|Year : 2010 | Volume
| Issue : 4 | Page : 202-204
Estimation of thorium lung burden in mineral separation plant workers by Thoron-in-breath measurements
Sujata Radhakrishnan1, K Sreekumar1, Esai Selvan2, RM Tripathi1, VD Puranik1
1 Health Physics Unit, Indian Rare Earths Ltd., Environmental Assessment Division, BARC, Mumbai, India
2 IRE Ltd., Manavalakurichi, India
|Date of Web Publication||1-Dec-2011|
Health Physics Unit, Indian Rare Earths Ltd., Environmental Assessment Division, BARC, Mumbai
Source of Support: None, Conflict of Interest: None
The Mineral Separation Plant (MSP) of M/s Indian Rare Earths Ltd at Manavalakurichi in Tamil Nadu is engaged in the processing of beach sands to separate ilmenite, monazite, rutile, sillimanite, garnet and zircon. The mining and mineral separation of these sands involves occupational radiation exposure. Inhalation of airborne particulates of monazite is the major route of internal exposure. Thoron - in - breath measurements are carried out on the workers of the plant. From this, the Th lung burden is calculated assuming 9% exhalation of thoron generated in the lung. Th burdens for workers from different operations of the plant range from 8.5Bq to 95Bq. This corresponds to less than 15% of Annual Limit for Intake (ALI) for thorium. The data validates the control measures adopted in the plant to keep the exposures As Low As Reasonably Achievable (ALARA).
Keywords: Thoron-in-breath, mineral separation plant, thorium burden
|How to cite this article:|
Radhakrishnan S, Sreekumar K, Selvan E, Tripathi R M, Puranik V D. Estimation of thorium lung burden in mineral separation plant workers by Thoron-in-breath measurements. Radiat Prot Environ 2010;33:202-4
|How to cite this URL:|
Radhakrishnan S, Sreekumar K, Selvan E, Tripathi R M, Puranik V D. Estimation of thorium lung burden in mineral separation plant workers by Thoron-in-breath measurements. Radiat Prot Environ [serial online] 2010 [cited 2020 Mar 29];33:202-4. Available from: http://www.rpe.org.in/text.asp?2010/33/4/202/90469
| 1. Introduction|| |
The beach sands of Midalam - Muttam coastal belt in Tamil Nadu are endowed with rich deposits of heavy minerals like ilmenite (46%), zircon (4-6%), rutile(2-7%), monazite(1-2%), garnet(7-14%) and silliminate(2-3%). Indian Rare Earths Ltd., an Undertaking of the Department of Atomic Energy has a plant at Manavalakurichi (MK) in this area where mining and mineral separation is carried out. Surface mining, collection of beach washings and dredge mining are the mining methods adopted. The mineral separation plant makes use of the differences in the electrical and magnetic properties and differences in specific gravity of the constituent minerals to separate them. The dredged sand is concentrated by slurrying in water and passing down through spirals. The dried concentrate is passed through a series of high tension electric separators and magnetic separators of varying intensities. Fine separation of some minerals is effected by wet tabling and froth floatation also. During final stages of monazite separation, air tabling also is adopted (Pillai and Khan, 2003). Monazite is radiologically the most significant mineral as it contains about 8-9% thorium as ThO 2 and 0.35% uranium. This can give rise to occupational radiation exposure to the workers.
In monazite, 232 Th exist in secular equilibrium with it's daughter products and do not exhibit any appreciable change in radioactivity with respect to time. During mineral separation stage this equilibrium is not disturbed. However the vigorous physical treatment processes can give rise to considerable dust (and consequently radioactivity) concentrations. Monazite tends to preferentially concentrate in airborne dust as it is softer than the titanium and zircon bearing minerals (Hewson, 1997). The high energy betas and gammas from the thorium daughter products constitute external radiation hazard. The internal hazards encountered in the processing of beach minerals and monazite are mainly by way of inhalation of mineral dust, long lived particulate air activity (mainly Th), thoron gas ( 220 Rn) and the daughter products of thoron. The present study has been carried out on nearly 200 workers of the mineral separation plant who are chronically exposed to these hazards. Measurement of Thoron in the exhaled breath of the worker is an indirect method of estimating the body burden with regard to Th.
| 2. Materials and Methods|| |
A modified double filter unit was used for the estimation of thoron (Mayya et al,1986). The person breathes in thoron free air from a delay chamber and exhales into a double filter unit through a specially designed mouthpiece. Thoron that enters the double filter unit decays and the particulate daughter products are collected in an end filter. The filter paper is counted for specified time to estimate the exhaled thoron and assuming the exhalation rate, thorium retained in the lung is calculated. Thoron emanating from the lung can be calculated using the relation Q Ra =KDE -1 Z -1 where K is a non-dimensional factor which depends primarily on the length of the D.F. unit, flow rate and the diffusion coefficient of the decay product atoms. D is the number of disintegrations observed on the second filter for a certain counting period and sampling time, E is the efficiency of the counter, Z is a theoretical factor for thoron daughters and depends upon the sampling time, counting duration and the decay constants of thoron daughters, Th-B and Th-C but independent of the physical parameters of the counting system. It has the dimension of time (seconds). The breath of each subject was sampled for 30 minutes. Alpha counting was done for 16 hours after a delay time of 5 hours. Under the experimental conditions, K=2.72, E=0.25 and Z=900s. Assuming an exhalation rate of 9% for Thoron (Rundo et al., 1959) and series equilibrium conditions, 232 Th activity can be calculated by dividing Q Ra by 0.09. The Minimum Detectable Activity (MDA) for the set up works out to 5 Bq at 2σ confidence level.
| 3. Results and Discussion|| |
[Table 1] gives the dust, Thoron daughter activity and 232 Th in different locations of the plant. It is seen that the average 232 Th activity is much lower than the DAC value of 0.22 Bq m -3 for Mineral Separation Plant. The average dust concentration is 1.8±0.6 Bqm -3 (respirable=25%) which is also well below the TLV (TLV= 1mgm -3 ). The concentration of Thoron daughters is negligible and therefore not significant (DAC=1000 mWL).
|Table 1: Dust, Thoron daughter activity and 232Th in different locations of Mineral Separation Plant (Annual Report, IRE MK, 2004-2008)|
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[Table 2] gives the data on Th lung burden estimated by Thoron in breath measurement for workers of the Mineral Separation Plant. The results are grouped according to the number of years of service viz 1-5 years, 6-10 years etc, up to 30 years. It is seen that the Thorium burden increases with increase in the number of years of exposure. The maximum burden is obtained for workers who have put in 26-30 years of service. For these workers the Thorium burden ranges from 25.7-84.6 Bq giving a mean of 41.9±17.7 Bq. The Annual Limit of Intake (ALI) for Th in MSP is 534 Bq. The thorium lung burden for the workers is less than 15% of ALI. For the workers with less than 6 years of of employment, the average Th burden is 16.7±4.9 Bq. A number of dust control measures have been implemented in the plant over a period of time which is reflected in the reduction of dust and air activity concentrations in recent years.
Terry et al (1997) estimated geometric mean thorium lung burden of mineral sand workers from thoron in breath measurements as 10 Bq(GSD=2.2).
Hewson (1997) estimated the thorium lung burden of mineral sand workers in Western Australia by thoron-in-breath measurements. He found reasonably good correlation between estimates of thorium intake derived from air sampling and retrospective assessment and estimates from bioassay measurements for long-term workers.
Analysis of the data also shows that maximum thorium burden is found in workers of MSP followed by workers of HUP (Heavies Upgradation Plant). This is followed by workers engaged in Mining, Packing and Maintenance sections. Thus, the nature of the processes involved i.e.: dry/wet operations, the material being handled also influence the thorium burden of the workers.
For IRE, MK the average annual individual exposure based on ambient air dosimetry and TLD measurements was in the range 5.15 mSv to 6.98 mSv for the period 2004-2008 of which internal exposure accounts for approximately 20% of the total exposure.(Annual Reports, IRE MK- 2004-2008)
| 4. Conclusions|| |
The thorium lung burdens of workers from the mineral separation plant of IRE, MK range from 8.5-9.5 Bq. The Th burden data validates the efficiency of control measures being carried out in the plant to limit the radiation exposure of workers to ALARA. Thoron-in-breath measurement is therefore an useful tool for determining the thorium burden in workers of the MSP in order to evaluate individual exposures and thereby to ensure that the ventilation conditions and other control measures adopted to limit the exposures are adequate.
| 5. Acknowledgements|| |
The authors are extremely grateful to H.S. Kushwaha, Director H.S&E Group, BARC for his encouragement. The support received from the management and workers of IRE ltd, Manavalakurichi is gratefully acknowledged. The authors are also thankful to Dr. C. G. Maniyan and Dr. P.M.B. Pillai (Ex-BARC) for their suggestions and useful discussions.
| 6. References|| |
- Annual Reports of Health Physics Unit, IRE, Manavalakurichi, 2004-2008.
- Hewson G.S. (1997), Inhalation and retention of thorium dusts by mineral sands workers, Ann.Occup.Hyg., Vol. 41, Supplement 1, 92-98.
- Mayya Y.S., Prasad S.K., Nambiar P.P.V.J., Kotrappa P., Somasundaram S. (1986), Measurement of 220 Rn in Exhaled Breath of Th Plant Workers, Health Physics, 51(6), 737-744
- Pillai P.M.B. and Khan A.H. (2003), Radiological Safety Environmental Surveillance during the mining and milling of beach minerals and processing of monazite, Radiation Protection and Environment, Vol.26, (3 and 4), 523-532.
- Rundo J, Ward W.M. and Jenson P.G. (1959), Measurement of thoron in exhaled breath, Phys. Med. Biol., 3, 101-110.
- Terry K.W, Hewson G.S and Burns P.A (1995), Further Thorium Lung Burden Data on Mineral Sands workers, Radiation Protection Dosimetry 59, 291-298.
[Table 1], [Table 2]