Health Physics Questions and Answers - Question 460
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.
(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).