Intake and Dose Assessment for Mixtures of Radionuclides

DOE-STD-1121-98

0.9

0.8

0.7

0.6

0.5

0.1

Tim e Betw een Bioassay Measurements, T (units of T1/2)

Figure 6. Time at Which Expectation Value of Intake Occurs as a Fraction of Interval

Midpoint for a Single Exponential Retention Function

7.4.2 Intake and Dose Assessment for Mixtures of Radionuclides

Mixtures of radionuclides can pose difficulties in assessment due to bioassay methods for different

nuclides having different sensitivities. When the isotopic composition of a mixture can be reasonably

known or assumed, an effective approach to bioassay and simplified intake and dose assessment can be to

select an indicator nuclide for the mixture and then base intake and dose assessments on the isotopic

activity ratios of each nuclide in the mixture relative to the indicator nuclide.

Example 7.1. Radionuclide Mixture: Sludge from Tanks Containing High Level Waste

High-level waste tank sludge mixtures may consist of predominantly mixed fission product radioactivity (mainly

137

Cs and 90Sr) with trace amounts of transuranics. From the bioassay perspective, the only readily detectable

nuclides may be the fission products. However, the contribution to total dose may be far more significant from

the minute quantities of transuranics. A logical nuclide for bioassay would be 137Cs because of its ease of

measurement by whole body counting and its relatively well-established biokinetic behavior. Detection of 137Cs

would result in estimating the intake of that nuclide using the standard biokinetic model for 137Cs in the total

body. Once the 137Cs intake was obtained, that result would be multiplied by the isotope ratio of each nuclide in

the mixture relative to 137Cs to give the intake of that nuclide. Doses can then be evaluated by calculating the

contribution from each nuclide intake.

When using indicator radionuclides and isotope ratios for mixtures, it is important to remember that

the activity ratio at the time of bioassay is not necessarily the same as the activity ratio at the time of

intake. If activity ratios in bioassay measurements at times following intake are being compared to those

in a smear sample or other source term sample, it is necessary to consider the differing biokinetic

behavior of the nuclides that are involved in the intake.