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Key Technical Issue: Radionuclide Transport |
Subissue 2
Radionuclide Transport Through Alluvium |
PMR Approach |
| 1a. |
For the estimation of radionuclide transport through alluvium, the Department of Energy (DOE) has determined, through performance assessment (PA) calculations, whether radionuclide attenuation processes such as sorption, precipitation, radioactive decay, and colloidal filtration are important to performance. |
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PA calculations indicate these processes are important. Laboratory data on sorption, matrix diffusion and colloids discussed in Section 3.1.4; conceptual models of solute transport, Section 3.2.4; mathematical models of radionuclide transport, Section 3.3.1; sorption and matrix diffusion sub-model, Section 3.3.4.2; assumptions and limitations associated with radionuclide transport processes, Section 3.5.2; abstraction of radionuclide transport in Total System PA, Section 3.6.3. |
| 1b. |
For the estimation of radionuclide transport through alluvium, DOE has (i) assumed the sorption coefficient is zero and radionuclides travel at the rate of groundwater flow, if it has been found that radionuclide attenuation is unimportant to performance and it can be demonstrated that this assumption is conservative, in which case Criteria 2 and 3 do not have to be met or (ii) demonstrated that Criterion 2 or 3 has been met, if radionuclide attenuation in alluvium is important to performance or if an assumption that the sorption coefficient is zero in alluvium is not conservative.
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For radionuclides with essentially no sorption potential (e.g., carbon, chlorine, iodine), the sorption coefficient (Kd) is assumed to be zero in alluvium. Laboratory data on sorption, matrix diffusion, and colloids discussed in Section 3.1.4; tracer tests, Section 3.1.3.2; conceptual models of solute transport, Section 3.2.4; mathematical models of radionuclide transport, Section 3.3.1; sorption and matrix diffusion sub-model, Section 3.3.4.2; assumptions and limitations associated with radionuclide transport processes, Section 3.5.2; abstraction of radionuclide transport in Total System Performance Assessment (TSPA), Section 3.6.3.3. |
| 2a. |
For the valid application of the Kd approach, using equation (1) Rf = 1+  bKd/n, DOE has demonstrated that the flow path acts as a single continuum porous medium. If the flow cannot be shown to be a single continuum porous medium, then the acceptance criteria for radionuclide transport in fractured rock apply.
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Fluid flow in the alluvium is likely to be well represented using a porous continuum conceptual model. Data to quantify the alluvial portion of the flow path are sparse, and hydrologic parameters used in numerical models should be considered to be bounding (Section 3.2.4.1.2). |
| 2b. |
For the valid application of the Kd approach, using equation (1) Rf = 1+ bKd/n, DOE has demonstrated that appropriate values for the parameters, Kd, n or  , and b have been adequately considered (e.g., experimentally determined or measured).
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Transport of sorbing solutes in porous media is a subject that has been well studied. Sorption coefficients onto alluvium from the Nye County Early Warning Drilling Program (NCEWDP) wells have been measured for a few key radionuclides; for the remaining radionuclides, sorption coefficients have been estimated based on the corresponding values measured for crushed tuff. Recent evaluations for the sorption of neptunium, technetium, and iodine have been accomplished for alluvium. Laboratory data on sorption, matrix diffusion, and colloids are discussed in Section 3.1.4. In the alluvium, n is modeled stochastically and b is modeled as a constant. |