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

excretion at long time (after 2,000 days post intake) predicted by the model still
underestimates the experimental results. In this time range the subject’s excretion is
characterized by the enhancement that made this case a particularly interesting contamination
event.
The Americium burden in lungs at long time, due to the 241 Pu decay, overestimates the
experimental findings in the scenario 1 (up to a factor 4) and underestimates them in the
scenario 3 (by about a factor 12). Therefore such fraction of Americium seems to behave in
the respiratory tract with an absorption type ranging from the type S (the absorption type for
the father Plutonium in the scenario 1) and the type M (the reference absorption type for
Americium adopted in the scenario 3). In order to improve model’s predictions of Americium
lung retention, the absorption parameter determining the Americium retention in lungs at long
time (s t) was varied in the range from 0.0001 d -1 (the reference value for absorption type S) to
0.005 d -1 (the reference value for absorption type M). It was found that model’s predictions
for the Americium burden in lungs significantly improve their agreement with the
experimental findings at long time if a value 0.001 d -1 is assumed for s t (Figure 3.2.7). The
other absorption parameters (s p and s pt) were set to the default values for the absorption type
S.
Such modification may have a reasonable physiological explanation:
• The Americium just formed after Plutonium decay behaves at short time in the respiratory
tract as the father (Plutonium default values of s p and s pt for the absorption type S are
therefore assumed);
• At longer time Americium own specific metabolic characteristics gets more and more
evident (therefore the value of s t has reasonably a value between the default values for S
and M absorption types).
Therefore such approach can be considered more realistic in comparison to the
assumption of an Americium ingrowth with all the same absorption parameters of the father
Plutonium (as usually assumed by ICRP and here in the scenario 1) or with its own default
absorption characteristics (as in case of a direct inhalation).
On the basis of the previous assumed value of s t for Americium ingrowth from 241 Pu
decay, the intake of 241 Am directly inhaled was evaluated as 9 kBq. The intakes of all the
other involved radionuclides are easily evaluated by means of the their ratio at the moment of
the intake. The relating value of the F target function calculated on lung measurements is 50%
and 1% of the value calculated in case of the scenarios 1 and 3 respectively. This
quantitatively points out the improvement of the model’s predictions of Americium lung
retention when the absorption parameters are changed according to the previous assumptions.
The model’s predictions for the urinary excretion of Plutonium are equal to those ones
evaluated in case of the scenario 1 (Figure 3.2.4) because only the default s t absorption
parameter for Americium ingrowth was modified and no parameter relating to Plutonium was
modified.
Model’s predictions for the lymph nodes burden of Americium were compared to few
available measurements too and a good agreement was found even for this set of data.
At conclusion of the analysis of the most reliable measurements for modelling and
testing purposes, a good agreement of model’s predictions with the experimental data was
obtained. Furthermore the considered modification (the change of the s t absorption parameter
for Americium ingrowth from Plutonium decay) seems to be reasonably acceptable from a
physiological point of view too. However the enhancement of Plutonium urinary excretion at
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