Development of the Fiber-Optic Sensor with a Large Surface Area to
Measure Radioactive Contamination in Soil at Nuclear Facility Site
Arim Lee, Chan Hee Park, Rinah Kim, Hanyoung Joo and Joo Hyun Moon
Department of Nuclear, Energy System Engineering, Dongguk University, Gyeongju, Republic of Korea
Keywords: Fiber-optic Sensor, Decommissioning, Nuclear Facility Site, Residual Radioactivity.
Abstract: Once decommissioning of a nuclear facility is completed, it shall be confirmed if its site meets the site
release criteria. For this the residual radioactivity in the site should be measured and assessed. This paper
developed and characterized the fiber-optic sensor with a large surface area for measurement of
radioactivity in soil at nuclear facility site, which is less time-consuming due to ease of measurement. The
fiber-optic sensor consisted of a radiation sensing probe including scintillator panel and light guide and a
transmission optical fiber. The light measuring system was assembled combining it with photomultiplier
tube, pre-amplifier, multichannel analyser and display. Several measurements using the light measuring
system showed that, as for measuring time for measurement of cesium-137 source, 1,800 sec was the optical
measurement duration, and as for reflector, aluminium foil was the best.
1 INTRODUCTION
Once decommissioning of a nuclear facility is
completed, it shall be confirmed if its site meets the
site release criteria for site release or clearance of
items from regulatory control. For this, the residual
radioactivity in the site should be measured and
assessed.
The residual radioactivity assessment consists of
(1) taking samples from contaminated area, (2)
measurements of radioactivity in the samples and (3)
statistical analysis of the measurement data. The
appropriateness of taking samples is a key factor in
determining the assessment’s reliability.
Radioactivity in soil is commonly measured by
using a small portable radiation detector or in-situ
gamma spectroscopy, which takes a long time.
Therefore, this paper developed and characterized
the fiber-optic sensor with a large surface area for
measurement of radioactivity in soil at nuclear
facility site, which is less time-consuming due to
ease of measurement.
2 EXPERIMENTAL SETUP
To identify what types of radiation are emitted from
soil at nuclear facility site, related documents and
radiation survey records of TRIGA research reactor
site in Korea were examined. In the middle of
decommissioning of nuclear facilities, cesium-137,
cobalt-60, iodine-129, -131, and thorium are usually
radionuclides of main concern. (Lawrence E. Boing,
2013). TRIGA site radiation survey records showed
that cesium and cobalt were main radionuclides in
soil at the site as shown in Table 1. (Gyenam Kim,
2003). Based on the examinations, cesium-137,
gamma emitter, was selected as a representative
radionuclide to be detected by using the fiber-optic
sensor developed in this paper.
Table 1: Main radionuclides in soil at TRIGA site.
Radionuclide Radioactivity(Bq/kg)
Co-60 644.4 ± 42.6
Cs-134 88.7 ± 4.1
Cs-137 88.7 ± 4.1
Sr-90 <9.7
Cr-51 <5.9
Fe-59 <4.2
LYSO:Ce was selected as scintillator suitable for
measuring gamma ray emitted from cesium-137. Its
characteristics are listed in Table 2(Chan Hee Park
et al, 2014). As shown in Fig.1, sensing probe of the
fiber-optic sensor was rectangular solid shape, made
171
Lee A., Park C., Kim R., Joo H. and Moon J..
Development of the Fiber-Optic Sensor with a Large Surface Area to Measure Radioactive Contamination in Soil at Nuclear Facility Site.
DOI: 10.5220/0005431101710175
In Proceedings of the 3rd International Conference on Photonics, Optics and Laser Technology (OSENS-2015), pages 171-175
ISBN: 978-989-758-092-5
Copyright
c
2015 SCITEPRESS (Science and Technology Publications, Lda.)
of LYSO:Ce crystal with the size of 105 mm (length)
× 91 mm (width) × 3 mm (height).
Table 2: Characteristics of scintillator LYSO:Ce.
Densit
y
(g/cm
3
)
Wavelengt
h
peak(nm)
Light
yield(%)
∗relative
to
NaI:Tl
Effective
number
Decay
time
(ns)
7.20 420 73-75 65 42
Light guide that guides lights emitted due to
interactions between scintillator and radiation was
attached to top-side of the scintillator. Before
fabricating the light guide, the optimal mixing ratio
between epoxy-resin YD-128 (Kukdo Co., Korea)
and hardener D-230 (Kukdo Co., Korea) was
determined to be 5:1, in terms of transmittance, light
self-absorbance and hardness, referring to Table 3
(Chan Hee Park, 2013). Using the mixture, the light
guide was fabricated with a rectangular solid shape
consistent with size of the scintillator for easy to
combine scintillator with light guide. Its size was
105 mm (length) × 91 mm (width) × 30 mm (height),
as shown in Fig. 1.
Table 3: Properties change according to mixing ratio
between YD-128 and D-230.
YD-128
+ D-230
Transmittance
Self -
absorbance
Hardness
50.0g +
10g
1 5 Excellent
47.5g +
12g
3 3 Excellent
45.0g +
14g
4 2 Good
42.5g +
16g
2 4 Good
40.0g +
18g
5 1 Bad
The fiber-optic sensor was fabricated by embedding
the transmission optical fiber in the light guide
before it solidified. To minimize the light loss due to
incomplete connection between them, the
transmission optical fiber was submerged into a
depth of 10mm at the centre of the light guide.
The transmission optical fiber used in this paper
was commercial-grade plastic multimode fiber
(Edmund Optics Co. ESKA® acrylic fiber optics).
The core of the optical fiber was made of poly-
methyl methacrylate (PMMA) and the cladding was
made of a fluorine series polymer. The length of the
transmission optical fiber was determined to be 1m
considering the distance to measurement site from
source. The optical fiber was coated with double
layers of heat shrinkable tube to block lights from
the outside.
Figure 1: Sensing probe of the fiber-optic sensor.
In this study, reflector was used to block lights
from the outside and prevent lights generated by
interactions between scintillator and radiation from
escaping from the light guide. The 4 types of
reflector were considered: Dupont's TYVEK-1025D;
TYVEK-1056D; aluminium foil and Teflon tape.
(W.Bugg, 2014). The surfaces of scintillator and
light guide were wrapped with the reflector.
The light-measuring system used in this paper
consisted of a radiation sensing probe, transmission
optical fiber, photomultiplier tube (Hamamatsu,
R1924A) that converts optical signals into electrical
signals, pre-amplifier (Hamamatsu, Amplifier Unit
C319) that amplifies electrical signals, multichannel
analyser (MCA) that analyse amplified signals and
display that shows transmitted signals. The Genie
2006 was used to analyse the electrical signals. The
experimental setup is shown in Fig.2.
Figure 2: Experimental setup.
3 RESULTS
First, several measurements were made and analysed
using the light-measuring setup shown in Fig. 2 to
PHOTOPTICS2015-InternationalConferenceonPhotonics,OpticsandLaserTechnology
172
find the optimal measuring time. As measuring time
increases, more radiation is counted, but more
background radiation is also accumulated. Hence,
the measuring time should be optimized.
Measurements of gamma radiation from cesium-137
source (1 μCi) were made for the three different
measuring times: 600, 1,000, and 1,800 sec. The
measurement results without reflector were depicted
in Fig. 3 to Fig. 5.
The net counts, dividing total counts by
measuring time, were compared. The net counts in
counts per second were 663 for 1,800 sec, 390 for
1,000 sec, and 414 for 600 sec, respectively, as
shown in Fig.6. The net counts for the case of 1800
sec were the highest. Based on the comparison,
measuring time for the fiber-optic sensor developed
in this paper was chosen to be 1,800 sec.
Figure 3: Measurements results for 600sec.
Figure 4: Measurement results for 1000sec.
To improve detection efficiency of the fiber-optic
sensor, it is necessary to collect the optical signals,
that is, lights generated by interactions between
radiation and scintillator as much as possible as well
as to reduce loss of the optical signals. For this,
reflector was used as described before. To find the
best reflector, measurements of cesium-137 source
were made for the 4 different reflectors: Dupont's
TYVEK-1025D; TYVEK-1056D; Aluminium Foil
and Teflon tape. The reference measurements were
the measurement without reflector.
Figure 5: Measurement results for 1800sec.
Figure 6: Total counts and net counts (counts per second)
for the three different measuring times.
Tyvek 1056D and 1025D are commonly used as
reflector. The light guide was surrounded by two
layers of both reflectors. The measurements with
Tyvek 1056D were higher, but the measurements
with Tyvek 1025D were lower than the reference
measurements. As for Aluminium Foil, common
household aluminum foil was used. The light guide
was covered with 3 layers of Aluminium Foil with
the thickness of 18μm. The measurements with
Aluminum Foil reflector were about twice as high as
the reference measurements. Teflon tape is also
common reflector material. The measurements with
Teflon tape were lower than the reference
measurements. Measurements for the four different
reflectors are shown in Fig. 7 to Fig.10.
DevelopmentoftheFiber-OpticSensorwithaLargeSurfaceAreatoMeasureRadioactiveContaminationinSoilat
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Figure 7: Comparison between measurements with Tyvek
1056D and the reference measurements.
Figure 8: Comparison between measurements with Tyvek
1025D and the reference measurements.
Figure 9: Comparison between measurements with
Aluminum Foil and the reference measurements.
Measurements with the four different reflectors
showed that the Aluminum Foil was the best
reflector in terms of detection efficiency of the fiber-
optic sensor.
Figure 10: Comparison between measurements with
Teflon tape and the reference measurements.
Figure 11: Comparison of measurements with the four
different reflectors.
If the total count for the reference measurements is
regarded as 1, normalized counts for the 4 different
reflectors are shown in Table 4.
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Table 4: Normalized counts for the 4 different reflectors.
Reflector Normalized count
Reference 1.000
Tyvek 1056D 1.492
Tyvek1025D 0.733
Aluminum foil 2.119
Teflon tape 0.508
4 CONCLUSIONS
This study developed and characterized a fiber-optic
sensor with a large surface area to measure
radioactive contamination in soil at nuclear facility
site.
The fiber-optic sensor consisted of a radiation
sensing probe including scintillator panel, light
guide and a transmission optical fiber. It was
fabricated by embedding the transmission optical
fiber in the light guide. The light measuring system
was assembled combining it with photomultiplier
tube, pre-amplifier, multichannel analyser and a
display unit.
The light measuring system was used to measure
gamma radiation from cesium-137 source. Several
measurements were made to characterize the fiber-
optic sensor with a large surface area. They showed
that as for measuring time, 1,800 sec was the
optimal measurement duration, and as for reflector,
aluminium foil was the best.
This fiber-optic sensor is expected to be useful in
measuring gamma radiations resulting from the
radioactive soil at nuclear facility site, along with the
experimental results such as the best reflector and
measuring time. To improve the detection efficiency
of the fiber-optic sensor, the best geometry of light
guide should be studied.
ACKNOWLEDGEMENTS
This study was supported by a National Research
Foundation of Korea(NRF) grant funded by the
Korea government is Ministry of Science, ICT and
Future Planning (MSIP, Research Project No.
2012M2A8A1027833 and No. 22012M2B2B1055499).
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Chan Hee Park., Joo Hyun Moon., Bum Kyoung Seo.,
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