IAEA-CN-87/61 XA0103313 DISPOSAL OF PHOSPHOGYPSUM WASTE CONTAINING ENHANCED LEVELS OF RADIOACTIVITY Fernando P. Carvalho Nuclear and Technological Institute Department of Radiological and Nuclear Safety E.N. 10, P-2686-953 Sacavem, Portugal From production of phosphoric acid based on the reaction of fosforite with sulphuric acid, manufacturers either have released the phosphogypsum containing uranium decay products into the aquatic environment or have stockpiled the phosphogypsum on land. In Portugal two factories have produced phosphoric acid by this wet chemistry method during several decades, from the 30s till the late 80s. The radioactivity remaining in the phosphogypsum depends upon the composition of the raw material used and upon the efficiency of the chemical reaction method. In one factory, using mainly fosforites from Syria and Tunisia, 226Ra concentrations in the gypsum were at about 600 Bq kg"1 and at about the same level for 210Pb and 210Po. In another factory, using mainly fosforites imported from Morocco, radionuclide concentrations in gypsum were higher, at about 1000 Bq kg"1 for the same radionuclides. Phosphogypsum waste stockpiled on land and uncovered may undergo weathering, including the slow dissolution of calcium sulphate by rain water. This process may be accompanied with partial dissolution of 226Ra, which leaches from the stockpiles, whereas the less soluble 210Pb and 210Po nuclides may remain in the gypsum. In one place, the stockpiles of phosphogypsum have been exposed in the open air for years until recent coverage with soil and vegetation. This remedial action to confine the phosphogypsum have reduced surface runoff, radium leaching and waste disposal. It may have contributed also to reduce radon emmanation. In another site, the gypsum stockpiles are still uncovered in the open air. The disposal site was a former salt evaporation bassin with compact, highly impermeable, fine grained grounds. The gypsum stockpiles are sorrounded by ditches to retain rain water drainage. In the water accumulated in the ditches high concentrations of 226Ra were measured as well as relatively high concentrations of 210Pb and 210Po, although these ones associated mainly to suspended partides. With time phosphogypsum piles become compact and are not easily dispersed by the wind. Radon emmanation and radionuclide leaching by rainwater are the main dispersion routes of radionuclides. To reduce environmental contamination the use of a sorptive layer of clay and coverage with soil may be appropriate means to avoid contamination of underground aquifers through percolation and to reduce radon emmanation into surface air. Final management decisions were not made as yet, but testing is underway to advise on the best solutions to confine phosphogypsum contained radioactivity. 67 REFERENCES [1] Carvalho (1995). 2!0Pb and 210Po in sediments and suspended matter in the Tagus estuary, Portugal. Local enhancement of natural levels by wastes from phosphate ore processing industry. The Science of the Total Environment 159:201-214. [2] Carvalho, F.P. (1997). Distribution, cycling and mean residence time of 226Ra, 210Pb and 210Po in the Tagus estuary. The Science of the Total Environment 196:151-161. 68.