Issue

Footnote 5 of Appendix D to Part 835, "Surface Radioactivity Values," reads as follows:

"This category of radionuclides includes mixed fission products, including the Sr-90 that is present in them. It does not apply to Sr-90, which has been separated from the other fission products or mixtures where the Sr-90 has been enriched."

Questions have been asked concerning how this provision is to be applied.

Introduction

The occupational radiation protection standards were promulgated previous to title 10, Code of Federal Regulations, part 835 (10CFR835) in Department of Energy (DOE) Order 5480.11, which was published December 21, 1988. In drafting this Order, it was decided to include "Surface Radioactivity Guides," which consisted of a table of removable and total (fixed + removable) contamination values in units of disintegrations per minute (dpm)/ 100 centimeters (cm)². The table was based on the Nuclear Regulatory Commission's (NRC) Regulatory Guide 1.86.

This footnote was added in DOE Order 5480.11 and later in 10CFR835, Appendix D,to the table of surface radioactivity values to resolve concerns that the total surface radioactivity limit for beta-gamma entities includes mixed fission products, one component of which is strontium-90 (SR-90), which could not be easily measured using frisking instruments, or otherwise assessed. However, facilities now contain items or areas that were radioactively contaminated with mixed fission products, which were produced many years ago. As a result, the majority of the shorter lived isotopes have decayed away and a major radionuclide now present is SR-90.

Discussion

In adopting the Regulatory Guide 1.86 values, DOE assessed potential doses from structures contaminated with Sr-90 at these values. At the 1000 dpm/100 cm² level, independent dose assessments conducted by the Office of Nuclear Energy and the Office of Environment, Safety and Health estimated maximum doses associated with occupying a structure uniformly contaminated at this level to be between 0.02 and 0.1 millirem (mrem) in a year. Estimates of Cesium (Cs)-137 associated doses at concentrations equal to the last category (5000 dpm/100 cm² for a similarly conservative scenario) were calculated to be between 3 and 7 mrem in a year. A mixture containing 50 percent Sr-90 and 50 percent other fission products would be expected to produce a dose significantly lower than the Cs-137 dose.

More recent conservative dose estimates conducted by the NRC (NUREG-1500) and the Environmental Protection Agency (EPA) (draft report for 40CFR196 cleanup standards) indicate that the doses for Sr-90 on surfaces of structures at values of 1000 dpm/100 cm² would be 1.5 mrem/yr (NRC) and about 1 mrem/yr (EPA). These estimates are conservative but suggest that Sr-90 surface limits in Regulatory Guide 1.86, DOE Orders, and 10CFR835 are conservatively protective. They further suggest that broad application of the footnote to concentrations of fission products that contain as much as 50 percent Sr-90 will not significantly increase risks beyond that expected from fission products without Sr-90 present.

Furthermore, addressing the Sr-90 fraction separately will significantly complicate measurement protocols; however, it will not add substantially to risk reduction. The archival documentation supporting Orders DOE 5480.11 and DOE 5400.5 contains information on the specific interpretation of the footnote, "It does not apply to Sr-90 which has been separated from other fission products or mixtures where Sr-90 is enriched." However, on the basis of information retrieved from the archives and this analysis, the Department believes it to be protective, implementable, and cost effective to establish a Sr-90 fraction of 50 percent in mixed fission products for the purpose of implementing surveys consistent with the table in 10CFR835, appendix D.

As long as the activity fraction of the Sr-90 is approximately 50 percent or less of the total fission product activity, compliance with the mixed fission product limits is sufficient to demonstrate compliance with the table values.

For situations where the fraction of Sr-90 exceeds 50 percent of the mixed fission products, the Department considered applying the sum of the fractions rule. However, it believes that the process will be too complicated and, instead, is recommending an incremental approach that is generally consistent with the sum of the fractions approach but will simplify protocols while meeting the intent of the table.

Technical Position

Based on the information presented above, the following application of footnote 5 in appendix D of 10CFR835 to Sr-90 combined with mixed fission products is considered an acceptable approach for compliance with 10CFR835:

1. Where the Sr-90 fraction is 50 percent or less, the mixed fission products surface activity values apply.

2. Where Sr-90 fraction is between 50 percent and 90 percent of the total activity, surface radioactivity values should be 3000 dpm/100 cm² or less.

3. Where the Sr-90 fraction exceeds 90 percent the total activity, Sr-90 surface radioactivity values should be applied to the material.

1. EPA Draft Background Document for 40CFR96 Cleanup Standards.

2. NRC, NUREG-1500, "Working Draft Regulatory Guide on Release Criteria for Decommissioning: NRC Staff's Draft for Comment."

3. DOE memorandum from Carl G. Welty to John Tseng, EH-23, "Analysis of Surface Contamination Limits Adopted from Regulatory Guide 1.86," September 21, 1988.