Fluid Flow, Heat Transfer, and Solute Transport at Nuclear Waste Storage Tanks in the Hanford Vadose Zone

on the Columbia River plateau, a semiarid region in south-central Washington (Fig. 1), the Hanford Site At the Hanford Site, highly radioactive and chemically aggressive served as a plutonium production facility for nuclear waste fluids have leaked from underground storage tanks into the vadose zone. This paper addresses hydrogeological issues at the weapons from 1944 to the end of the Cold War era. A 241-SX tank farm, especially focusing on Tank SX-108, which is one total of 177 large underground tanks were constructed of the highest heat load, supernate density and ionic strength tanks in the Hanford vadose zone to store waste fluid streams at Hanford and a known leaker. The behavior of contaminants in from the plutonium extraction facilities. Many of the the unsaturated zone near SX-108 is determined by an interplay of older single-shell tanks have leaked radioactive waste multiphase fluid flow and heat transfer processes with reactive chemi- fluids, posing a contamination hazard for the underlying cal transport in a complex geological setting. Numerical simulation aquifer and ultimately the Columbia River. Our analysis studies were performed to obtain a better understanding of mass and specifically addresses the 241-SX tank farm (Fig. 2), energy transport in the unique hydrogeologic system created by the SX tank farm. Problem parameters are patterned after conditions where highly radioactive and chemically aggressive at Tank SX-108, and measured data were used whenever possible. aqueous fluids of high ionic strength have leaked into Borrowing from techniques developed in geothermal and petroleum the vadose zone. Of interest is the nature and extent of reservoir engineering, our simulations feature a comprehensive de- subsurface contamination; the past, present, and future scription of multiphase processes, including boiling and condensation migration of contaminants; and hydrogeologic issues phenomena, and precipitation and dissolution of solids. We find that posed by possible future remedial actions. the thermal perturbation from the tank causes large-scale redistribu- Construction and operation of the SX storage tanks tion of moisture and alters water seepage patterns. During periods since the mid 1950s involved massive perturbations of of high heat load, fluid and heat flow near the tank are dominated by vapor–liquid counterflow (heat pipe), which provides a much more the natural hydrogeologic system. At the SX tank farm, efficient mechanism than heat conduction for dissipating tank heat. the ground was excavated down to 15.4 m depth, and The heat pipe mechanism is also very effective in concentrating dis- 15 large cylindrically shaped storage tanks with approxisolved solids near the heat source, where salts may precipitate even mate dimensions of 13.6-m height and 11.8-m radius if they were only present in small concentrations in ambient fluids. were emplaced in a regular pattern with 30.4-m spacing Tank leaks that released aqueous fluids of high ionic strength into between tank centers (Fig. 2, 3). The excavated material the vadose zone were also modeled. The heat load causes formation was then backfilled, and a gravel layer 1.8 m thick was dry-out beneath the tank, which is accompanied by precipitation of placed on top. The latter led to a substantial reduction solutes. These may become remobilized at a later time when tank temperatures decline and previously dried out regions are rewetted. in evapotranspiration and increase in net infiltration. Simulated temperature and moisture distributions compare well with Water migration in the unsaturated zone was profoundly borehole measurements performed in 2000. The temperature maxi- altered by the umbrella effect of the tanks that diverts mum observed beneath Tank SX-108 can be explained from past water around the tank perimeters and by altered hydrothermal history of the tank; it is not necessary to invoke heat genera- geologic properties in the backfilled region. Additional tion from leaked radioactive contaminants. A novel composite me- effects arose from the heat generated by the radioactive dium model is used to explore effects of moisture tension–dependent wastes in the tanks. Temperatures in several tanks rose anisotropy, which is shown to have important impacts on fluid flow to well above the nominal boiling point of 100C, in one and solute transport in the Hanford sediments.