method in this project did not follow the method description for measuring radon indoor
above ground, this could make it hard to compare the values however there is no method description for
measurement of radon below ground.
4.7 Uranium estimates and radon concentrations
The report also aimed to investigate if measurements of uranium content and radon concentration could
have a correlation. In Figure 17 the radon concentration and the corresponding uranium content for each
measurement location is shown. No conclusions could be drawn from the result, which supports the
statement that radon concentration cannot be inferred from gamma dose rate.
24
Outlook
In 1.5.1 Pulsed ionization chamber it is discussed that the pulsed ionization chamber (alphaGUARD) is
not optimal for constructing long-term mean values. The suggestion should be to do a long-term
measurement with an etched track detector instead to be on the safe side but it would be unnecessary to
measure more than this projects location for highest value for starters. If that long-term measurement
would show a mean value below the exposure limit it could be concluded that the blue line from
Kymlinge to Stadshagen is below the limit.
To more thoroughly investigate the health effects, the radon daughters and particle content should be
investigated as well. However such measurements are more difficult to perform and better equipment
would be needed.
25
Acknowledgment
This project was financially supported by the Swedish Radiation Safety Authority (SSM), the Swedish
Work Environment Authority and the Public Transport Administration (Stockholm läns landstig); I
would like to thank both authorities for this opportunity.
I am deeply grateful to my supervisor Magnus Ahnesjö at the Swedish Radiation Safety Authority for
all the support, patience, encouragement and guidance during all my work.
I would also like to thank my second supervisor Kirlna Skeppström at the Swedish Radiation Safety
Authority for her comments along the work.
Many thanks go to Catrin Tholinsson and Erik Wåhlin at the Swedish Radiation Safety Authority for
the help with the instruments, showing me how the software was operated, commenting my work and
telling me about the uncertainty estimations.
Thanks to Amir Mansourian and all his TSM at Strukton Rail AB for helping me getting down in the
subway tunnels and being so flexible with working hours.
Thanks to Fanny Nordin at BergAB for the help of finding appropriate locations to measure and
determine all the rock types at the measurement locations.
Thanks to Bo Baudin and Maria Röjvall at Public Transport Administration (Stockholm läns landsting)
for agreeing and helping the project getting started by telling me how to get access to the subway tunnels.
Thanks to Janez Marinko from the Swedish Work Environment Authority for his help in the initial phase
of the work and landing us the necessary measurement equipment needed for the project.
Finally, I want to thank my supervisor Mats Isaksson at the Department of Radiation Physics, University
of Gothenburg, for taking his time reading through my work and making suggestions along the way.
Thank you!
26
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29
Appendix A
In Table 7 the mean values of humidity for the measurement locations are listed.
Table 7 – the mean values of the relative humidity for the measurement locations. The table also gives the
measurements maximums, minimums and the standard deviation.
Measurement location
Humidity [%]
Maximum
value [%]
Minimum
value [%]
Västra skogen – Stadshagen, 4+600
Västra skogen, 5+150
Solna C – Västra skogen, 5+400
Solna C – Näckrosen, 7+000
Näckrosen – Hallonbergen, 8+100
Hallonbergen – Kymlinge, 8+950
Duvbo – Rissne, 9+420
Hallonbergen – Kymlinge, 9+980
Hallonbergen – Kymlinge, 10+300
The Rissne depot
Hallonbergen – Kymlinge, 10+300,
middle of transport tunnel
Hallonbergen – Kymlinge, 10+300,
end of transport tunnel
63
63
73
81
65
60
71
64
70
57
89
73
77
80
86
73
63
76
75
83
66
94
43
45
63
67
47
41
50
53
53
42
64
Standard
deviation
[%]
8
8
3
4
5
3
6
7
8
7
6
81
91
58
6
In Table 8 the mean values of temperature for the measurement locations are listed.
Table 8 – the mean values of the temperature for the measurement locations. The table also gives the measurements
maximums, minimums and the standard deviation.
Measurement location
Temperature [°C]
Maximum
value [°C]
Minimum
value [°C]
Västra skogen – Stadshagen, 4+600
Västra skogen, 5+150
Solna C – Västra skogen, 5+400
Solna C – Näckrosen, 7+000
Näckrosen – Hallonbergen, 8+100
Hallonbergen – Kymlinge, 8+950
Duvbo – Rissne, 9+420
Hallonbergen – Kymlinge, 9+980
Hallonbergen – Kymlinge, 10+300
The Rissne depot
Hallonbergen – Kymlinge, 10+300,
middle of transport tunnel
Hallonbergen – Kymlinge, 10+300,
end of transport tunnel
14.4
14.6
12.7
13.1
12.6
13.7
14.7
13.2
12.1
13.1
9.1
15.6
15.2
15.8
15.4
13.8
15.4
16.3
13.6
13.6
14.7
11.4
11.2
13.8
9.3
10
11.1
10.2
12.8
12.5
11.2
12.6
8.8
Standard
deviation
[°C]
1.0
0.5
1.2
0.8
0.6
1.3
0.4
0.2
0.3
0.3
0.4
7.6
10.5
6.6
0.7
In Table 9 the mean values of air pressure for the measurement locations are listed.
Table 9 – the mean values of the air pressure for the measurement locations. The table also gives the measurements
maximums, minimums and the standard deviation.
Measurement location
Air pressure
[mbar]
998
998
1003
1003
998
998
1019
1003
1002
1011
1020
Maximum
value
[mbar]
1013
1013
1017
1017
1008
1009
1034
1021
1020
1031
1027
Minimum
value
[mbar]
989
990
993
993
972
972
1002
983
980
999
1007
Standard
deviation
[mbar]
6
6
6
6
8
9
7
11
11
8
7
Västra skogen – Stadshagen, 4+600
Västra skogen, 5+150
Solna C – Västra skogen, 5+400
Solna C – Näckrosen, 7+000
Näckrosen – Hallonbergen, 8+100
Hallonbergen – Kymlinge, 8+950
Duvbo – Rissne, 9+420
Hallonbergen – Kymlinge, 9+980
Hallonbergen – Kymlinge, 10+300
The Rissne depot
Hallonbergen – Kymlinge, 10+300,
middle of transport tunnel
Hallonbergen – Kymlinge, 10+300,
end of transport tunnel
1018
1025
1004
7
In Figure 19 the radon concentrations and humidity is shown for location 5+150.
Figure 19 - the radon concentrations and humidity is shown over time for the location of 5+150.
In Figure 20 the radon concentrations and humidity is shown for location 5+400.
Figure 20 - the radon concentrations and humidity is shown over time for the location of 5+400.
In Figure 21 the radon concentrations and humidity is shown for location 7+000.
Figure 21 - the radon concentrations and humidity is shown over time for the location of 7+000.
In Figure 22 the radon concentrations and humidity is shown for location 8+100.
Figure 22 - the radon concentrations and humidity is shown over time for the location of 8+100.
In Figure 23 the radon concentrations and humidity is shown for location 8+950.
Figure 23 - the radon concentrations and humidity is shown over time for the location of 8+950.
In Figure 24 the radon concentrations and humidity is shown for location 9+420.
Figure 24 - the radon concentrations and humidity is shown over time for the location of 9+420.
In Figure 25 the radon concentrations and humidity is shown for location 9+980.
Figure 25 - the radon concentrations and humidity is shown over time for the location of 9+980.
In Figure 26 the radon concentrations and humidity is shown for location 10+300 end of the tunnel.
Figure 26 - the radon concentrations and humidity is shown over time for the location of 10+300 end of the tunnel.
In Figure 27 the radon concentrations and humidity is shown for location 10+300 middle of the tunnel.
Figure 27 - the radon concentrations and humidity is shown over time for the location of 10+300 middle of the tunnel.
In Figure 28 the radon concentrations and humidity is shown for location Rissne depot.
Figure 28 - the radon concentrations and humidity is shown over time for the location of Rissne depot.
In Figure 29 the radon concentrations and temperature is shown for location 4+600.
Figure 29 - the radon concentrations and temperature is shown over time for the location of 4+600.
In Figure 30 the radon concentrations and temperature is shown for location 5+150.
Figure 30 - the radon concentrations and temperature is shown over time for the location of 5+150.
In Figure 31 the radon concentrations and temperature is shown for location 7+000.
Figure 31 - the radon concentrations and temperature is shown over time for the location of 7+000.
In Figure 32 the radon concentrations and temperature is shown for location 8+950.
Figure 32 - the radon concentrations and temperature is shown over time for the location of 8+950.
In Figure 33 the radon concentrations and temperature is shown for location 9+420.
Figure 33 - the radon concentrations and temperature is shown over time for the location of 9+420.
In Figure 34 the radon concentrations and temperature is shown for location 10+300.
Figure 34 - the radon concentrations and temperature is shown over time for the location of 10+300
In Figure 35 the radon concentrations and temperature is shown for location 10+300 end of the tunnel.
Figure 35 - the radon concentrations and temperature is shown over time for the location of 10+300 end of the tunnel.
In Figure 36 the radon concentrations and temperature is shown for location 10+300 middle of the
tunnel.
Figure 36 - the radon concentrations and temperature is shown over time for the location of 10+300 middle of the
tunnel.
In Figure 37 the radon concentrations and temperature is shown for location Rissne depot.
Figure 37 - the radon concentrations and temperature is shown over time for the location of Rissne depot.
In Figure 38 the radon concentrations and air pressure is shown for location 4+600.
Figure 38 - the radon concentrations and air pressure is shown over time for the location of 4+600.
In Figure 39 the radon concentrations and air pressure is shown for location 5+150.
Figure 39 - the radon concentrations and air pressure is shown over time for the location of 5+150.
In Figure 40 the radon concentrations and air pressure is shown for location 5+400.
Figure 40 - the radon concentrations and air pressure is shown over time for the location of 5+400.
In Figure 41 the radon concentrations and air pressure is shown for location 7+000.
Figure 41 - the radon concentrations and air pressure is shown over time for the location of 7+000.
In Figure 42 the radon concentrations and air pressure is shown for location 8+100.
Figure 42 - the radon concentrations and air pressure is shown over time for the location of 8+100.
In Figure 43 the radon concentrations and air pressure is shown for location 8+950.
Figure 43 - the radon concentrations and air pressure is shown over time for the location of 8+950.
In Figure 44 the radon concentrations and air pressure is shown for location 9+420.
Figure 44 - the radon concentrations and air pressure is shown over time for the location of 9+420.
In Figure 45 the radon concentrations and air pressure is shown for location 9+980.
Figure 45 - the radon concentrations and air pressure is shown over time for the location of 9+980.
In Figure 46 the radon concentrations and air pressure is shown for location 10+300 end of the tunnel.
Figure 46 - the radon concentrations and air pressure is shown over time for the location of 10+300 end of the tunnel.
In Figure 47 the radon concentrations and air pressure is shown for location 10+300 middle of the
tunnel.
Figure 47 - the radon concentrations and air pressure is shown over time for the location of 10+300 middle of the
tunnel.
In Figure 48 the radon concentrations and air pressure is shown for location Rissne depot.
Figure 48 - the radon concentrations and air pressure is shown over time for the location of Rissne depot.