Air Monitoring cont'd - doe-hdbk-1184-2004

DOE-HDBK-1184-2004

If large non-respirable tritiated particulates are captured on an air filter, intake

amounts are somewhat overestimated. However, non-respirable particulates are

not efficiently suspended in breathable air and are therefore discriminated against

during air monitoring. Also, some dissolution can occur in the scintillation cocktail,

which measures the activity captured on a filter. Dissolved tritium is not subject to

self-absorption. Intake calculations are therefore overestimated if all activity

observed by LSC is taken as "insoluble" and treated as Type S particulate. Since

SAFs vary by a factor of ~10, dissolution in cocktail will not increase intake

computations by more than a factor of ~10. Either non-respirable particulate

collection or dissolution in cocktail will act to provide overestimation of intake (and

ultimately dose) from air monitoring results.

Intake (and dose rate) in this case may be expressed as follows:

D(t) ∝ Io + ∆Io

(Eq. 5-8)

where:

D(t) = dose rate to lung from observed intake at time t

Io = observed intake to lung from air monitoring results at time zero

∆Io = incremental observed activity from particle dissolution or non-

respirable particulates at time zero.

Shortfalls

The main shortfalls associated with this methodology for intake (and dose)

estimation are: 1) actual activity intakes are uncertain by a factor of 10 because of

self-absorption. 2) activity collected and measured on filters may over-represent

the intake because of capture of non-respirable particulates and dissolution in

scintillation cocktail.

As previously discussed in this handbook, intakes in terms of actual activity do not

need to be determined. Intakes of observed activity are adequate to provide dose

estimates as long as dose conversion factors are available which are described in

terms of observed activity. In Section 5.2.3, self-absorption is investigated in

detail. The section shows that actual activity of an intake is underrepresented by

the observed results from scintillation counting of an air filter sample, but that the

actual activity is correspondingly not fully available to impart dose to lung tissue. In

fact, the observed activity more nearly represents dose or dose potential, since only

that beta radiation that is not self-absorbed and escapes the particulate (i.e., that

beta radiation "observed" via LSC analysis) is available to impart dose.

In Section 5.2.4, the ICRP 66 lung model is used to tabulate dose conversion

factors for a given actual intake of tritiated particulate of various materials and

various particle size distributions, assuming an ICRP 78 biokinetic model and

Absorption Type S (slow). ICRP 66 dose, which does not account for self-

absorption, is corrected by a "self-absorption factor for energy" (SAFe) for the

various particulates to determine the actual dose from the actual intake. SAFe is

the fraction of the total tritium beta energy generated within a particulate which is

released from that particulate. The tritium beta activity expected to be observed in

LSC analysis from the actual intake is then determined using the "self-absorption

factor for beta particles" (SAF) for the various particulates. The result of these