Answer to Question #6859 Submitted to "Ask the Experts"

We are decommissioning a former radium-use site for the Navy. There is also ⁹⁰Sr present. Soil samples have indicated about 3 pCi/g ⁹⁰Sr compared with 500 pCi/g ²²⁶Ra. The release criteria for ⁹⁰Sr are as follows: 200 dpm loose/1,000 total. Are these release criteria too strict considering ²²⁶Ra is the predominant radionuclide? Also, if we have to use the ⁹⁰Sr limit, shouldn't we then use a ⁹⁰Sr source for our scalers, instead of the ⁹⁹Tc we currently use? We currently have to count swipes for five minutes to get an MDA (minimum detectable activity) of 200 dpm beta.

Thank you for your question pertaining to the final status survey design for a building contaminated with ²²⁶Ra and ⁹⁰Sr. It appears from your question that you are using the surface activity guidelines found in the NRC (Nuclear Regulatory Commission) Regulatory Guide 1.86. There are four groupings of guidelines in this document—²²⁶Ra belongs to the grouping that has a limit of 100 total dpm/100 cm² and 20 dpm/100 cm² removable, while ⁹⁰Sr has limits of 1,000 total dpm/100 cm² and 200 dpm/100 cm² removable. I should mention that many decommissioning projects performed under the NRC's regulatory authority must now comply with the license termination rule (LTR) promulgated in 1997. The release criteria for unrestricted release in the LTR are based on 25 mrem/y. The NRC published screening values that are based on the 25 mrem/y release criterion; for example, the ⁹⁰Sr limit is 8,700 dpm/100 cm². Also, under the new regulatory paradigm, there are no specific removable contamination limits; rather, there is a general dose modeling consideration that removable limits are no greater than 10 percent of the screening value. So it may be worthwhile to check with the appropriate regulatory authority to determine whether the new release criteria might apply to your project.

That being said, regardless of the surface activity guidelines being used, it is often the case that multiple radionuclides are present within a building survey unit. Because gross measurements of surface activity levels are usually performed during final status surveys, it is necessary to calculate a gross surface activity guideline. This guideline accounts for the relative fraction of the total activity contributed by each radionuclide. The specific equation can be found in Chapter 4 of the Multiagency Radiation Survey and Site Investigation Manual (MARSSIM).

Finally, the calibration source used should certainly reflect the nature of the radiations (e.g., alpha and beta radiation) being emitted from the radionuclides of concern. Additional guidance on calibration for surface activity measurement instruments can be found in Chapter 6 of the MARSSIM. Also, it may be helpful to point out that a number of useful references and presentations on this topic can be found on the MARSSIM Web page of the Decontamination and Decommissioning Science Consortium Web site.

Eric W. Abelquist, PhD, CHP