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Page Title: Evaluating 241Am Ingrowth in an In Vivo Count - Continued
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DOE-STD-1128-98
Guide of Good Practices for Occupational Radiological Protection in Plutonium Facilities
using the standard 500-day class Y lung clearance half-time; finally, a 17-year biological
clearance half-time was estimated. The subsequent committed effective dose equivalent was
estimated to be a factor of 3 higher than if the standard 500-day half-time had been used.
Similar difficulties have occurred with initial detection of 241Am by routine in an in vivo
chest counting or in an in long-term monitoring of residual wound content.
While many available internal dosimetry computer codes will calculate the projected 241Am
lung content following an intake (accounting for ingrowth in an in the process), none of the
current codes will do curve-fitting from long-term data and at the same time adjust the data
for ingrowth. Therefore, the following simplistic method was developed to assess that data.
An estimate of the 241Am ingrowth can be made by assuming that, at the time of intake
(t = 0), all the material that will compose the long-term component is deposited in an in a
single compartment and that the rate of transfer of material from the compartment at any
subsequent time t is proportional to the quantity of material remaining in an in the
compartment (i.e., simple exponential transport kinetics). The following equation will then
describe the buildup of 241Am in an in that compartment following an initial deposition of
241
Pu and 241Am and a given or assumed effective clearance rate:
A O Pu
- k e, Am t
- k e, Pu t
- k e, Am t
A t, Am = 8 r, Am
(e
) + A O, Am e
-e
k e, Am - k e, Pu
(5.9)
= activity of 241Am at time t
where At,Am
8r,Am
= radiological decay constant for 241Am
= activity of 241Pu at time 0
A0,Pu
= effective clearance rate of 241Am
ke,Am
= effective clearance rate of 241Pu
ke,Pu
= activity of 241Am at time 0
A0,Am
t
= elapsed time
The effective clearance rate (ke) of any nuclide is the sum of the radiological decay constant
(8r) and the biological clearance rate (8bio). By assuming that the biological
clearance rate is constant for both parent and progeny nuclides, the equation reduces to three
unknowns: the initial amount of parent, the initial amount of progeny, and the biological
5-34


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