K.S.Parthasarathy
July 2, 2009
Radioactivity of phosphogypsum to be studied
Phosphogypsum may contain radioactive materials such as uranium-238 and radium-226
The AERB has recently issued a safety directive on the use of phosphogypsum
If you visit any fertilizer factory, you may see large quantities of phosphogypsum (PG) in its premises. It is produced when rock phosphate is treated with sulphuric acid. Each ton of phosphoric acid leaves behind nearly five tons of PG. In many countries, the building industry extensively uses PG in producing cement, wallboard, and other building materials.
Phosphogypsum is not an innocuous material. Besides many heavy elements, it may contain significant quantities of radioactive materials such as uranium-238 and radium-226. Phosphogypsum produced from imported rock phosphates contains typically activity concentrations of U-238 in the range 0.1-0.2 Bq/g and Ra-226 in the range 0.5-1.3 Bq/g. (Bq is a unit of radioactivity. In a radioactive material having a radioactivity of one Bq, one atom disintegrates every sec).
The Atomic Energy Regulatory Board (AERB) has been examining the radiological safety implications of adding phosphogypsum in building and construction materials and in using it in agriculture.
Based on the principles followed internationally, the Board has recently issued a safety directive on the use of phosphogypsum.
Analysing content
The Board directed that all rock phosphate processing industries shall analyze Uranium-238 and Radium-226 content in each imported consignment of rock phosphate as well as in the phosphogypsum produced from its processing and shall report the results to AERB on a quarterly basis. AERB will review this data for a period of about two years for deciding on the frequency of such analysis in future.
AERB decided that its approval is not required for selling phosphogypsum for its use in building and construction materials, if the activity concentration of Ra-226 in it is less than or equal to 1 Bq/g.
If Ra-226 concentration in phosphogypsum is more than one Bq/g, the seller must mix it with other ingredients such that the Ra-226 activity concentration in bulk material is less than or equal to 1.0 Bq/g.
According to the International Atomic Energy Agency (IAEA), at one Bq/g, we need not regulate the material as the radiation doses to persons involved will be insignificant, irrespective of the quantity of material whether it is in its natural state or has been subjected to some form of processing.
AERB stated that its approval is not required for manufacturing and use of phosphogypsum panels or blocks, if they have Ra-226 activity less than 40 kBq/square metre area of any surface of the panels/blocks.
The possible annual increase in radiation dose to a person living in a building made with such panels is sufficiently low to qualify for exemption as per guidelines accepted by the European Commission on Radiation Protection.
The activity levels prescribed by AERB are such that they do not present an unreasonable radiation hazard to anyone.
Further, AERB decided that there need be no restriction for use of phosphogypsum in agricultural applications from the radiological safety considerations.
Twelve fertilizer plants in India presently process rock phosphates imported from countries such as Jordan, China, Morocco, Egypt, Senegal, Togo and others for production of phosphoric acid / fertilizers.
Annual generation
According to Building Materials and Technology Promotion Council, Indian companies generate 4.5 million tonnes of phosphogypsum annually. Over 10 million tonnes gets accumulated at plant sites. In Florida, U.S., alone, more than 900 million tons of PG is stacked in more than 25 stacks. Thirty million tons of PG is produced each year.
AERB received queries from the Ministry of Chemicals and Fertilizers, the Ministry of Agriculture and some of the fertilizer plants regarding restrictions based on radiological safety considerations, if any, on use of phosphogypsum in building and construction materials and in agriculture respectively.
As is the practice evolved by AERB from its inception, the Board issued the directive after a comprehensive and in-depth review of all aspects and after broad consultation with all stakeholders. The inputs needed to arrive at the directive came from extensive research by the scientists from the Bhabha Atomic Research Centre and from the deliberations of specialists in related fields.
K.S. PARTHASARATHY
(Raja Ramanna Fellow, Department of Atomic Energy)
ksparth@yahoo.co.uk
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