Rapid Migration of Radionuclides Leaked from High-Level Water Tanks: A Study of Salinity Gradients, Wetted Path Geometry and Water Vapor Transport

Ward, Anderson L. ; Gee, Glendon W. ; Selker, John S. ; Cooper, Caly

Oregon State University Research Office (United States)

关键词: Water Vapor;    Radioactive Waste Storage;    Radioactive Waste Facilities;    Storage Facilities;    Radionuclide Migration;   

DOI  :  10.2172/794075
RP-ID  :  DOE/ER/14925
RP-ID  :  FG07-98ER14925
RP-ID  :  794075

美国|英语

来源: UNT Digital Library

【 摘 要 】

The basis of this study was the hypothesis that the physical and chemical properties of hypersaline tank waste could lead to wetting from instability and fingered flow following a tank leak. Thus, the goal of this project was to develop an understanding of the impacts of the properties of hypersaline fluids on transport through the unsaturated zone beneath Hanford's Tank Farms. There were three specific objectives (i) to develop an improved conceptualization of hypersaline fluid transport in laboratory (ii) to identify the degree to which field conditions mimic the flow processes observed in the laboratory and (iii) to provide a validation data set to establish the degree to which the conceptual models, embodied in a numerical simulator, could explain the observed field behavior. As hypothesized, high ionic strength solutions entering homogeneous pre-wetted porous media formed unstable wetting fronts a typical of low ionic strength infiltration. In the field, this mechanism could force flow in vertical flow paths, 5-15 cm in width, bypassing much of the media and leading to waste penetration to greater depths than would be predicted by current conceptual models. Preferential flow may lead to highly accelerated transport through large homogeneous units, and must be included in any conservative analysis of tank waste losses through coarse-textured units. However, numerical description of fingered flow using current techniques has been unreliable, thereby precluding tank-scale 3-D simulation of these processes. A new approach based on nonzero, hysteretic contact angles and fluid-dependent liquid entry has been developed for the continuum scale modeling of fingered flow. This approach has been coupled with and adaptive-grid finite-difference solver to permit the prediction of finger formation and persistence form sub centimeter scales to the filed scale using both scalar and vector processors. Although laboratory experiments demonstrated that elevated surface tension of imbibing solutions can enhance vertical fingered flow, this phenomenon was not observed in the field. Field tests of showed that the fingered flow behavior was overwhelmed by the variability in texture resulting from differences in the depositional environment. Field plumes were characterized by lateral spreading with an average width to depth aspect ratio of 4. For both vertical fingers and lateral flow, the high ionic strength contributed to the vapor phase dilution of the waste, which increased waste volume and pushed the wetting from well beyond what would have occurred if the volume of material had remained unchanged from that initially released into the system. It was also observed that following significant vapor-phase dilution of the waste simulants that streams of colloids were ejected from the sediment surfaces. It was shown that due to the high-sodium content of the tank wastes the colloids were deflocculated below a critical salt concentration in Hanford sediment s. The released colloids, which at the site would be expected to carry the bulk of the sorbed heavy metals and radioisotopes, were mobile though coarse Hanford sediments, but clogged finer layers. The developments resulting from this study are already being applied at Hanford in the nonisothermal prediction of the hypersaline, high pH waste migration in tank farms and in the development of inverse methods for history matching under DOE's Groundwater/Vadose Zone Integration Project at Hanford.