INTERNAL DOSE EVALUATION PROGRAM

Internal doses are not directly measured but are estimated or calculated based on knowledge of the material to which a worker may be exposed and its known or assumed biokinetic behavior. The common approach to internal dosimetry is to calculate an occupational intake based on worker bioassay

measurements or workplace air-sample data and assumed breathing rates. Once an intake is calculated, appropriate equivalent doses to organs and tissues of concern can be estimated by using fundamental dosimetry principles, by various intake-to-dose conversion factors, that incorporate assumed biokinetic models, or by use of an appropriate computer code. Intake-to-dose conversion factors can be found in ICRP Publication 68. Further discussion on intake and dose assessment is provided in Section 5.8.

Participation in internal dose evaluation programs (which include routine bioassay programs) is required for conditions identified in 10 CFR 835.402(c). The internal dose evaluation program must address both general workplace conditions and individual intakes. Workplace conditions are monitored through air and surface contamination monitoring programs. Individual monitoring for intakes is commonly performed using bioassay procedures. Bioassay monitoring includes both direct (in vivo) measurements of radioactivity in the body and indirect (in vitro) measurements of material excreted or removed from the body.

a bioassay program is generally based on the possibility that a single intake causing a dose in excess of 0.1 rem committed effective dose might occur.

Indications of intake include (but are not limited to) detection of facial or nasal contamination, positive air monitoring or sampling results that may indicate internal exposure, or any wound in which contamination is detected or suspected. The most common internal exposure monitoring program for workers is the bioassay program, which must be designed for the specific nuclides and forms of material at a particular facility. Likely candidates for internal exposure monitoring include personnel who may be routinely exposed to surface or airborne contamination, or those identified by workplace indicators.

Workplace monitoring for potential internal exposures is performed to verify the adequacy of containment and work practices. This monitoring includes air sampling, continuous air monitoring, personal contamination surveys, and workplace contamination surveys. Facilities are to be designed and operated to minimize internal exposure. Details regarding workplace monitoring and control practices are discussed in Chapter 4, Contamination Control.

5.1.1 Performance Capabilities for Internal Exposure Monitoring

Bioassay programs must be capable of showing compliance with the 5-rem/year stochastic and 50-rem/year deterministic dose limits of 10 CFR 835.202. 10 CFR 835.402(c)(1) identifies 0.1 rem committed effective dose for all likely intakes as a level above which workers must participate in a bioassay program.

Therefore, ideally, such bioassay monitoring programs should be capable of detecting individual doses at that level. To meet this requirement, reliance must be placed on workplace monitoring to identify potential intakes at the time they occur so that special bioassay monitoring can be initiated.

Performance capabilities for bioassay and internal dosimetry programs can be expressed as the minimum detectable dose, based on some combination of minimum detectable activity and frequency of measurement or time post-intake at which the measurement is made. The term "minimum detectable dose"

is preferred over any variants of the occasionally encountered terms "dose-missed" or "potentially

undetected dose," which were usually defined as the same thing. The connotation of the latter terms is that of an actual intake which was not detected, whereas the intent was to define a measure of program

sensitivity to doses that might have gone undetected had an intake occurred. The preferred term "minimum detectable dose" (MDD) ties the concept to the recognized terminology of minimum detectable activity (MDA).

The MDD for a bioassay program must meet the aforementioned dose limit requirements of 10 CFR 835.202. A design goal of 0.1 rem committed effective dose from all intakes of similar nuclides in a year is desirable but unrealistic for a routine program. To meet these requirements, bioassay programs should have measurement sensitivities (i.e., MDAs for bioassay measurements) established based on the material to which workers might be exposed. Examples of such sensitivities are given in Tables 5-1 through 5-3 for pure ²³⁸Umonitored by urinalysis, fecal analysis, and lung counting, respectively. The bioassay goals are calculated by multiplying the intake (nCi) by the intake retention fraction (IRF) and by a correction factor of 2,220 dpm/nCi, where intake is the dose limit divided by a calculated dose conversion factor (rem/nCi). For simplicity, in the tables, the dose limit goal is based on the 50-rem committed equivalent dose; the other goals are based on the 0.1-rem committed effective dose monitoring threshold. For type M uranium-238, the stochastic limit (5 rem) is slightly more restrictive, by approximately 20%.

If, for some reason, the goal of 0.1 rem committed effective dose cannot be met through routine bioassay, the radiation protection organization should take the following administrative actions:

• document the adequate control measures for auditing purposes,

• upgrade bioassay measurement systems and workplace monitoring practices to provide state-of-the-art measurements, and

• ensure that internal dose assessments use state-of-the-art technology.

All confirmed occupational intakes of uranium, regardless of magnitude, should be assessed. The results of all bioassay and other measurements needed to demonstrate the quality of measurements and dose assessment should be recorded and maintained. The recording and reporting requirements for internal dosimetry data are set forth in Section 3.2.9 of this technical standard

Table 5.1. Urine Bioassay Goals

for

U Days Post-Intake Urine Intake Retention

Fraction

(a) The goals reflect the activity in a 24 hour urine void corresponding to either a 50 rem committed equivalent dose or a 0.1 rem committed effective dose.

(b) IRF values obtained from “Intake Retention Functions Developed from Models Used in the Determination of Dose Coefficients Developed for ICRP Publication 68 – Particulate Inhalation” (Potter 2002).