United States Nuclear Regulatory Commission - Protecting People and the Environment

U.S. Nuclear Regulatory Commission

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

HPPOS-122 PDR-9111210281

Title: Clarification of Regulatory Guide 1.21, Section

C.10, "Sensitivity"

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

Applicability: Reactors

Page Last Reviewed/Updated Thursday, March 29, 2012