The in vivo measurement is an efficient method to estimate the retention of activity in case of internal contamination. However, it is currently limited by the use of physical phantoms for the calibration, not enabling to reproduce neither the morphology of the measured person nor the actual distribution of the contamination. The current method of calibration therefore leads to significant systematic uncertainties on the quantification of the contamination. To improve the in vivo measurement, the Laboratory of Internal Dose Assessment (LEDI, IRSN) has developped an original numerical calibration method with the OEDIPE software. It is based on voxel phantoms created from the medical images of persons, and associated with the MCNPX Monte Carlo code of particle transport,.The first version of this software enabled to model simple homogeneous sources and to better estimate the systematic uncertainties in the lung counting of actinides due to the detector position and to the heterogeneous distribution of activity inside the lungs. However, it was not possible to take into account the dynamic feature, and often heterogeneous distribution between body organs and tissues of the activity. Still, the efficiency of the detection system depends on the distribution of the source of activity. The main purpose of the thesis work is to answer to the question: what is the influence of the biokinetics of the radionuclides on the in vivo counting?
To answer it, it was necessary to deeply modify OEDIPE. This new development enabled to model the source of activity more realistically from the reference biokinetic models defined by the ICRP.
The first part of the work consisted in developing the numerical tools needed to integrate the biokinetics in OEDIPE. Then, a methodology was developed to quantify its influence on the in vivo counting from the results of simulations.
This method was carried out and validated on the model of the in vivo counting system of the LEDI.
Finally, the procedure was applied to the in vivo counting system of the medical laboratory of AREVA NC La Hague and to a real case of contamination. This work enabled to study and quantify the incomplete knowledge of the body distribution of activity as another systematic source of uncertainty,.Discrepancies of the order of 50% were found in the estimation of the retention of activity from the lung measurement of the 59.54 keV ray of Am-241 in the first days following the contamination. The developed method will be used in the laboratory of AREVA NC La Hague and can be applied in every laboratory dedicated to the in vivo counting of nuclear workers, to correct the efficiency calibration depending on the biokinetics. By mitigating the associated source of uncertainty, this work will therefore contribute to optimizing the estimation of the internal dose.