Thermochemical and Continuum Modeling to Understand the Chemical Composition of PWR Fuel CRUD

Computational modeling of Chalk River Undesirable Deposits (CRUD) allows for the prediction of associated phenomena that impact nuclear power plant performance, reliability, and safety. It also provides insight into the physical mechanisms by which CRUD forms and affects plant performance. A major concern in pressurized water reactors (PWRs) is Axial Offset Anomaly (AOA) which is caused by CRUD’s proficiency at trapping boron within the reactor core. The ability to predict AOA and other phenomena requires a detailed explanation of the chemical composition of CRUD. By pairing computational models that can simulate the structure and species trapping with detailed thermochemical models, the compounds that makeup CRUD are determined. Among these thermodynamically predicted compounds is Ni2FeBO5, a mineral named bonaccordite, the formation of which provides a boron retention mechanism. Accordingly, bonaccordite has been found in CRUD samples from fuel linked to very extreme AOA. In this dissertation, thermochemical models are detailed for PWR primary loop chemistry up to the saturation temperature and are implemented using CALPHAD modeling. Likely solid precipitation reactions are identified, and those reactions are incorporated into the multiphysics continuum modeling code MAMBA. An assessment of the kinetic rates of the reactions are determined by Bayesian calibration of the MAMBA model using observational data from CRUD samples. The modeling is able to demonstrate the composition of CRUD scrapes obtained from plant data. This model contributes to the understanding of CRUD formation and composition and allows for the prediction of phenomena such as AOA

Heat and mass transfer analysis for crud coated PWR fuel

In water-cooled nuclear reactors, various species are present in the coolant, either in ionic solution, or entrained as very fine particles. Most arise from corrosion of primary circuit surfaces, or from chemicals, such as boric acid, lithium hydroxide, zinc and hydrogen, deliberately added to the coolant. These materials deposit on the surfaces of fuel pins, typically in the upper regions of the core, forming what is generally termed “crud”. This thesis reports a study of the thermal-hydraulic consequences of this deposit. These crud deposits are generally found to contain a large population of through-thickness chimneys, and it is believed that this gives rise to a wick-boiling mechanism of heat transfer. A coupled two-dimensional model of the processes of heat conduction, advection and species diffusion in the crud has been developed. An iterative scheme has been employed to solve the set of coupled equations of each process. The wick boiling process has been found to be an efficient heat transfer mode, taking away about 80% of the heat generated. It has also been found that consideration of heat transfer in the clad can increase the predicted solute concentration in the crud. The effects of some important parameters, such as chimney density, chimney radius, porosity of the crud, crud thickness, clad heat flux and boron concentration in the coolant have been investigated. The fuel thermal performance has been characterized in terms of an effective crud thermal conductivity, and the non-linear dependence this has on parameters such as crud thickness and chimney density had been determined. Lastly, it is observed that plausible pore sizes of the crud, coupled with higher temperatures in the crud, may be such that a film of vapour is generated at the base of the crud. Initial estimates are presented of the cladding temperatures and solute concentration that may be generated as a consequence of this vapour layer

Application of Differential Scanning Calorimetry to Characterize Thin Film Deposition Processes

With the recent increase in awareness on the environmental impact of industrial coating processes, chromate-based coating processes have been elevated to the rank of the technologies targeted by the EPA for rapid replacement by environmentally friendly processes. Therefore, there is a clear need for advances in coating technologies to identify alternative industrial practices. This thesis characterizes a process developed at Cleveland State University as an alternative deposition technique to generate uniform coatings onto solid substrates. A kinetic analysis to extract scale up parameters involved in the reaction kinetics leading to high-performance coatings is demonstrated in this research. The work consists of thermal characterization of deposition experiments using Modulated Differential Scanning Calorimeter (MDSC), complemented with preliminary finite-element-modeling (FEM) of fluid flow and transport phenomena in the vicinity of the deposition assembly. MDSC is capable of using linear and modulated heating rates. Modulation over imposes a sinusoidal heating profile to a linear heating rate. Therefore, modulation combines two conventional DSC experiments into one. Modulation provides the ability to differentiate reversibility from irreversibility in transitions. This study intends to study both the advantages and disadvantages of the modulation compared to conventional DSC in the analysis of thin film deposition. A protocol to analyze deposition reaction kinetics using a conventional DSC was formulated in this research. While modulation was unable to produce results that could be compared to the conventional DSC, further in-depth studies need to be completed. This research outlines the experimental procedure to analyze deposition reactions via conventional DSC, and a kinetic analysis procedure to extract reaction kinetics is demonstrated. This research successfully demonstrated that the deposition mechanism can be characterized via DSC experiments. Further studies are anticipated to lead to scale-up criter

Electrolysis in reduced gravitational environments: current research perspectives and future applications

Electrochemical energy conversion technologies play a crucial role in space missions, for example, in the Environmental Control and Life Support System (ECLSS) on the International Space Station (ISS). They are also vitally important for future long-term space travel for oxygen, fuel and chemical production, where a re-supply of resources from Earth is not possible. Here, we provide an overview of currently existing electrolytic energy conversion technologies for space applications such as proton exchange membrane (PEM) and alkaline electrolyzer systems. We discuss the governing interfacial processes in these devices influenced by reduced gravitation and provide an outlook on future applications of electrolysis systems in, e.g., in-situ resource utilization (ISRU) technologies. A perspective of computational modelling to predict the impact of the reduced gravitational environment on governing electrochemical processes is also discussed and experimental suggestions to better understand efficiency-impacting processes such as gas bubble formation and detachment in reduced gravitational environments are outlined

Multiphysics Assessment of Accident Tolerant Fuel, Cladding, and Core Structural Material Concepts

The severe accident at the Fukushima-Daiichi nuclear power plant in 2011 ignited a global research and development effort to replace traditionally-used materials in Light Water Reactors (LWRs) with Accident Tolerant Fuel (ATF) materials. These materials are intended to extend the coping time of nuclear power plants during severe accident scenarios, but must undergo thorough safety and performance evaluations before being implemented. Four ATF concepts are analyzed in this dissertation using state-of-the-art computer modeling tools: (1) iron-chromium-aluminum (FeCrAl) fuel rod cladding, (2) silicon carbide (SiC) fiber-reinforced, SiC matrix composite (SiC/SiC) boiling water reactor (BWR) channel boxes, (3) mixed thorium mononitride (ThN) and uranium mononitride (UN) fuel, (4) and UO2 [uranium dioxide] with embedded high thermal conductivity Mo inserts. The goals and approaches used for each study differed, and portions of this dissertation focused on verifying the accuracy of advanced modeling tools. Although each ATF evaluation is distinct, the underlying theme is the enhancement of safety, efficiency, and economic competitiveness of nuclear power through the use of advanced modeling techniques applied to material characterization studies. Results from the evaluations show the pros and cons of each ATF concept and highlight areas of needed modeling development. Comparisons of simulated and experimental critical heat flux (CHF) data for FeCrAl cladding and subsequent sensitivity analyses emphasized differences between real-world and simulated post-CHF phenomena. The Virtual Environment for Reactor Applications (VERA) multiphysics modeling suite was verified against other widely-used modeling tools for BWR application, and its advanced features were used to generate boundary conditions in SiC/SiC channel boxes used for deformation analyses. Several ThN-UN mixtures were analyzed using reactor physics and thermal hydraulic techniques and were shown to significantly increase the margin to fuel melt compared with UO2 [uranium dioxide] in LWRs. Mo inserts for UO2 [uranium dioxide] were optimized using sensitivity regression techniques and were also shown to significantly increase the margin to fuel melt compared with traditional UO2 [uranium dioxide]

CIPS Validation Data Plan

This report documents analysis, findings and recommendations resulted from a task 'CIPS Validation Data Plan (VDP)' formulated as an POR4 activity in the CASL VUQ Focus Area (FA), to develop a Validation Data Plan (VDP) for Crud-Induced Power Shift (CIPS) challenge problem, and provide guidance for the CIPS VDP implementation. The main reason and motivation for this task to be carried at this time in the VUQ FA is to bring together (i) knowledge of modern view and capability in VUQ, (ii) knowledge of physical processes that govern the CIPS, and (iii) knowledge of codes, models, and data available, used, potentially accessible, and/or being developed in CASL for CIPS prediction, to devise a practical VDP that effectively supports the CASL's mission in CIPS applications

CIPS Validation Data Plan

This report documents analysis, findings and recommendations resulted from a task 'CIPS Validation Data Plan (VDP)' formulated as an POR4 activity in the CASL VUQ Focus Area (FA), to develop a Validation Data Plan (VDP) for Crud-Induced Power Shift (CIPS) challenge problem, and provide guidance for the CIPS VDP implementation. The main reason and motivation for this task to be carried at this time in the VUQ FA is to bring together (i) knowledge of modern view and capability in VUQ, (ii) knowledge of physical processes that govern the CIPS, and (iii) knowledge of codes, models, and data available, used, potentially accessible, and/or being developed in CASL for CIPS prediction, to devise a practical VDP that effectively supports the CASL's mission in CIPS applications