Answer to Question #11491 Submitted to "Ask the Experts"

International Commission on Radiological Protection (ICRP) Publication 78, Individual Monitoring for Internal Exposure of Workers, is an international guidance document containing ingestion and inhalation dose conversion factors (DCFs) for various radionuclides. These DCFs were specifically generated for an adult worker. Tables A12.2 and A12.3 of ICRP 78 contain ingestion DCFs for plutonium-239 (²³⁹Pu) and are broken down by compounds (unspecified compounds, insoluble oxides, and nitrates).

ICRP Publication 72, Age-Dependent Doses to the Members of the Public From Intake of Radionuclides—Part 5 Compilation of Ingestion and Inhalation Coefficients, is a compilation of age-dependent committed effective DCFs for ingestion and inhalation of radionuclides for members of the public. Table A.1 from ICRP 72 contains ingestion DCFs for ²³⁹Pu but does not contain a breakdown by compounds like ICRP 78.

Comparing the two reports (Table 1), I note that the ICRP 72 ingestion DCF in sieverts per becquerel (Sv Bq^-1) for an adult member of the public is identical to the ICRP 78 ingestion DCF for an adult worker using an unspecified compound of ²³⁹Pu.

Table 1. Comparison of Ingestion DCFs for ²³⁹Pu

^aUnspecified compounds are not explicitly stated in ICRP 72 but are inferred from ICRP78

I would like to know what to do if the plutonium exists as an insoluble oxide compound. ICRP 78 provides an ingestion DCF for an insoluble oxide form (9.0 × 10^-9 Sv Bq^-1), but ICRP 72 does not. Given that the ingestion DCF for an unspecified compound is the same in both ICRP 72 and ICRP 78 (2.5 × 10^-7 Sv Bq^-1), could the insoluble oxide DCF from ICRP 78 (9.0 × 10^-9 Sv Bq^-1) be applied to an adult member of the public?

And a follow-on question: if you can use the 9.0 × 10^-9 Sv Bq^-1 DCF for an adult member of the public based on an insoluble oxide compound, what would the ingestion DCFs be for other age groups outlined in ICRP 72?

Yes, you can reasonably assume that the DCF for an insoluble plutonium oxide as provided in ICRP 78 (9 × 10^-9 Sv Bq^-1) may be applied to an adult member of the public. In fact, you can find the same ingestion DCF in earlier ICRP publications; e.g., Table B.1 of ICRP Publication 68, Dose Coefficients for Intakes of Radionuclides by Workers, shows the value of 9 × 10^-9 Sv Bq^-1 for an absorption fraction (f₁) value of 1.0 × 10^-5.

To make a reasonable estimate of what the appropriate age-dependent DCFs would be for other age groups, one could use the ICRP 72 data (for the f₁ value of 5.0 × 10^-4) to normalize the DCF for each age group to the DCF value for the adult and then multiply the adult DCF value for the insoluble plutonium oxide, 9.0 × 10^-9 Sv Bq^-1, by the normalized value for each age group of interest. I have done this in Table 2.

Table 2. Projected Age-Dependent Normalized DCFs

^a Normalized to ICRP 78 adult DCF of 9.0 × 10^-9 Sv Bq^-1
b Age-dependent value for insoluble oxide

I believe that any uncertainties that result from using this method would be small compared to other uncertainties associated with the biokinetic models themselves and individual variability, and I would feel comfortable using the resulting values. So far as I am aware there is no formal recommendation by any authoritative group for the best approach to handle this situation.

George Chabot, PhD

References
International Commission on Radiological Protection. Dose coefficients for intakes of radionuclides by workers. Oxford: Pergamon Press; ICRP Publication 68; Ann ICRP 24(4); 1994.

International Commission on Radiological Protection. Age-dependent doses to the members of the public from intake of radionuclides—Part 5 compilation of ingestion and inhalation coefficients. Oxford: Pergamon Press; ICRP Publication 72; Ann ICRP 26(1); 1995.

International Commission on Radiological Protection. Individual monitoring for internal exposure of workers (preface and glossary missing). Oxford: Pergamon Press; ICRP Publication 78; Ann ICRP 27(3–4); 1997.