United States Nuclear Regulatory Commission - Protecting People and the Environment

U.S. Nuclear Regulatory Commission

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).

Page Last Reviewed/Updated Thursday, March 29, 2012