Osvaldo Pensado

An In-Situ Galvanically Coupled Multielectrode Array Sensor for Localized Corrosion

Abstract A localized corrosion sensor consisting of multiple, corrodible, miniature electrodes was tested in different chemical environments. The miniature electrodes were coupled together by connecting each of them to a common joint through independent resistors, with each electrode simulating an area of a corroding metal. In a localized corrosion environment, anodic currents flow into the more corroding electrode, and cathodic currents flow out of the less or noncorroding electrodes. These currents are measured from the voltages across the resistors. The variation among the galvanic currents measured from the miniature electrodes responded well to changes in the environment with respect to localized corrosion. It was demonstrated that statistical parameters derived from the currents flowing through the miniature electrodes, such as the standard deviation or the 90th percentile anodic value, can be used as effective indictors for localized corrosion. Measurement showed the following order of corrosivenes...

The electrochemical behavior of lithium in alkaline aqueous electrolytes

The passive film on lithium in contact with KOH solution comprises a bilayer structure, consisting of a defective barrier layer and a porous outer layer. Thermodynamic analyses indicate that LiH is the barrier layer phase that forms on lithium in aqueous alkaline solutions under open circuit conditions. Hydrated LiOH, which form as the outer layer of the passive film, also regulates the voltage dependence of the anodic dissolution rate. A steady-state model, based on the Point Defect Model, was developed to account for polarization data, hydrogen evolution data, and the measured faradaic efficiency of the lithium/water system. The steady-state model was extended to account for electrochemical impedance data. The impedance model indicates that some properties (e.g. porosity) of the outer layer depend on the electrolyte concentration, and that the porosity of the outer layer exerts significant influence over the kinetics of electro-dissolution of lithium in aqueous solution.

Electrochemical Behavior of Lithium in Alkaline Aqueous Electrolytes: III. Impedance Model

An impedance model for the electrochemical dissolution of lithium in alkaline solutions is presented. The construction technique of the impedance function depends on calibration to steady-state properties, described in Part II of this series. The model, which is based on the point defect model for the growth and breakdown of passive films, is used to identify effects of various electrolyte solutes on the properties of the lithium film. The high frequency experimental impedance data are explained by the existence of a capacitance that is voltage and frequency dependent, a property that is theoretically rationalized. It is concluded that electrolyte solutes influence the rate of water transport through the outer layer, rate constants, the polarization of the harrier layer/outer layer interface, and the porosity of the outer layer. Based on the shape of predicted impedance signatures, it is suggested that the derived impedance equation may be applicable to other systems.

Time-domain random-walk algorithms for simulating radionuclide transport in fractured porous rock

Abstract Time-domain random-walk (TDRW) algorithms are efficient methods for simulating solute transport along one-dimensional pathways. New extensions of the TDRW algorithm accommodate decay and ingrowth of radionuclides in a decay chain and time-dependent transport velocities. Tests using equilibrium sorption and matrix diffusion retention models demonstrate that the extended TDRW algorithm is accurate and computationally efficient. When combined with stochastic simulation of transport properties, the resulting algorithm, Particle on Random Streamline Segment (PORSS), also captures the effects of random spatial variations in transport velocities, including the effects of very broad velocity distributions. When used in combination with discrete fracture network simulations, the PORSS algorithm provides an accurate and practical method for simulating radionuclide transport at the geosphere scale without invoking the advection-dispersion equation.

Estimated Effects of Temperature-Relative Humidity Variations on the Composition of In-Drift Water in the Potential Nuclear Waste Repository at Yucca Mountain, Nevada

We define four distinct thermohydrochemical environments for drip shield and waste package corrosion in the potential nuclear waste repository, referred to here as the Dry, Seepage + Evaporation, Seepage + Condensation + Evaporation, and the Seepage + Condensation environments. These environments are bounded by temperature and relative humidity conditions at drift wall and drip shield/waste package surfaces judged most likely to initiate fundamental changes in the quantity and/or chemistry of in-drift waters. The duration in which different environments might exist is evaluated by comparing simulated, time-dependent temperature and relative humidity curves for three different locations within repository drift 25. In-drift conditions and processes postulated to cause drip shield/waste package corrosion are evaluated within the context of the thermohydrochemical environments by various means, including analytical calculations and geochemical simulations. Of the four environments considered here, the Seepage + Evaporation environment presents the most significant potential for aqueous corrosion of drip shield and waste package materials, and may persist for approximately 500 years in center drift locations. The likelihood for corrosion in other thermohydrochemical environments is significantly lower, but may increase with the acquisition of new data or the demonstration of extenuating circumstances.

Degradation of high-level waste glass under simulated repository conditions

The internal waste package environment that contains corrosion products from the containers may have a significant influence on waste form degradation. Leaching experiments of two simulated high-level waste glasses (WVDP Ref. 6 and DWPF Blend 1) were carried out in aqueous solutions of FeCl2 and FeCl3 at temperatures of 40, 70, and 90 °C that simulate the possible aqueous environment inside the breached waste packages. Both solution pH and the presence of corrosion species such as iron chloride were found to be major contributing factors to the net glass durability. Model abstraction and performance assessment analyses using rate expressions showed that the presence of corrosion products enhances glass dissolution and increases the subsequent release of radionuclides to the environment.

Independent Evaluation of Waste Package Corrosion Performance Under Potential Repository Conditions

Abstract Long-term corrosion performance of the waste package is among the key engineered barrier system attributes of a potential high-level waste repository at Yucca Mountain, Nevada. Waste package degradation processes are evaluated on the basis of independent investigations conducted at the Center for Nuclear Waste Regulatory Analyses. This paper summarizes the results of laboratory measurements and model analyses focused on uniform, localized, and microbially influenced corrosion, and stress corrosion cracking of Alloy 22 (UNS N06022).

Corrosion Behavior of Waste Package and Drip Shield Materials

Abstract The susceptibility to various forms of corrosion that could be experienced by the alloys considered by the U.S. Department of Energy for the waste package and drip shield, the principal components of the engineered barrier system for the proposed repository at Yucca Mountain, is evaluated on the basis of experimental studies conducted at the Center for Nuclear Waste Regulatory Analyses. Environmental, metallurgical, and mechanical conditions for the occurrence of uniform corrosion, localized corrosion, and environmentally assisted cracking of Alloy 22 (58Ni–22Cr–13Mo–3W–4Fe), the preferred material for the outer container, and Titanium-Grade 7 (Ti–0.15 Pd), the alloy proposed for the drip shield, are reported.

Spatial Variability and Parametric Uncertainty in Performance Assessment Models

The problem of defining an appropriate treatment of distribution functions (which could represent spatial variability or parametric uncertainty) is examined based on a generic performance assessment model for a high-level waste repository. The generic model incorporated source term models available in GoldSim® , the TDRW code for contaminant transport in sparse fracture networks with a complex fracture-matrix interaction process, and a biosphere dose model known as BDOSE™. Using the GoldSim framework, several Monte Carlo sampling approaches and transport conceptualizations were evaluated to explore the effect of various treatments of spatial variability and parametric uncertainty on dose estimates. Results from a model employing a representative source and ensemble-averaged pathway properties were compared to results from a model allowing for stochastic variation of transport properties along streamline segments (i.e., explicit representation of spatial variability within a Monte Carlo realization). We concluded that the sampling approach and the definition of an ensemble representative do influence consequence estimates. In the examples analyzed in this paper, approaches considering limited variability of a transport resistance parameter along a streamline increased the frequency of fast pathways resulting in relatively high dose estimates, while those allowing for broad variability along streamlines increased the frequency of “bottlenecks” reducing dose estimates. On this basis, simplified approaches with limited consideration of variability may suffice for intended uses of the performance assessment model, such as evaluation of site safety.Copyright

Estimates of mean consequences and confidence bounds on the mean associated with low-probability seismic events in total system performance assessments

An approach is described to estimate mean consequences and confidence bounds on the mean of seismic events with low probability of breaching components of the engineered barrier system. The approach is aimed at complementing total system performance assessment models used to understand consequences of scenarios leading to radionuclide releases in geologic nuclear waste repository systems. The objective is to develop an efficient approach to estimate mean consequences associated with seismic events of low probability, employing data from a performance assessment model with a modest number of Monte Carlo realizations. The derived equations and formulas were tested with results from a specific performance assessment model. The derived equations appear to be one method to estimate mean consequences without having to use a large number of realizations.