United States Nuclear Regulatory Commission - Protecting People and the Environment

Clarification of Regulatory Guide 1.21, Section C.10, "Sensitivity"

HPPOS-122 PDR-9111210281

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 ratios.

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

Applicability: Reactors

Page Last Reviewed/Updated Friday, June 12, 2015