Assessment of the solubility of thorium and uranium from black sand of Camargue in both simulated lung and gut fluids for dose calculation after internal exposure

Titre du congrès :IRPA 2006 2nd European IRPA Congress on Radiation Protection Ville du congrès :Paris Date du congrès :15/05/2006

  In the south of France, some beaches exhibit a high dose rate (1-3 uSv h"1) due to enrichment in naturel radioactivity, mainly thorium (Th) and uranium (U) from zircon and apatite heavy minerais présent in the sand. The full assessment of radiological risk also requires internai exposure of people frequenting thèse beaches requires some informations on the human bioavailability of U and Th of this sand. In such conditions, the internai exposure either by inhalation or ingestion of sand was studied and the consécutive dose delivered was calculated with two différent scénarios in each case of exposure.
Preliminary studies consisted in the détermination of the activities of ^U and 232Th using gamma spectrometry, the morphology of sand grains with scanning électron microscopy, the détermination of the minerai phases by X-ray diffraction and the assessment of the inhalable fraction by sieving.
In vitro assays of solubility were then performed either on the fraction of sand considered as inhalable with lung simulated fluid and on whole sand sample with both gut and intestinal simulated fluids. Kinetics of dissolution of uranium and thorium were determined after ICP-MS analyses and the remaining fraction of sand were analysed by gamma spectrametry to assess. the acbvity of ^U, za2Th and their descendais: after the dremicat tieauntstL
The first important conclusion is that the inhalable fraction, i.e. particles with a géométrie diameter ranging between 20-50 um, was tiny (û.0û2x>) in studied sand. sample. The inhalation risk is therefore very weak even if the main radioactivity is carried by this fine fraction. Uranium and thorium as well as their descendants présent a Type M of solubility. Doses hâve been calculated from différent cases of acute or chrome exposure according to the expérimental results.
In case of ingestion, the solubility of thorium and uranium in the gastrointestinal fluids was found to be generally very low with a maximum solubility of 0.5% of the initial mass of radioélément in the sample of sand. Then the worst hypothesis studied yields to a dose assessment of 0.018 mSv.(g_swallowed sand)'1 that is roughly 50 times less than the légal annual dose admitted for public.

As a conclusion, the radiological delivered dose after internai exposure, Le. inhalation and ingestion of black sand, seems to be very weak especially compare to the extemal exposure.