U.S. Nuclear Regulatory Commission
Clarification of Regulatory Guide 1.21, Section C.10, "Sensitivity"
Title: Clarification of Regulatory Guide 1.21, Section
See the memorandum from L. K. Cohen to J. T. Sutherland
dated October 5, 1977. It clarifies the provision in
Regulatory Guide (RG) 1.21 that allows determination of
concentrations of certain radionuclides based on
measurements of other radionuclides and predetermined
Provided below are answers to specific questions raised on
Section C.10 of RG 1.21 which states: "... it may be more
appropriate to calculate releases of such radionuclides to
those radionuclides which are routinely identified and
measured. Measurements should be made periodically to
establish and assure the continued validity of the ratios
used. Any reported data determined by this method should
be clearly identified."
1. Should the nuclides to be considered include all 10
CFR 20 Table II nuclides?
No. This statement was inserted in RG 1.21 to cover
situations during routine analyses, where a particular
radionuclide or radionuclides predominated a mixture or had
a gamma energy spectrum which interfered with other gamma
energies. Under these circumstances, it would be difficult
to measure certain radionuclides which are known to be
present from more detailed extensive analyses. The
techniques depends upon having a data base of detailed,
thorough analyses, perhaps performed with better
sensitivity and resolution. For example, periodically, the
licensee should make a long measurement on a sample with
GeLi system. Information from these analyses would be then
used to generate ratios and calculate other radionuclides
unresolved in the NaI spectrum.
2. Should the nuclides to be ratio'd be based upon the
isotopic inventory of a composite batch (weekly, monthly,
quarterly, yearly) or single batch (preceding batch, or
reference batch to be selected by licensee)?
The makeup of the composite to determine ratios depend upon
the variability of the isotope mixture and ratios observed
in the past data. If the mixture is stable, then quarterly
composited samples may be sufficient, if not, then more
extensive sampling and analyses may be necessary.
3. If a reference batch, selected by the licensee, is
acceptable - what documentation requirements are necessary?
The licensee must provide documentation to demonstrate that
the batch is representative of the effluent streams being
analyzed. The licensee must also provide and document a
series of analyses over a reasonable length of time to
demonstrate the stability of the isotopic mixture.
4. Where should the ratio based sample be obtained
(primary coolant, secondary systems)?
The sample should be collected from an effluent stream that
assures a representative sample. It is meaningless to
calculate ratios from isotopic mixtures of the primary
coolant for determining airborne effluents.
Regulatory references: Regulatory Guide 1.21
Subject codes: 7.3, 10.1