Question 460: Appendix B contains only one derived air
concentration (DAC) value for each radionuclide. The DAC
provided in Appendix B is derived from the more limiting of
the stochastic or the non-stochastic annual limit on intake
(ALI). In Regulatory Guide 8.34 (Section 3.3) the NRC
provides guidance that the stochastic DAC should be used,
in preference to the non-stochastic DAC, to calculate the
committed effective dose equivalent (CEDE). This
Regulatory Guide further provides a method for deriving
stochastic DACs for radionuclides that only have the
non-stochastic DAC listed in Appendix B. In addition,
Regulatory Guide 8.7 (Section 2.2) provides guidance that
if the CEDE does not exceed 1 rem, then organ doses, which
utilize non-stochastic DACs for calculation, need not be
calculated. Some licensees have concluded, from their
prospective evaluations of potential internal dose to
workers at their facility, that workers are not likely to
exceed 10% of an ALI (i.e., are not likely to exceed 500
mrem CEDE). For the situation where the licensee has
concluded that workers are not likely to exceed 10% of an
ALI, may the licensee derive and use stochastic DACs, in
lieu of the non-stochastic DACs listed in Appendix B, for
(a) posting and (b) exposure control purposes? Such an
approach, employing the stochastic DACs, would allow
licensees to more appropriately assess and control
exposures commensurate with the applicable radiological
conditions, than would be the case if the more
conservative, non-stochastic DACs were used. For example,
in evaluating the use of respirators with regard to keeping
the total effective dose equivalent (TEDE) ALARA, the use
of stochastic DACs, and respective calculated internal dose
projections, would provide a more valid comparison with
projected doses from external sources of exposure, than
would be afforded through the use of non-stochastic DACs.
Answer: (a) No, with respect to posting of "airborne
radioactivity areas" in accordance with the provisions of
10 CFR 20.1902 (d) and the definition of "airborne
radioactivity area" in 10 CFR 20.1003. The use of
stochastic DACs in lieu of non-stochastic DACs listed in
Appendix B would require an exemption, under the provisions
of 10 CFR 20.2301 [applications for exemptions], from the
posting requirements of 10 CFR 20.1902 (d) [posting of
airborne radioactivity areas] .
(b) It is not possible to answer the general question with
respect to "exposure control purposes," without having an
explanation of what is meant by this term. However, in
regard to the specific example given, the use of a
stochastic DACs, and respective calculated internal dose
projections, is acceptable in evaluating the use of
respirators with regard to keeping the total effective dose
equivalent (TEDE) ALARA, when this results in a more valid
comparison with projected doses from external sources of
exposure than would be afforded through the use of
non-stochastic DACs. Note: See related Question 459
concerning the meaning of the word "applicable" in the
phrase "applicable ALIs" in 10 CFR 20.1502. (References:
10 CFR 20.1902, 10 CFR 20.1502, 10 CFR 20.1003).