Optimizing measuring protocols for 129I et 129I/127I. Establishing a methodology for environmental samples.
Carole FRECHOU, doctorate thesis of the University of Paris XI-Orsay, speciality: radiochemistry, defended in 2000.
129I is a natural long-lived isotope, with a half-life of 15,7 million years, also artificially produced in nuclear power plant It is then released in the liquid and gaseous effluents of the nuclear fuel reprocessing plants. 129I is integrated in all biological compartments at different activity levels, depending on their distance from the emission source and their ability to metabolise iodine.
Performances of the different 129I and 129I/127I measurement techniques available : Radiochemical Neutron Activation Analysis, Accelerator Mass Spectrometry, direct g?X spectrometry and liquid scintillation were evaluated. Associated radiochemical preparation steps of the two first techniques were optimized and adapted to the characteristics of the major environmental matrices.
In a first step, the radiochemical protocols were developed and validated. In a second step, intercomparison exercises have been lead on various environmental samples presenting different 129I activity levels. They showed the good agreement between the results given by the three techniques on different environmental matrices with activities between 0,2 and 200 Bq.kg-1 dry weight.
As a conclusion, a methodology for the measurement of 129I and 129I/127I ratio in environmental samples is proposed. It includes a decisional diagram taking into account the characteristics of the matrices, the detection imits and the answer delay.
A study on the losses of 129I during the calcination of an algae was lead by direct g?X spectrometry and application studies were made to measure 129I levels in different biological compartments issued from various locations: "I activity interspecific variation in different species of seaweeds from the French channel coast under the relative influence of La Hague, 129I levels in bovine thyroids from the Cotentin area and 129I in vegetal samples collected around the nuclear fuel reprocessing plant of Marcoule.