In-Drift Natural Convection and Condensation

The Yucca Mountain repository configuration consists of waste packages stored inside of underground tunnels, or drifts. The waste packages generate heat due to radioactive decay, and moisture flows into and out of the drifts in liquid and vapor form. Heat and mass transfer within the drifts, including interaction with the surrounding rock, are potentially important processes for the performance of the repository. The present report documents models for in-drift heat and mass transfer during the post-closure period. Pre-closure, or ventilated, conditions are documented in a separate report (BSC 2004 [DIRS 169862])

Deliquescence of NaCl–NaNO(3), KNO(3)–NaNO(3), and NaCl–KNO(3 )salt mixtures from 90 to 120°C

We conducted reversed deliquescence experiments in saturated NaCl–NaNO(3)–H(2)O, KNO(3)–NaNO(3)–H(2)O, and NaCl–KNO(3)–H(2)O systems from 90 to 120°C as a function of relative humidity and solution composition. NaCl, NaNO(3), and KNO(3 )represent members of dust salt assemblages that are likely to deliquesce and form concentrated brines on high-level radioactive waste package surfaces in a repository environment at Yucca Mountain, NV. Discrepancy between model prediction and experiment can be as high as 8% for relative humidity and 50% for dissolved ion concentration. The discrepancy is attributed primarily to the use of 25°C models for Cl–NO(3 )and K–NO(3 )ion interactions in the current Yucca Mountain Project high-temperature Pitzer model to describe the nonideal behavior of these highly concentrated solutions

Evaporative evolution of a Na–Cl–NO(3)–K–Ca–SO(4)–Mg–Si brine at 95°C: Experiments and modeling relevant to Yucca Mountain, Nevada

A synthetic Topopah Spring Tuff water representative of one type of pore water at Yucca Mountain, NV was evaporated at 95°C in a series of experiments to determine the geochemical controls for brines that may form on, and possibly impact upon the long-term integrity of waste containers and drip shields at the designated high-level, nuclear-waste repository. Solution chemistry, condensed vapor chemistry, and precipitate mineralogy were used to identify important chemical divides and to validate geochemical calculations of evaporating water chemistry using a high temperature Pitzer thermodynamic database. The water evolved toward a complex "sulfate type" brine that contained about 45 mol % Na, 40 mol % Cl, 9 mol % NO(3), 5 mol % K, and less than 1 mol % each of SO(4), Ca, Mg, ∑CO(2)(aq), F, and Si. All measured ions in the condensed vapor phase were below detection limits. The mineral precipitates identified were halite, anhydrite, bassanite, niter, and nitratine. Trends in the solution composition and identification of CaSO(4 )solids suggest that fluorite, carbonate, sulfate, and magnesium-silicate precipitation control the aqueous solution composition of sulfate type waters by removing fluoride, calcium, and magnesium during the early stages of evaporation. In most cases, the high temperature Pitzer database, used by EQ3/6 geochemical code, sufficiently predicts water composition and mineral precipitation during evaporation. Predicted solution compositions are generally within a factor of 2 of the experimental values. The model predicts that sepiolite, bassanite, amorphous silica, calcite, halite, and brucite are the solubility controlling mineral phases

Effect of reducing groundwater on the retardation of redox-sensitive radionuclides

Laboratory batch sorption experiments were used to investigate variations in the retardation behavior of redox-sensitive radionuclides. Water-rock compositions were designed to simulate subsurface conditions at the Nevada Test Site (NTS), where a suite of radionuclides were deposited as a result of underground nuclear testing. Experimental redox conditions were controlled by varying the oxygen content inside an enclosed glove box and by adding reductants into the testing solutions

Evaluating the Moisture Conditions in the Fractured Rock at Yucca Mountain: The Impact of Natural Convection Processes in Heated Emplacement Drifts

The energy output of the high-level radioactive waste to be emplaced in the proposed geologic repository at Yucca Mountain, Nevada, will strongly affect the thermal-hydrological (TH) conditions in the near-drift fractured rock. Heating of rock water to above-boiling conditions will induce large water saturation changes and flux perturbations close to the waste emplacement tunnels (drifts) that will last several thousand years. Understanding these perturbations is important for the performance of the repository, because they could increase, for example, the amount of formation water seeping into the open drifts and contacting waste packages. Recent computational fluid dynamics (CFD) analysis has demonstrated that the drifts will act as important conduits for gas flows driven by natural convection. As a result, vapor generated from boiling of formation water near elevated-temperature sections of the drifts may effectively be transported to cooler end sections (where no waste is emplaced), would condense there, and subsequently drain into underlying rock units. Thus, natural convection processes have great potential for reducing the near-drift moisture content in heated drift sections, which has positive ramifications for repository performance. To study these processes, we have developed a new simulation method that couples existing tools for simulating TH conditions in the fractured formation with modules that approximate natural convection and evaporation conditions in heated emplacement drifts. The new method is applied to evaluate the future TH conditions at Yucca Mountain in a three-dimensional model domain comprising a representative emplacement drift and the surrounding fractured rock

Modeling seepage into heated waste emplacement tunnels in unsaturated fractured rock

Predicting the amount of water that may seep into waste emplacement tunnels (drifts) is important for assessing the performance of the proposed geologic repository for high-level radioactive waste at Yucca Mountain, Nevada. The repository will be located in thick, partially saturated fractured tuff that-for the first several hundred years after emplacement-will be heated to above-boiling temperatures as a result of heat generation from the decay of radioactive waste. Heating of rock water to above-boiling conditions induces water saturation changes and perturbs water fluxes that affect the potential for water seepage into drifts. In this paper, we describe numerical analyses of the coupled thermal-hydrological (TH) processes in the vicinity of waste emplacement drifts, evaluate the potential of seepage during the heating phase of the repository, and discuss the implications for the performance of the site. In addition to the capillary barrier at the rock-drift interface-independent of the thermal conditions-a second barrier exists to downward percolation at above-boiling conditions. This barrier is caused by vaporization of water in the fractured rock overlying the repository. A TOUGH2 dual-permeability simulation model was developed to analyze the combined effect of these two barriers; it accounts for all relevant TH processes in response to heating, while incorporating the capillary barrier condition at the drift wall. Model results are presented for a variety of simulation cases that cover the expected variability and uncertainty of relevant rock properties and boundary conditions