58. Calculation of Monitoring Data for ²³⁹Pu Using ICRP Dosimetric Models for Internal Exposure

Nobuhito Ishigure, Takashi Nakano, Hiroko Enomoto and Jiro Inaba

Keywords: monitoring quantities, Pu-239, ICRP, respiratory tract model, biokinetic model

Plutonium is one of the most important elements from the viewpoint of radiotoxicology and radiation protection. The International Commission on Radiological Protection (ICRP) recently revised the respiratory tract model (Publication 66) and biokinetic models for selected elements including plutonium (Publication 67). Using these new models, the monitoring quantities for inhaled ²³⁹Pu such as lung retention, daily urinary and fecal excretion rate have been computed.

The following ICRP default values for the physical characteristics of the radioactive aerosols were used: AMAD (Activity Median Aerodynamic Diameter) = 5 m and 1 m, geometric standard deviation = 2.5, particle density = 3 g/cm³, particle shape factor = 1.5. The subject exposed to the aerosols is the ICRP reference worker doing light work: light exercise with the ventilation rate of 1.5 m³/h for 5.5 h + sitting with the ventilation rate of 0.54 m³/h for 2.5 h.

As well as the monitoring quantities, the conversion coefficients from the measured monitoring quantities to the effective dose have been obtained. An example for the daily fecal excretion rate is shown in Fig.20, (a) is for Type M compounds and (b) for Type S compounds. It is shown that the estimated effective dose based on the feces analysis depends significantly on the types of compounds and the AMAD of aerosol particles.

In addition to the graphical representation, the computed monitoring quantities have been represented approximately by summation of exponential functions. For example, the fecal excretion function F(t)of inhaled ²³⁹Pu of type M compounds with the AMAD of 5 m is approximated as

F(t)=2.269exp(-1.018t) + 5.71810^-4exp(-3.01810^-2t)
+ 1.50010^-4exp(-2.32210^-1t) + 4.77110^-5exp(-1.26610^-2t)
+ 1.85910^-5exp(-3.69910^-3t) + 1.31710^-7exp(-3.75310^-7t)
- 3.540exp(-1.602t)

Fig.20. Conversion coefficients (Sv/(Bq/d)) for effective dose per daily fecal excretion rate of inhaled ²³⁹Pu;
(a) for Type M and (b) for Type S compounds.

Publications:
Ishigure, N., Nakano, T., Enomoto, H. and Inaba, J.: J. Health Physics, 33,415-423, 1998 (in Japanese, English abstract available).