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Year : 2010  |  Volume : 33  |  Issue : 4  |  Page : 209-210  

Optimization of occupational dose in thorium nitrate processing

1 Environmental Assessment Division, BARC, Mumbai, India
2 IREL, OSCOM, India

Date of Web Publication1-Dec-2011

Correspondence Address:
D Vidya Sagar
Environmental Assessment Division, BARC, Mumbai
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Source of Support: None, Conflict of Interest: None

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Thorium plant at, IREL OSCOM produces thorium nitrate (mantle grade) from thorium concentrate/thorium oxalate obtained from the Rare Earths plant at Udyogamandal. The process involves external and internal exposure of workers due to the handling of radioactive material. Scientific management, involving safe handling of radioactive material and process modifications has been responsible to optimize the occupational exposures to low levels. The paper describes the various safety measures and process modifications that resulted in the per capita dose reduction, from 10.4 mSv to 3.6 mSv, during the last one decade.

Keywords: Thorium concentrate, optimization of dose, occupational dose

How to cite this article:
Sagar D V, Tripathy S K, Tripathi R M, Puranik V D, Choudhury L K. Optimization of occupational dose in thorium nitrate processing. Radiat Prot Environ 2010;33:209-10

How to cite this URL:
Sagar D V, Tripathy S K, Tripathi R M, Puranik V D, Choudhury L K. Optimization of occupational dose in thorium nitrate processing. Radiat Prot Environ [serial online] 2010 [cited 2021 Aug 3];33:209-10. Available from: https://www.rpe.org.in/text.asp?2010/33/4/209/90473

  1. Introduction Top

The thorium plant at OSCOM, based on solvent extraction process, was commissioned in 1992. The thorium concentrate (ThO 2 : 26-28%, U 3 O 8 :0.6-0.7%) produced at the Rare Earths Plant, Udyogamandal was used as the feed material till mid 1999. Thereafter, as the feed material got changed to thorium oxalate (ThO 2 : 38-40%, U 3 O 8 : Nil) the existing process at OSCOM was modified since 2000.

The modified process involves conversion of thorium oxalate to thorium hydroxide by treatment with dilute caustic soda solution followed by soda ash solution in a mechanical agitated vessel at a temperature of 50 o C. The thorium hydroxide slurry so produced is filtered to recover the thorium hydroxide cake. It is then subjected to dissolution with nitric acid. The thorium and rare earths values are converted to corresponding nitrates. The conditioned solution is pumped to the solvent extraction section wherein thorium is extracted using 40% TBP as thorium nitrate and the rare earth come out along with the raffinate. The thorium nitrate solution is evaporated in two stages, crystallized and centrifuged to get the final product of thorium nitrate.

  2. Radiological Aspects Top

The activity inventory of thorium nitrate and radioactive waste, before and after the process modification, is presented in [Table 1]. It is seen that, the generation of solid waste is significantly reduced, by a factor of about 4, after introducing thorium oxalate processing in the plant. [Table 2] provides data on external gamma radiation fields in some important operational locations. The reduction in the radiation fields after switching over to thorium oxalate process is very obvious.
Table 1: Inventory of thorium nitrate and effluents

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Table 2: Radiation fields in operational locations

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2.1 Occupational dose

The average per capita occupational radiation dose, since starting the operations in 1992 is provided in [Table 3]. There has been a steady decline in the doses over the entire operational period of the plant. However, the steady values achieved after 2000 is of special significance, as it signifies the overwhelming effect of process modification. Increased awareness on radiation safety and improved house keeping also significantly contributed to the dose reduction.

The trend of collective dose over the years is shown graphically in [Figure 1].
Table 3: Occupational dose, 1992-2008

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Figure 1: The dose reduction achieved over the years

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It is clearly noticeable that the occupational dose incurred in the plant has reduced by a factor of about 5, during the operational period under review. Though change in process modification played the most important role in reducing the occupational exposures other measures such as improved house-keeping, disposal of solid wastes, augmentation of ventilation system, zoning, smooth surface finish, adequate decontamination, improved awareness on radiation safety, etc helped in achieving the desired dose levels in the plant. Similar reduction in occupational exposures have been reported in earlier studies (Paul, et al, 1984; Saha and Vidya Sagar, 1996; Paul, et al, 1997)

  3. Conclusions Top

Chemical processing of thorium involves handling of large quantities of low level radioactive material. Very often, conventional techniques are employed to handle large sources. Therefore, such operations may involve a significant component of avoidable exposures. Health physics experience gained in operational situations, helps to bring down such exposures in a scientifically sustainable manner.

The thorium nitrate plant at OSCOM is a typical example where operational Health Physics experience was taken into account while implementing process modifications. Proper adherence to radiation safety requirements resulted in substantial savings in occupational and environmental exposures. The fact that occupational dose could be reduced to an extent of about 80%, within a span of about 10 years, shows the result of persistent efforts put up by the Health Physics Unit ably supported by the plant management.

  4. Acknowledgements Top

The authors are very much thankful to Dr. H.S. Kushwaha, Director, Health, Safety & Environment, BARC, Mumbai. Thanks are also due to Sri. M.S.Roy, Head, OSCOM for providing the facilities.

  5. References Top

  1. Paul A.C., Pillai K.C. and Soman S.D.(1984), Occupational and environmental radiation exposure from monazite processing in India, International Radiation Protection Association (IRPA), Berlin (West), Proc. of the Conf. 1984.
  2. Paul A.C., Pillai, P.M.B., Maniyan C.G., Haridasan P.P., Sujata Radhakrishnan and Kamala Rudran (1997), Occupational and environmental radiation exposures in the beach mineral and monazite processing in India, 23 rd IARP Conference, Amritsar, 1997.
  3. Saha S.C. and Vidya Sagar D.(1996), Management of radioactivity and nitrate concentration in liquid effluents at new thorium plant of IRE, LTD, OSCOM, Orissa; Paper at the National Symposium on Coastal Zone Management, Berhampur, 1996.


  [Figure 1]

  [Table 1], [Table 2], [Table 3]


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  In this article
1. Introduction
2. Radiological ...
3. Conclusions
4. Acknowledgements
5. References
Article Figures
Article Tables

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