Journal of Radioanalydcal and Nuclear Chemiatry, Articles, VoL 177, No. I (1994) 121-~126 INTERPRETATION OF THORIUM BIOASSAY DATA L- i . Q. C, JULIAO: A. M: G. F. AZEREDO,i . S. SANTOS;DiR, MELO; B.M. DANTAS,J. L. LIPSZTEIN Instituto de Radioprote~do e Dosimetria/CNEN, A ~. :Salvador Allende, s/n*, via 9'Barra da Tijuccr CEP 22780:!60 ~ de Janeiro (Brazil) (Received October1, 1993) This study is a comparisonbetweenbioassay data of thorium.exposedworkers from two different facilities. The first of these facilities is a monazite sand extraction plant. Isotopic equilibrium between Zn'rh and ~ was not observed in excreta smnplesof these workers. The second facility is a gas mantle factory. An isotopic equilibrium between ~ and Z~rh was observed in excreta samples. Whole body counter m e s s ~ e n t s have indicated a very low intake of thoriumthroughinhalation. As die concentrationof thoriumin feces was ~teryhigh we concluded that die main pathway of entrance of the nuclide was ingestion, mainly via contamination through dirty hands. The comparison between the bioassayresults of workersfrom the two facilities shows that the lack of Th isotopicequilibriumobservedin the exeretionfrom the workersat the monazite sand plant possiblyoccurreddue to an additional Th intake by ingestion of contaminatedfresh food. This is presumablybecause ZnRa is more effw, iently taken up.from the soil by plants, in comparisonto :ruTh or ~ and subsequently,ruth grows in from its immediate ptn'ent, ~Ra. Thorium is an element of potential interest for the Brazilian nuclear industry because of its possible use as a fissile material in breeder reactors. In Brazil the largest deposits of the nuclide occur on the coast as monazitc sand. The Brazilian monazite sand is made up of four minerals: iknenite (FeTiO3), ruffle (TiO2), zirconite (SiT_.tO4) and monazite, an orthophosphatr of rare earth containing up to 6% ThO 2 and 0.3% of U30 v The Brazilian monazite sand extraction plant processes about 100 ton/month of the sand to obtain a concentrate of monazite. The physical methods t of treatment consist of magnetic separation of ilmenite, electrostatic separation of zirconite and gravimetric concentration of monazite up to 99%. The monazite concentrate is leached to obtain the rare earth chloride and also Th and U precipitate as by-products. The Th precipitate is purified and concentrated to obtain the thorium nitxate, which is used in the production of gas mantles. In Brazil, thorium is of great interest in terms of radiological protection, 2 especially in the monazite sand extraction and purification of monazite, and in gas mantle facilities. Elsevier Sequoia S. A., Lausanne Akad~miai Kiad@, Budapest