Low-Level Radioactive Waste Classification, Characterization, and Assessment: Waste Streams and Neutron-Activated Metals (NUREG/CR-6567, PNNL-11659)

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Publication Information

Manuscript Completed: July 2000
Date Published: August 2000

Prepared by:
D.E. Robertson, C.W. Thomas, S.L. Pratt, E.A. Lepel, V.W. Thomas
Pacific Northwest National Laboratory
P.O. Box 999
Richland, Washington 99352

P. R. Reed, NRC Project Manager

Prepared for:
Division of Risk Analysis and Applications
Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001

NRC Job Code L1808

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Abstract

This study was conducted to provide NRC, other federal and state regulatory agencies, nuclear utilities, and low-level radioactive waste (LLW) managers with further information on radionuclide source terms that will improve the health and safety aspects of LLW management and disposal. The goal of this work was to enhance the understanding of the occurrence, distribution, and assessment of the radiological impacts of radionuclides associated with neutron-activated metals and other LLW streams from commercial nuclear power stations. This study focused on identifying and characterizing a group of very long-lived radionuclides that are not specified in 10 CFR Part 61, but which are present in significant concentrations in various types of LLW materials generated at commercial nuclear power stations. The concentrations of ¹⁰Be, ³⁶Cl, ⁹³Mo, ^93mNb, ^108mAg, ^113mCd, and ^121mSn (as well as the specified 10 CFR Part 61 radionuclides) have been measured in a variety of neutron-activated metal and spent primary demineralization resin LLW samples obtained from U.S. nuclear power stations. This work required the development of new or modified radiochemical separation, purification, and counting procedures to measure these radionuclides in the LLW materials of interest. Of this group of radionuclides, the ¹⁰Be, ³⁶Cl, and ^108mAg appear to be present in some types of LLW materials in sufficient quantities to warrant further investigations to better assess their radiological and environmental impacts associated with LLW disposal. It is recommended that further studies be focused on: 1) providing an accurate assessment of the total quantities of these radionuclides in LLW from these sources, 2) determine the leaching characteristics of these LLW materials, 3) determine the migration behavior and environmental pathways of these radionuclides upon release from LLW disposal facilities, and 4) provide performance assessment modelers with the necessary radiological/geochemical information to better predict the potential impacts from disposal of this group of radionuclides. Activity scaling factors have been developed for all of the additional radionuclides measured in this study. The use of activity scaling factors for estimating the concentration of a radionuclide in LLW is permitted by 10 CFR 61.55, and this method is desirable for radionuclides such as those listed above that are difficult to measure directly in LLW materials. Consistent scaling factors for a wide variety of LLW materials were determined for ⁹³Mo and ^93mNb relative to ⁶⁰Co over a concentration range of four orders of magnitude. Generic scaling factors for the other long-lived radionuclides measured in this study showed much greater variability due to the unique composition of some of the materials. However, in some cases useful scaling factors were obtained when similar LLW materials were considered as a group. In addition to these radionuclides, an assessment was conducted to provide an updated understanding of the occurrence, distribution, and activity scaling factors of ¹⁴C, ⁹⁹Tc, and ¹²⁹I, as well as an evaluation of the ⁵⁹Ni/⁶³Ni activity ratio in LLW. This assessment has shown that the concentrations and associated activity scaling factors of ⁹⁹Tc and ¹²⁹I are several orders of magnitude lower than values published earlier in nuclear power industry data bases, thereby significantly reducing the radiological/environmental impacts associated with their disposal in LLW. The assessment of the ⁵⁹Ni/⁶³Ni activity ratio in LLW indicated that early reported values of this ratio were too high by up to factors of 5-10. This was probably due to systematic and/or random uncertainties in their radiochemical analyses. More recent analyses and assessments indicate that this ratio should range from about 0.006 to 0.012, depending upon the neutron flux and irradiation time. Activity scaling factors for ¹⁰Be, ³⁶Cl, ⁹³Mo, ^93mNb, ^108mAg, ^113mCd, and ^121mSn relative to ⁶⁰Co were determined for a variety of activated metal and LLW samples. These scaling factors are believed to be the first published in the literature for these radionuclides.