Absence of radionuclides

["CLAUDE A. FLORY" <FLORYCA@GWSMTP.NU.COM>]

The old 10CFR20 had a method in Appendix B to determine the absence of any
radionuclides in a mixture.  If each 'absent' radionuclide was less than 10% of
it's MPC and the sum ratio of 'absent' radionuclides to their respective MPCs
were less than 25%, than those radionuclides were absent from the mixture.  The
new Part 20 requires that the absence of the radionuclides be demonstrated. 
These appears to be another case of "if it's detectable, it's there."  Have any
NRC licensees had to 'demonstrate' the absence of radionuclides in a mixture? 
If so, how?

Claude Flory
Northeast Utilities
floryca@gwsmtp.nu.com