The rationale for the selected bioassay measurement frequency should also be documented.
It is appropriate to evaluate the probability of intake and to modify the sampling frequency
based on that probability.
The frequency of bioassay measurements should normally not be decreased because
analytical results are below the detection level. The bioassay program should be maintained
to confirm the proper functioning of the overall internal exposure control program and to
document the absence of significant intakes of radionuclides.
Frequency Based on Program Sensitivity
The minimum detectable dose concept refers to the potential dose associated with an
MDA bioassay measurement at a given time interval post-intake. The pattern of
retention of activity in the body, the MDA for a bioassay measurement technique,
and the frequency with which that technique is applied define a quantity of intake
that could go undetected by the bioassay program. An intake of such a magnitude
would not be detected if it occurred immediately after a bioassay measurement and if
it were eliminated from the body at such a rate that nothing was detected during the
next scheduled measurement. The dose resulting from such an intake would be the
MDD for that particular measurement technique and frequency.
Estimates of MDD in terms of CEDE should be documented for each measurement
technique, MDA, and frequency. Retention functions specific to the various chemical
forms and particle size distributions found in the facility should be used. Examples of
MDD tabulations can be found in La Bone et al. (1993) and Carbaugh et al. (1994).
In establishing MDD tables, it is important to consider dose contributions from all
appropriate radionuclides in any mixture, rather than just the dose contribution from
the bioassay indicator nuclide.
Frequency Based on Potential Risk of Intake
As discussed in Section 5.3.2, although plutonium workers are not generally
considered to be at high risk of incurring intakes that might result in CEDEs of 100
mrem or more, any plutonium worker can be considered to have the potential for
such an intake. However, having the potential for intake does not mean that they are
likely to incur an intake.
Workers who have the highest potential risk for an intake are those most closely
working with plutonium or plutonium-contaminated material. Typically, these
workers are glove-box workers, maintenance workers, and operational health physics
surveillance staff. These workers should be on a routine plutonium or americium
bioassay program, including urinalysis and in vivo measurements. Such programs are
relatively insensitive compared to the 100-mrem CEDE goal and are a safety net
intended to catch intakes of significance relative to regulatory limits, rather than
substantially lower administrative levels. Selection of bioassay frequency depends on
the facility experience with potential intakes, the perceived likelihood of intake, and
the MDD of a program. Annual urinalyses and in vivo chest counts are fairly typical.
More frequent (e.g., semi-annual or quarterly) measurements may permit more timely
review of workplace indicators in the event that an abnormal bioassay result is
obtained, but do not necessarily mean a more sensitive program.