Special Analysis Air Pathway Modeling of E-Area Low-Level Waste Facility

Abstract

This investigation was initiated to address a concern expressed by the Department of Energy's Low Level Waste Disposal Facility Federal Review Group (LFRG) Review Team during their review of the 2008 E-Area Performance Assessment (PA) [13]. The concern was the potential for overlapping of atmospheric plumes, emanating from the soil surface above SRS LLW disposal facilities within the E-Area, to contribute to the dose received by a member of the public during the Institutional Control (IC) period. The implication of this concern was that the dose to the maximally-exposed individual (MEI) located at the SRS boundary might be underestimated during this time interval. To address this concern a re-analysis of the atmospheric pathway releases from E-Area was required. A new atmospheric release model (ARM) capable of addressing the LFRG plume overlap concern was developed.The conceptual approach to assessing the atmospheric dose to a member of the public from E-Area LLW disposal facilities is to perform sub-surface simulations of the release of volatile radionuclides from the waste zones through the overlying engineered barriers to determine a flux rate at the land surface. Then, atmospheric dispersion of the radionuclide flux is simulated in an atmospheric transport model [1] to compute air concentrations in the vicinity of the hypothetically exposed individual. A re-analysis of this portion of the atmospheric pathway releases was not undertaken in this SA, however the Dose Release Factors (DRF's) computed in a separate study [ 9] were retained to convert the new surface emanation fluxes (computed with the ARM) into doses received by the MEI at the appropriate points of compliance (POC). In this way, the dose received by the MEI could be compared to the maximum permissible dose level, defined in DOE Order 435.1 [5] as being 10 mrem/yr.

Air Pathway Dose Modeling for the E-Area Low-Level Waste Facility

“…Air pathway disposal limits were subsequently developed for these 15 radionuclides in the 2008 PA. In a 2011 special analysis (SA) (Hiergesell and Taylor 2011), this list of 15 radionuclides was reevaluated to assess the potential for plume overlap in the air pathway. This analysis reduced the list of radionuclides requiring disposal limits to H-3 and C-14 by incorporating a mechanism to partition volatile contaminants across the water-air interface within the partially saturated pore space of the engineered and natural materials through which vapor phase transport occurs.…”

Recommended Henry’s Law Constants for Non-Groundwater Pathways Models in GoldSim

“…Agreement between OLI and Gorensek (2015) for CO2 is excellent at both pH 5.4 and 8.23. The two soil/waste conditions above were chosen to be consistent with assumptions used by Hiergesell and Taylor (2011) in the air pathway modeling special analysis for the E-Area low-level waste facility. Soil Condition A (pH 5.4, Eh 0.37 V, background groundwater) is judged to be representative of a noncementitious slit or engineered trench, for example, while Oxidized Region III (pH 8.23, Eh 0.73 V, calcite saturation) is thought to approximate conditions in an oxidized cementitious environment (e.g., CIG trench).…”

Dose Calculation Methodology and Data for Solid Waste Performance Assessment and Composite Analysis at the Savannah River Site

“…These DRF's represent the impact at the facility and site boundary of releasing one curie of each volatile radionuclide to the atmosphere from the ELLWF disposal units. The DRF's were used by Hiergesell and Taylor (2011) in the most recent analysis of the Air Pathway dose from the ELLWF. The DRF values have been included in the "Dose Coefficients" worksheet as shown in Figure 10-9.…”