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

DOE-HDBK-1184-2004

calculations is a table of actual dose conversion factors for intakes, based on

observed data, for a variety of materials and particulate sizes. Because actual

activity is not needed to assign dose from air sampling data, uncertainty in actual

activity (because of uncertainty in particle size and hence SAFs) is not a shortfall to

the air monitoring method for dose estimation. In addition, uncertainty in observed

activity is only a function of the LSC assay technique, which is expected to be small

and comparable to the uncertainty when LSC is used to analyze in urine or fecal

bioassay samples.

The remaining expected shortfall is an overrepresentation of tritiated particulate

intake because of capture of non-respirable particulates or dissolution in scintillation

cocktail. These factors are not expected to increase dose calculations significantly.

Non-respirable particulates will tend to be less available to sampling than respirable

particulates, due to rapid settling. Dissolution in scintillation cocktail would have to

occur within the few-hour period prior to analysis during which samples are

immersed in the cocktail. These factors would serve to exaggerate intake

estimates. The extent of this overestimation is the only significant uncertainty in

the intake determination from air sampling methodology.

Another uncertainty occurs in the air sampling method which affects all methods.

For a given intake, an expected dissolution rate must be chosen for the

determination of the DCF. If Type S is chosen, when in fact captured particulates

on an air-sampling filter more rapidly dissolve in the lung, this constitutes dose

overestimation. Dose can be overestimated by a factor of 50 for this reason. The

typical overestimate is expected to be a factor of less than 10, because the fastest

dissolving particulates lose tritium to air during sampling and prior to analysis.

For the case of materials with extremely slow dissolution rates, such as hafnium

tritide, the assumption of Type S is reasonable (Cheng 1999a), and would not lead

to significantly underestimated doses. For Type S and Type "Super S" materials,

mechanical clearance is the predominant removal mechanism, as opposed to

dissolution. ("Super S" is a term used to describe any material where the

absorption rate for tritium is less than that of Type S.) Section 5.2.2 shows that

dose conversion factors for Hf, as derived from Type S and "Super S" input data,

differ by only a few percent.

Detectable intake (and dose) is small in the case of air sampling. Section 5.2.2

determines an air concentration value (ACV) for Type S tritiated particulate which,

when inhaled for one hour, imparts a CEDE of 2.5 10-5 Sv (2.5 mrem); this value

is shown to be 4.8 104 Bq observed/m3. Sub-millirem doses are measurable and

can be readily assigned in the case of air monitoring. Sensitivity is not a shortfall,

but is in fact an advantage to air monitoring.

An additional shortfall for analyzing air samples using LSC is self-absorption of

tritium beta radiation due to dust loading. An effective compensatory measure for

this shortfall is the use of detergent in preparing air samples to remove dust from

samples and suspend it in the counting solution.

The ability to obtain air samples that are representative of the monitored individual's

breathing zone is another potential shortfall of air monitoring. However, according

to U. S. Nuclear Regulatory Commission Regulatory Guide 8.25, samplers located

within about 1 foot of the worker's head may be accepted as representative without