Nuclear Science and Technology, Vol. 3, No. 4 (2013), pp. 49-55
Parallel radon and thoron measurements in mineral sand mining area of Ha Tinh province
Bui Dae Dung', Trinh Van Giap, Tibor Kovacs, Tran Ngoc Toan, Le Dinh Cuong, Tran Khanh Minh, Nguyen Huu Quyet, Nguyen Van Khanh
'institute for Nuclear Science and Technology 179 - Hoang Quoc Viet, Nghia Do - Cau Giay, Ha Noi
Email: [email protected]. vn (Received 18 November, accepted for publication 7 March)
Abstract In this study, we have tested the possibility of parallel measurement of radon (^^^Rn) and thoron (^^"Rn or Tn) by alpha track detectors at the extraction and processing of black sands that contains high content of Zircon and Monazite minerals in a coastal area of the Ha Tinh province. A pair of cylindrical cups RADUET, designed to separately measure Rn and Tn with CR-39 detectors (Radosys Company Ltd - Hungary), were used for the test. The results showed that concentrations of indoor Rn and Tn in the black sand extraction area are not high (28 ± 6 Bq/m^ for Rn, 14 ± 5 Bq/m' for Tn). The Rn and Tn concentrations in the indoor air of the titanium processing plant are 34 ± 6 Bq/m^ for Rn, 58 ± 11 Bq/m^ for Tn, and are higher than those concentrations in the residential houses outside the plant (20 ± 4 Bq/m^ for Rn, 16 ± 8 Bq/m^ for Tn) but still comparable to the average concentrations of indoor Rn and Tn reported by the UNSCEAR 2006. At Zircon processing plant, indoor Rn concentrations are similar to those in the titanium processing plant but Tn concentrations in the houses for separating Rutile and Zircon are very high. Indoor Tn concentration is especially high in the house for separating Zircon where it could reach 2931 Bq/m^, hundred times higher than that in the surrounding residential houses and that ofthe world average published by UNSCEAR 2006.
Keywords: Coastal black mineral sand. Zirconium, Monazite. Radon (Rn), Thoron (Tn), Alpha Track Detectors - A TD. RADUET.
I. INTRODUCTION amount of radiation. Monazite - a rare earth phosphate containing a series of rare earth Besides Ilmenite (FeTi03) and Rutile ^,^^^^^^^ (particularly Cerium and (Ti03). coastal black mineral sands contain Lanthanum) and 5-12% (usually around 7%) Zircon (ZrSi04) and Monazite (RePO, where _, „ „ . t i. . , .
\. 4/ ^1^ Xenotime - yttrtura phosphates also have Re stands for rare earth metals) in which . .- i • j TU -n, • i
trace amounts of U and Th. These minerals radioactive elements like uranium and thorium • •.- . j - ,• •., , ,innn
j^gyg significant radioactivity, up to 4000 are usually found. These minerals are mined in „ „ ^ ^ . . j . j - . , . . . , ..
aic usuoii; 1" Bq/kg, and most of them are found in the tail of manv countries to produce millions of tons of . , . . /• i j . iiiaiij wuiju V mineral processing wastes (including Zircon and Titanium (Ti) each year. Zircon is ^^^^^^.^ ,„ ^ ^ ^ ^^^^^ , ^ , , , ; , , ^ „ „ y ^ p„, used primarily in the foundry, refractoty ;„,„ „ , , , e landfills or as construction materials nmHiiction and ceramics industries. Zircon . , , , . c proauciioii ai.u ll^^j j ^ j j ^ jl^^ increase of exposure to the usually has high radioactivity of U and Th but .^^^
there are no attempts to remove these
radionuclides from Zircon due to economic Long term working with naturally reasons, and therefore, refractory ceramic and occurring radioactive materials (NORM) such porcelain products should contain a certain as U and Th could cause harmful effects to
©2013 Viemam Atomic Energy Societ>' and Viemam Atomic Energy Institute
PARALLEL RADON AND THORON MEASUREMENTS IN MINERAL SAND MINING AREA OF..
health [2], NORM can affect workers by inhalation or ingestion of radioactive dust in the air as well as by inhalation of radon (Rn) and Thoron (Tn) isotopes and their progeny. Rn ('"'Rn) is a radioactive isotope gas from the U- 238 radioactive series with a half-life of 3.8 days, which is often considered as the main contributor to internal dose in humans.
Historically, contribution of Tn (""Rn), a radioactive isotope gas from the Th-232 series is often ignored, mainly because the Tn half- life is relatively short (T, 2= 55 s), and the Tn concentration is low in living environment when compared with Rn.
However, recent statements about the contribution of Tn on the dose have to be reconsidered. Studies [3] have shown that people living in areas with high Th background in Brazil, China and India are receiving a high dose from Tn. In addition, recent studies [4]
indicate that workers in black mineral sands mining also received a high dose from Tn, up to 180mSv/year.
Radon concentration can be measured in many ways with different types of devices [5, 6]. A number of Tn measurement techniques have also been developed. In principle, Tn measurement should also include measurement of Rn, simultaneously. There are two types of parallel Rn and Tn measurement methods:
Instantaneous and cumulative measurements.
Instantaneous methods use continuous metering equipments that give out Rn/Tn concentration. Cumulative methods mainly use passive measurement devices such as alpha track detector (ATD), activated carbon. TLD to absorb Rn/Tn and their decay products. From radiation dose measurement perspective, passive measurement devices are recommended for use because they give a mean value of a longer time period that could average the daily
fluctuations of weather conditions affecting the variations of Rn and Tn concentration in the environment. Parallel Rn and Tn measurements using ATD is probably the most popular method because it is suitable for long term measurements from 1 week to I year.
We have developed and tested the procedures for parallel Rn and Tn measurements by ATD in a mineral sand mining area of the Ha Tinh province. The purpose of the test was mainly to verify the conformity ofthe developed procedures, and to have initial assessment of indoor Rn and Tn concentrations in different houses and offices, inside and outside the areas of mineral sand extraction and processing sites. This paper reports the findings ofthe test.
n. MATERIAL AND METHODS A. The study area
The mineral sand mining activity in Ha Tinh started in the years 1989-1990 when several small companies were founded to extract the valuable mineral sand. Every company exploited a rich in Titanium small mine less than 20 ha. The poor quality ore (TiOi less than 5%) was left over. Later, these companies were imified to form a mining and processing enterprise named as MITRACO HA TINH [7].
Nowadays even the poor quality ore is processed using advanced technology and suitable for each kind of ore. MITRACO has obtained many products with different qualities that suit the requirements of every customer.
Besides the traditional products such as Ilmenite and Rutile, MITRACO is now the only company in Vietnam that produces commercial zircon with ZrOi concentration of more than 65% and grain size of less than 1 fam
BUI DAC DUNG et al.
tiiat is suitable for most ceramics and electronics industries [7].
The production line includes three main processes at three sites [7]. Our test was conducted in these sites as follow:
• At the mineral sand extracrion site (Cam Hoa Commune, Cam Xuyen DistricL Ha Tinh Province) where the mineral sand layers are excavated and put into the wet gravitational spirals that separate hea\y minerals from the usual sand minerals. Heavy minerals are shifted to the Titanium Processing Plant.
• At the Titaniiun Processing Plant (Cam Xuyen cit}. Cam Xuyen District Ha Tinh Province) w here electrostatic separation techniques are used to separate conductors (Rutile and Ilmenite) frx>m the non-conductors (Zircon and Monazite). Rutile and Ilmenite are kept for further processing into commercial products. The tailings that contain Zircon and Monazite are shifted to the nearb\" Zircon Processing Plant.
• At the Zircon Processing Plant located closely to tiie Titanium Processing Plant where magnetic separation machines are used to separate the non-magnetic minerals (Zircon) fiT)m the magnetic minerals (Monazite).
Monazite remains in the tailings wastes.
B. Rn and Tn measurement tedmiqoes RADUET chambers made b\ Radosys Ltd - Hungar)' and CR-39 detectors, designed to measure separately Rn and Tn. were used for the investigation [8]. TTK two chambers have bottom radius of 2.45 cm. and 2.60 cm high (Figure 1)- In one monitoring site one pair of chambers w'as installed. One chamber, designated to measure both Rn and Tn. has open holes around with a particulate filter to prevent dust and RnfTn progenies from passing through but allowing both Rn and Tn to diffuse
freely into the chamber. Another chamber is for measuring Rn only. It is a closed chamber that b\ air convection from outside to inside, allowing more than 95% of Rn and less than 1 % of Tn to diffuse inside (due to the shorter half-life of Tu). ATD are small pieces of CR- 39 Ixlcm size and thickness of 100pm [8].
The 25 investigated points were set up as shown in Figiu^ 2, including:
• At the mineral sand extraction site: 3 measuring points in the ore extraction stations.
1 point in the temporary shift house for workers. I measuring points in the administration office.
• At Titanium Processing Plant: 2 points in the input stock house. 2 points in separated Ilmenite stock house. I point in the Ilmenite separating factory. I point in the administration office.
• At Zircon Processing Plant: 01 point in the factory for super fine Zircon preparation. 1 point in Zircon stock house. I point in the Rutile separating factory, 1 point in the wet separating factor\'. 01 point in the Zircon separating factory. 01 point in the technical department and 01 point in the executive office.
• At the smrounding. closely located to the Titanium Processing and Zircon Processing Plants, residential houses: 07 measuring points.
The exposure time was 3 months. After 3 months one pair of chambers were har\ested but replaced by another pair. Total measured time for one measuring point was 1 year with 4 detector pairs. During exposure time, the R.'^DL'ET chambers were hung on a stable rod.
20 cm away from any wall, at the height of 1.5 to 2 m from the floor.
After recovering, the chambers were returned back to the laboratory where the
PARALLEL RADON AND THORON MEASUREMENTS IN MINERAL SAND MINING AREA OF..
exposured CR-39 chips were etched in 6 M NaOH solution, at constant temperature of 80o C for 4 hours. Track density counting was performed in OLIMPUS CX2I microscope.
The concentrations of Rn and Tn were calculated using conversion factor given by Radosys Ltd. [8] and verified by exposing test detector pairs in Rn and Tn calibrating chambers at the Institute of Radiochemistry and Radioecology, of the University of Pannonia, Hungary.
m . RESULTS AND DISCUSSION Yearly average concentration of Rn and Tn are given in Table 1. The results show that indoor Rn and Tn concentrations in the residential houses and in the black sand extraction area are not high, in the range of 14- 43 Bq/m^ for Rn and 8-17 Bq/m' for Tn. These values are similar to the world average indoor Rn (40 Bq/m^) and Tn (15 Bq/m^) concentrations published by UNSCEAR [9],
At the Titanium processing plant Rn and Tn concentrations in the indoor air are higher than that in the surrounding residential houses.
The Rn concentration is in the range of 18-55 Bq/m^ (average 34 B q W ) , Tn is from 32-118 Bq/m"" (average 58 Bq/m"") but still comparable to the average concentration of the world published by UNSCEAR [9].
At Zircon processing plant, Rn concentrations are similar to Rn concentrations in the Titanium processing plant (25-36 B q W , average 30 Bq/m") but Tn concentrations in the factories for separating Rutile and Zircon are high (138 and 2931 B q W , respectively).
Tn is especially high in the factory for separating Zircon where Tn concentration could reach 2931 Bq/m^ hundred times higher than that in the surrounding residential houses and the world average Tn concentration published by UNSCEAR [9].
Fig. 1. RADUET chambers designed to separately measure Rn and Tn, made by Radosys Company Ltd - Hungarian. One chamber has open holes around for total Rn and Tn measurement the other chamber is
sealed for air convecnon from outside to inside only and is used for measuring Rn [8]
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At the mineral sand extraction site (Cam Hoa Commune. Cam Xuyen District Ha Tinh Province): 5 points ( K T 1 - K T 5 )
--v.^ THIEN CAM 13KM
QL1A
At Cam Xuyen city.
Cam Xuyen District, Ha Tinh Province:
- 6 points (Tl-1, Tl-2, T2-I, T2-2, T3 and T4) in the Titanium Processing Plant:
- 7 points (Zl - Z7) in the Zircon Processing Plant; and
7 points (NDl-1,2.;
ND2-1.2.3: and ND3- 1,2) in the surrounding residenfial houses.
f^MHW-^KMwi^'-:lH^j-^-:M^F^
Fig. 2: Location ofthe investigated points in the area.
PARALLEL RADON AND THORON MEASUREMENTS IN MINERAL SAND MINING AREA OF.
Table I: Yearly average indoor concentration of Rn and Tn in the Ha Tinh black mineral sand mining areas.
No. Measured
point Description ofthe point At tile mineral sand extraction sit
1 2 3 4 5
KTI KT2 KT3 KT4 KT5
Administration office Temporary shift house for workers Ore extraction station Ore extraction station Ore extraction station Average At Titanium Processing Factory
6 7 8 9 10 11
Tl-1 Tl-2 T2-1 T2-2 T3 T4
Input stock house Input stock house Ilmenite stock house Ilmenite stock house Ilmenite separating house
Administration office Average
\t Zircon Processing Factory
12 13 14 15 16 17 18
Zl Z2 Z3 Z4 Z5 Z6 Z7
House for super fine Zircon preparation Zircon stock house Rutile separating house Wet separating house Zircon separating house Technical Department Executive office Average M the surrotmding residential houses
19 20 21 22 23 24 25
NDl-1 NDl-2 ND2-1 ND2-2 ND2-3 ND3-1 ND3-2
Living room Living room Living room Living room Living room Living room Living room Average
Rn concentration (Bq/m*)
29 43 25 24 18 28
±
±
±
±
±
± 6 7 6 6 5 6 29
55 18 22 25 53 34
±
±
±
±
±
±
± 5 6 5 6 6 7 6
35 26 25 27 36 34 26 30
±
±
±
±
±
±
±
± 6 6 5 5 5 7 6 5
19 17 25 24 18 26 14 20
±
±
±
±
±
±
±
± 3 3 3 4 5 5 5 4
Tn concentration (Bq/m')
14 8 13 17 17 14
±
±
±
±
±
± 5 5 3 7 4 5
33 43 118 39 80 32 38
±
±
±
±
±
±
± 8 8 24 7 14 5 11
24 60 138 36 2931
9 14 459
±
±
±
±
±
±
± J:
7 12 20 9 118
10 8 26
21 22 11 14 13 14 13 16
±
±
±
±
±
±
±
± 4 5 3 7 10 12 13 8 54
BUI DAC DUNG et al.
IV. CONCLUSIONS The test in the Ha Tinh black mineral sand mining areas proved that our developed procedures for parallel Rn and Tn measurements, using a pair of cylindrical cups RADUET, could be recommended for use in conducting surveys of indoor Rn and Tn concentrations in other parts of Vietnam, especially in those areas with expected high Tn concentrations.
At most of the places in the Ha Tinh black mineral sand mining areas, indoor Rn and Tn concentrations are similar to the average concentration of the world reported by UNSCEAR 2006. fricreased indoor Tn concentrations are observed at the factory for separating Zircon where Tn concentration could reach 2931 Bq/m^, the value that is hundred times higher than that in the surrounding residential houses and the world average Tn concentration published by UNSCEAR 2006.
REFERENCES
[1] International Atomic Energy Agency, "Extent of Environmental Contamination by Naturally Occurring Radioactive Material (NORM) and Technological Options for Mitigation'', Technical Reports Series No. 419, STI/DOC/010/419 (ISBN: 9201125038) (2003).
[2] Intemational Commission on Radiological Protection (ICRP), "ICRP Publication 66'", (1994).
[3] S. Tokonami, "Summary of dosimetry (radon and thoron) studies", Intemational Congress Series 1276, Elsevier, pp. 151- 154 (2005).
[4] Intemational Atomic Energy Agency, Intemational Labour Office, "Occupational Radiation Protection in the Mining and Processing of Raw Materials", Safety Standards Series No.RS-G-1.6, L ^ A , Vienna, (2004).
[5] CEI/IEC 61577-1, "Radiation protection instrumentation Radon and radon decay product measuring instruments Part I:
General requirements", (2000).
[6] CEVIEC 61577-2, "Radiation protection instrumentation Radon and radon decay product measuring instruments Part 2;
Specific requirements for radon measuring instruments", (2000).
[7] MITRACO HATINH: Report on Titanium advanced processing technology (in Vietnamese), Company report, (2003).
[8] RADOSYS, "User Manual RS_Man63- 080316", (2008).
[9] UNSCEAR Report Volume II, "Annex E:
Sources-to-effecls assessment for Rn in homes and workplaces", (2006).