The biokinetic model presented by the International Commission on Radiological Protection (ICRP) in the Publication 67 represents one of the basic tools far evaluating the risk of exposure to an internal contamination from Plutonium. However it is characterized by some assumptions that have no clear physiological explanation, but that were introduced to have a closer fit to the available data from studies on humans, particularly in relation to the urinary excretion at long time. Such assumptions are not only a correction of model's prediction of Plutonium urinary excretion, but it was showed that they are the leading process in determining the urinary excretion after already 100 days post intake. Yet, even with such corrections, this biokinetic model shows still difficulties in describing the metabolism of Plutonium in human body.In recent years, as data relating to the urinary excretion of Plutonium at long time become more and more available, an enhancement of the urinary excretion long after intake was observed. The application of such mode l on these actual cases would result in a significant overestimation of the dose, with important consequences not only from a scientific, but even from a legal point of view.Owing to these considerations the biokinetic model from ICRP was further developed. Particular attention was paid to the predictions of the urinary excretion, because this is the most common and feasible technique far monitoring the exposure to the risk of an internal contamination of Plutonium. The ICRP model was modified on the basis of the available data from studies of Plutonium metabolism in humans in order to get accurate predictions of the urinary excretion without the ICRP assumptions that can not be physiologically supported. The optimized model obtained from this analysis predicts also values for the fecal excretion and blood retention of Plutonium that agrees better to the available data than ICRP model does.Dose coefficients and analytical expressions that approximates the urinary excretion of Plutonium predicted by the optimized mode I were calculated in order to allow an easy implementation of the model in the monitoring routine of radiation protection.Furthermore a sensitivity analysis was carried out on the transfer rates of the model in order to point out the most significant parameters in determining the urinary excretion of Plutonium and, at a lesser extent, the fecal excretion and the blood retention.On the basis of the results of the sensitivity analysis an uncertainty analysis was also carried out to evaluate the range of variation of the model transfer rates that would reproduce the variability of the urinary excretion observed in humans. It is therefore possible to have an indication about the possible values of the transfer rates that can be assumed far a specific subject of the population.The optimized model was finally used to describe the urinary excretion of an actual case of contamination. This subject is today one of the most known and studied worldwide because a great number of data are available since his contamination in 1983. Furthermore he represents one the most significant examples of enhancement of the urinary excretion of Plutonium at long time after exposure.
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