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Highlights • An alternative method for tracking the Mark-V electrorefiner inventory is proposed. • The proposed method determines the phase and location of material unaccounted for (MUF). • A strategy of closing out the Mark-V electrorefiner inventory is proposed. Abstract This paper discusses the current practice of assessing the Mark-V electrorefiner inventory and a proposed new way of tracking this inventory. The current practice involves the estimation/calculation of the salt phase mass in the transferred material mass and the calculation of the holdup. The proposed method involves the mass measurement of the transferred materials and the total inventory volume measurement along with the consideration of metallic uranium phase in the inventory. Without estimating the involved salt inventory in the incoming and outgoing material streams, the proposed method yields the salt inventory agreeing with the current mass tracking practice. The estimated holdup in the current mass tracking practice is entirely explained with the introduced metallic uranium phase in the proposed model. The current practice of closing out the special nuclear material inventory is revisited with the insight gained from the proposed model. Suggestions are given to achieve the bounded uncertainty of the inventory estimates with the emerging in situ measurement technique on the molten salt density. [ABSTRACT FROM AUTHOR]

This technical note discusses radioactive decay computation with multiple source terms with heterogeneous introduction dates to the system. Two methods are considered: (1) decaying in sequence from the oldest dated source, ad finitium, to the final decay date and (2) decaying each source term to the final decay date and summing the resulting nuclides. We prove that radioactive decay computation using the prescribed two methods produces the same result. The algorithmic advantage of the second method over the first one is formally argued. The radioactive decay of 90Sr with multiple initial decay dates is given as an illustrative example. [ABSTRACT FROM AUTHOR]

Abstract The Mark-IV electrorefiner contains a molten-salt/molten-cadmium system for electrorefining uranium. The anode basket is loaded with chopped sodium-bonded metallic driver fuel. For the present investigation, the bulk of the fuel was U 10Zr binary alloy. The undissolved anode materials after electrorefining are sampled for chemical analysis. The estimation of the undissolved anode materials based on individual chemical analysis results are challenging due to inherently high sampling errors associated with heterogeneous undissolved material compositions. Investigation of chemical analysis data as a whole reveals noticeable trends in the compositions of the primary phases and materials. Based upon this observation, we propose a methodology for accounting the primary phases relying on the measurement of the total anode mass only. The methodology is based on an empirical model explaining the mass of the undissolved anode materials constructed from historical anode material analysis data. [ABSTRACT FROM AUTHOR]

A dissolution experiment is performed in order to examine the kinetics of uranium isotope equilibration in a molten salt and cadmium system. The salt contains a solute concentration of UCl 3 in LiCl-KCl eutectic and the cadmium contained dissolved uranium at 500 °C. A small piece of high enriched uranium is introduced to the cadmium, which perturbs the equilibrium with respect to the uranium enrichment of the salt and cadmium phases. The series of salt and cadmium samples trace how the enrichment of the two phases reach equilibrium (in the absence of a chemical driving force) over the course of approximately 24 h. A model linking dissolution and isotope exchange kinetics is developed to track the evolution of uranium isotope composition. With the assumption of unaccounted salt mass incurred from the dated sample reporting practice, the model explains the trend of the measured uranium isotope composition well. [ABSTRACT FROM AUTHOR]

In the field of pyroprocessing of used nuclear fuels, liquid cadmium is applied in different ways to recover transuranics from molten salt, while effecting some degree of separations between transuranics and lanthanides. A model of the separation performance between uranium and plutonium across the phase boundary between molten salt and liquid cadmium is presented. The salt is the LiCl-KCl eutectic with solute concentrations of UCl 3 and PuCl 3 . The cadmium is the liquid cadmium phase field from the Cd-U-Pu ternary alloy system. The model used to describe the behavior of uranium and plutonium in the salt/cadmium system is applied to describe the behavior of plutonium and lanthanide in the salt/cadmium system. [ABSTRACT FROM AUTHOR]

Reoxidation reaction of metallic uranium in the oxide reduction system for pyroprocessing of used nuclear fuels is quantitatively investigated via calculating thermodynamic equilibrium compositions. The calculations consider three operating parameters; LiCl/U ratio, LiO concentration, and excess metallic Li. The results suggest that decreasing LiCl/U ratio and LiO concentration reduce the extent of the reoxidation reaction. The presence of excess metallic Li suppresses the reoxidation reaction more effectively; the calculated amount of reoxidized U from pure metallic U is merely 0.6% with 9 wt% (vs. LiO) of excess Li. Excess Li can be synthesized in situ during the oxide reduction operation. [ABSTRACT FROM AUTHOR]

• Smoothed particle hydrodynamics (SPH) -based oxide reduction model is developed. • The model is verified and validated with 1D kinetic model and experimental data. • Multi-physics and multi-dimensional analysis of oxide reduction process is enabled. A common kinetic feature for oxide reduction chemical/electrochemical processes is oxygen transport via a porous metallic layer, which has been considered as a rate-determining step for reducing uranium oxides to metallic uranium. Accounting this kinetic behavior must involve the resolution of the moving reactive interface between shrinking oxide and expanding metal phases. This study presents a numerical model using smoothed particles hydrodynamics (SPH) to effectively deal with the evolution of the shrinking core reaction interface and oxygen transport via mass transfer of lithium oxide (Li 2 O) species in multiple mass transfer domains. We successfully validated the proposed model against a theoretical derivation for the oxide reduction process handling a shrinking oxide core with molten salt and metal ash medium on a simple planar geometry. Armed with successful validation results, the model examined a realistic reactant geometry to extend the arguments beyond the one-dimensional analyses, allowing the proposed model to apply to general application scenarios with multi-dimensional geometry. This study demonstrated that the proposed model could simulate and evaluate an arbitrary reaction basket design without iterative experimental trials, which is prohibitive for a scaled high-temperature molten salt study in an inert environment. The construct potentially provides not only deep insights on multiphysics behaviors governing the process dynamics but also a robust framework for evaluating and screening candidate basket designs in the most cost-effective manner. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, we address the problem of diagnosing the behaviors of interest in discrete-event systems. To this end, we introduce the notion of language-diagnosability, based on language specifications that generalizes diagnosability based on event specifications. A polynomial-time algorithm for verifying language-diagnosability is developed. Building upon the verification algorithm, we develop a polynomial-time algorithm for computing the worst case detection delay of a given system. The computation of the worst case detection delay involves the shortest path computation of a weighted, directed graph. We exploit a special weighting structure of the graph resulting from the verification algorithm, which enables an algorithm with a lower complexity than the commonly used Bellman–Ford shortest path algorithm. [Copyright &y& Elsevier]

Abstract: A theoretical framework and its practical implications for formulating and implementing model-based monitoring of discrete flow networks are discussed. Possible flows of items are described as discrete-event (DE) traces. Each trace defines the DE sequence(s) that are triggered when an entity follows a given flow-path, visiting tracking locations within the monitored system. To deal with alternative routing, creation of items, flow bifurcations and convergences are allowed. Given the set of possible discrete flows, a possible-behavior model—an interacting set of automata—is constructed, where each automation models the item discrete flow at each tracking location. In this model, which assumes total observability, event labels or symbols contain all the information required to unambiguously distinguish each discrete movement. Within the possible behavior, there is a special sub-behavior whose occurrence is required to be detected. The special behavior may be specified by the occurrence of routing events, such as faults or route violations, for example. These intermittent or non-persistent events may occur repeatedly. An observation mask is then defined, characterizing the observation configuration available for collecting item tracking data. The verification task is to determine whether this observation configuration is capable of detecting the specified special behavior. The assessment is accomplished by evaluating several observability notions, such as detectability and diagnosibility. If the corresponding property is satisfied, associated formal observers are constructed to perform the monitoring task at hand. The synthesis of observation masks may also be conducted to suggest optimal observation configurations (specifying number, type, and tracking locations of observation devices) guaranteeing the detection of special events and to construct associated monitoring agents. The developed framework, modeling methodology, and supporting techniques for defining and implementing discrete flow monitoring of entity movements are presented and illustrated with examples. [Copyright &y& Elsevier]

• Near real-time nuclear waste materials inventory assessment. • Utilizing chopper mass close out operation to assess materials phases in used metallic fuels. • Materials phases uncertainty characterization with respect to operation device performances. During the electrometallurgical pyroprocessing of used metallic nuclear fuel, the chopper operation cuts the fuel element into segments and distributes these segments between the anode basket and plenum can. This paper discusses a method of systematically assessing the mass balance of partitioned materials between the anode basket and plenum can. The proposed method fully accommodates the mass closeout measurements from the chopper operation and automates the assessment of chopper-induced material partitioning. With a set of reasonable assumptions, the proposed approach allows a formal uncertainty analysis on the partitioned masses as well. An illustrative calculation example adopted from the ongoing fissium fuel processing campaign is given. [ABSTRACT FROM AUTHOR]

This article describes a kinetic model for the reduction process of used oxide nuclear fuel with molten LiCl at 650°C. In this process, metal oxides, primarily UO 2 , react to form metallic U and Li 2 O. As UO 2 is gradually reduced, metallic U and Li 2 O form in molten LiCl, in and through which Li 2 O dissolves and diffuses. To capture the prescribed chemical and physical phenomena, two classical kinetic models are integrated sequentially to describe Li 2 O formation, dissolution, and diffusion behaviors. The first part of the integrated model uses a shrinking core kinetics to explain the formation, dissolution, and diffusion of Li 2 O while accounting for the expanding metallic uranium layer. The second part of the integrated model begins with the completion of Li 2 O dissolution and deals with the diffusion of excess Li 2 O in the metallic uranium layer. A Fick's second law without an external source describes the second part of the process behavior. The integrated model explains engineering scale kinetic measurement data from the literature with the effective Li 2 O diffusion coefficient range between 2 and 6 (10 − 5 c m 2 / s e c). [ABSTRACT FROM AUTHOR]

• A rotary riffling theory is developed to establish an input accountancy for used nuclear oxide fuels. • The developed rotary riffling theory utilizes particle size distribution information along with target species concentration statistics. • The developed theory uses particle size distribution and species concentration information. In establishing nuclear fuel input accountancy for electrometallurgical pyroprocessing treatment of used nuclear oxide fuel, a representative sampling method has been considered as a suitable alternative of methods pursuing homogenization of used nuclear oxide fuel feed such as mixing the pulverized fuels. In principle, the representative sampling method involves random particle selection from size-classified nuclear fuel materials to give quantified and controllable sampling uncertainty. For this purpose, the rotary riffling method is known to be superior in realizing sample randomization. To qualify the rotary riffling method as a tool to establish used nuclear fuel input accountancy, this paper first derives theoretical randomization performance tailored to the rotary riffling method and used nuclear fuel feed. The performance of an off-the-shelf rotary riffler was tested and validated with mixed alumina particle feeds of different sizes. [ABSTRACT FROM AUTHOR]

The liquid cadmium cathode is a pyroprocessing technology used for the recovery of group actinide metals from the molten chloride salts that support uranium electrorefining. This article presents a model for the performance of a liquid cadmium cathode based on considerations of salt/alloy equilibrium thermodynamics, Cd–U-Pu phase diagram behaviors, and mass balance constraints. The model demonstrates how the salt/cadmium system responds as incremental additions of uranium are made to the liquid cadmium cathode. [ABSTRACT FROM AUTHOR]

• A method for establishing input accountancy for a used oxide fuel is proposed. • The method is based on obtaining a representative sample. • A sufficient sample quantity for satisfying a sampling error requirement is derived. The method of input accountancy for a used oxide fuel reprocessing plant has been based on the sampling of a homogenized liquid stream of dissolved used oxide fuel. This article presents an alternative sampling strategy where samples are taken directly from solid oxides. The strategy is to obtain a representative sample while not requiring the homogenization of the input materials. A theoretical derivation is given to determine the sample quantity for satisfying the sampling error requirement. [ABSTRACT FROM AUTHOR]

This work examines the change in liquidus temperature of the salt used in the Mk-IV electrorefiner at the Idaho National Laboratory during processing campaigns of sodium-bonded driver fuels from Experimental Breeder Reactor II and the Fast Flux Test Facility reactor. Modeling and Simulation Tool for Electrochemical Recycling Systems (MASTERS), an INL proprietary pyroprocessing flowsheet simulation tool, was used to simulate the processing campaigns and determine the resulting composition changes of the Mark-IV electrorefiner salt. Surrogate salt samples simulating the Mk-IV electrorefiner salt during the fuel processing campaigns were prepared, and the thermal properties were measured via differential scanning calorimetry. Results from this study indicate that after processing approximately 2,150 kg of uranium metal from used fuel, the liquidus temperature of the molten salt exceeds 520 °C, which is a significant increase from 352 °C (the melting temperature of eutectic LiCl KCl). It was also found that the liquidus temperature is strongly dependent on the amount of NaCl accumulating in the Mk-IV electrorefiner salt. Thus, the assessment of the liquidus temperature during the subsequent fuel processing campaigns can be simplified by considering the ternary LiCl KCl NaCl system only. [ABSTRACT FROM AUTHOR]

• A multiphysics kinetic model of oxide reduction process for uranium oxides was presented coupled with active salt convection, based on smoothed particle hydrodynamics. • The effect of active salt convection on the reduction process was quantified for key reactant configuration parameters. • A reduction-time correlation was constructed with related dimensionless numbers, and several insights and implications in the use of pelletized oxide fuels were provided. Oxygen ion transport via molten salt continuum is a key kinetic feature of electrochemical reduction process for uranium oxides in molten salt. During the electrolysis, the evolution of oxygen ion concentration field in molten salt continuum appears from porous metallic layer to open bulk salt. Despite the important role affecting process throughput, its effect coupled with controlled salt movement was not adequately investigated. To capture the reactive oxygen ion transport phenomena, a multidimensional reduction model based on smoothed particle hydrodynamics (SPH) was combined with the advection-diffusion model. A pelletized oxide fuel bed capturing interconnected salt continuum is considered as a reactant material geometry, which allows forced convective delivery of molten salt through the reactant materials with a relatively low pressure head. In this study, reduction time of a pelletized-fuel bed was quantified with respect to key design parameters of the reactant material configuration. The simulation results demonstrated the advantage of salt convection which effectively removes oxygen ion products from the reactant materials and consequently accelerates the reduction process. Moreover, the reduction-time correlation of a single pelletized fuel was developed as a function of relevant dimensionless parameters and was extended to the application for a pelletized-fuel bed. Finally, several insights and implications in the use of a pelletized oxide fuel were presented. The developed SPH-based framework enabled multi-physics analysis of governing dynamics in appreciating and screening candidate reactant material configurations in a cost-effective manner. [ABSTRACT FROM AUTHOR]

Molten eutectic LiCl-KCl salt is a widely used electrolyte for electrorefining uranium from spent nuclear fuel. Due to the hygroscopic nature of this salt, such operations must be performed under controlled atmospheric conditions, and waste salts require careful storage to avoid deliquescence and corrosion of container materials. This study investigated a potential processing path for reducing the degree of deliquescence through dilution to varying extents with NaCl. The hydration behavior of LiCl-KCl salts diluted with NaCl was evaluated in terms of mass gain due to water absorption, degree of deliquescence (including first appearances of standing water), and evidence of corrosion to stainless steel containers in a humid air environment (40°C, 20% relative humidity). In this humid air environment, pure eutectic LiCl-KCl exhibited a 50 mass % increase due to water absorption and showed evidence of standing water after 24 h. Waste salt diluted with NaCl required loadings of 89 mass % NaCl in order to prevent deliquescence and exhibited a 3 mass % increase due to water absorption. After periodic observation for 48 h, standing water was observed near all ingots with the exception of the 89 mass % NaCl samples. Dilution with 89% NaCl was also found to reduce evidence of corrosion when stored in stainless steel crucibles. While dilution with NaCl greatly decreases steady-state hydration, the storage volume is increased ~10× through this procedure. [ABSTRACT FROM AUTHOR]

[Display omitted] • A two-heat-source crystallizer setup was designed to concentrate Cs+ into the salt liquid fraction. • The recovery of Cs+ from a chloride salt matrix (CsCl-NaCl-LiCl-KCl) using a thermally controlled solid-liquid separation was demonstrated. • High-temperature X-ray diffraction data showed that above the solidus temperature, the solid fraction does not exhibit the presence of any phase containing cesium. • Thermodynamic calculations indicated that a thermally controlled solid-liquid separation method holds the potential to recover up to 92.8% of the initial cesium chloride. • Elemental analysis of the liquid fraction indicated that cesium concentrates in the liquid portion, which agrees with the thermodynamic calculations. Pyroprocessing refers to a series of high-temperature molten-salt processes used to manage and recycle nuclear materials to reduce the waste volume and hazardous environmental impact of the final waste products. Fission products with high halide affinity, including cesium (Cs-137) with a half-life of 30 years, are dissolved into a molten-salt bath. The separation and recovery of cesium can benefit waste-stream minimization because Cs-137—a strong gamma-ray emitter—is a potential source of environmental contamination. This work proposes the recovery of Cs+ from a chloride-salt matrix using a thermally controlled solid-liquid separation. Thermodynamic calculations for the CsCl-NaCl-LiCl-KCl system were performed to predict the partitioning of Cs+ into liquid and solid phases as functions of salt composition and temperature. The selected composition simulated the formulation of high-level salt waste from used fuel treatment. These calculations showed that once the solidus temperature was reached, Cs+ partitions to the liquid phase. According to the thermodynamic calculations, a thermally controlled liquid-solid separation holds the potential to recover up to 92.8 % of the initial cesium chloride. The phase transition temperatures of the sample, including the solidus and liquidus temperatures, were determined using differential scanning calorimetry. The phase composition of the system was studied at various temperatures using high-temperature X-ray diffraction. A laboratory-scale crystallization apparatus was used to melt the sample partially, allowing the collection of the liquid fraction for elemental analysis. All results demonstrated that Cs+ in this system concentrates in the liquid phase and confirmed the viability of recovering CsCl through thermally controlled solid-liquid separation. [ABSTRACT FROM AUTHOR]

• Decomposition potentials based on thermodynamics were compared to measured values. • Cyclic voltammetry data were normalized arithmetically using an internal standard. • Normalization eliminates the need of a particular reference electrode. • Use of quasi-reference electrodes is suitable for short-term measurements. • Values of D , i 0 , and R ct provided confidence in use of quasi-reference electrodes. Electrochemical measurements of YCl 3 , ScCl 3 , GdCl 3 and MgCl 2 were obtained in molten LiCl-KCl eutectic at 773 K using a three-electrode configuration. Tungsten rods were used as the working and counter electrodes. Two quasi- and one reference electrode(s) (glassy carbon, nickel wire, and Ni/NiO) were used to collect electrochemical measurements of the analyte salts. Cyclic voltammetry data were analyzed to calculate values of diffusion coefficient, exchange current density, and charge transfer resistance and were determined to be on the order of 10−5 cm2 s−1, 10−2 A cm−2, and 100 Ω; respectively. In general, the values of diffusion coefficients were found to be consistent with those reported of high-temperature molten salts. Relatively large values of exchange current density corresponded with smaller values of charge transfer resistance. These values were found to be reasonable in comparison to results available in the literature. The measured cyclic voltammograms were normalized with respect to both lithium reduction and chloride ion oxidation potentials. Such a normalization technique is effective for comparing experimentally obtained cyclic voltammetry data to those that have been published. An analysis of experimentally obtained results indicates the relation of electrode reactions (e.g., differences of reduction potentials) are independent of the choice of reference electrode. Additionally, the choice of reference electrode did not affect the electrochemical window, exchange current density, or diffusion coefficient values. [ABSTRACT FROM AUTHOR]

This paper reports the hygroscopic properties of eutectic LiCl-KCl after absorption into zeolite-4A, up to salt loadings of 75 wt%. Samples of the salt occluded zeolite were hydrated in a humidity chamber at constant temperature and relative humidity for up to 100 h. At up to 45 wt% salt loading, the un-occluded phase of salt consisted primarily of NaCl, which forms when the Na+ ions present in the zeolite framework exchange with Li+ and K+ ions from the eutectic LiCl-KCl. This results in minimal water absorption and corrosion of contacted stainless steel. At greater than 45 wt% salt loading, water absorption and corrosion progressively worsened. The mixture has a significant amount of excess LiCl-KCl, making it highly hygroscopic. This study reveals an option for the intermediate treatment of waste salt from spent nuclear fuel electrorefiners that could facilitate it to be stored in a non-inert atmosphere for extended periods of time before final conversion into a permanent waste form. [ABSTRACT FROM AUTHOR]

In this paper, we apply an advanced safeguards approach and associated methods for process monitoring to a hypothetical nuclear material processing system. The assessment regarding the state of the processing facility is conducted at a system-centric level formulated in a hybrid framework. This utilizes architecture for integrating both time- and event-driven data and analysis for decision making. While the time-driven layers of the proposed architecture encompass more traditional process monitoring methods based on time series data and analysis, the event-driven layers encompass operation monitoring methods based on discrete event data and analysis. By integrating process- and operation-related information and methodologies within a unified framework, the task of anomaly detection is greatly improved. This is because decision-making can benefit from not only known time-series relationships among measured signals but also from known event sequence relationships among generated events. This available knowledge at both time series and discrete event layers can then be effectively used to synthesize observation solutions that optimally balance sensor and data processing requirements. The application of the proposed approach is then implemented on an illustrative monitored system based on pyroprocessing and results are discussed. [ABSTRACT FROM AUTHOR]

Experimental Breeder Reactor II (EBR-II) operated from 1964 to 1994 as one of the first sodium-cooled fast reactors to use sodium-bonded metallic driver fuels and blankets. EBR-II was a testbed for fuel development and reactor design principles. Following shutdown, it was decided to use electrometallurgical methods to treat the spent fuel and blanket materials for final disposition. This paper provides a brief history of U.S. liquid metal cooled reactor development with emphasis on EBR-II, and a technical description of the electrometallurgical methods. [ABSTRACT FROM AUTHOR]