Plutonium Biokinetics in Human Body A. Luciani - Kit-Bibliothek - FZK

i
ABSTRACT
The biokinetic model presented by the International Commission on Radiological
Protection (ICRP) in the Publication 67 represents one of the basic tools for evaluating the
risk following an 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 after exposure. Such assumptions are not only a
correction of model’s prediction of Plutonium urinary excretion, but it was shown 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 still shows 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 after
exposure become more and more available, an enhancement of the urinary excretion long
after intake was observed in some contaminated subjects. The application of the original
model 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 for monitoring the risk after 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't be physiologically supported. The
optimized model obtained from this analysis predicts also values for the fecal excretion and
blood retention of Plutonium that agree better to the available data than ICRP model does.
Dose coefficients and analytical expressions that approximate the urinary excretion of
Plutonium predicted by the optimized model were calculated in order to allow an easy
implementation of the model into the monitoring routine of radiation protection.
Furthermore a sensitivity analysis was carried out with respect to the transfer rates of
the model in order to point out the most significant parameters in determining the urinary
excretion of Plutonium and, to a smaller 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 for 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.
In occasion of the present work a complete set of measurements was carried out in the
Forschungszentrum Karlsruhe in order to assess the urinary and fecal excretion and blood
retention of Plutonium at the present time. Whole body counter and alpha-spectrometry
techniques were adopted for performing in vivo and in vitro measurements on organs and
bioassay samples, respectively. The measurements have confirmed the enhancement of
Plutonium urinary excretion observed in occasion of previous investigations of the same
subject. The application of the information about the possible inter-subjectual variation of the