submitted: 1. Xiao, W., Yu, L. (2019) Nondestructive evaluation with fully non-contact ACT-SLDV guided wave system, Proceedings of SPIE 2019 Smart Structures/NDE, Nondestructive Characterization and Monitoring of Advanced Materials, Aerospace, Civil Infrastructure, and Transportation XIII

A coordinated experimental and computational analysis was undertaken to investigate the temperature field, heat generation, and stress distribution within a spark plasma sintering (SPS) tooling-specimen system during single- and multipellet fabrication of uranium dioxide (UO2),fuel pellets. Different SPS tool assembly configurations consisting of spacers, punches, pellets, and a die with single or multiple cavities were analyzed using ANSYS,finite element (FE) software with coupled electro-thermo-mechanical modeling approach. For single-pellet manufacture, the importance of the die dimensions in relation to punch length and their influence on temperature distribution in the pellet were analyzed. The analysis was then extended to propose methods for reducing the overall power consumption of the SPS fabrication process by optimizing the dimensions and configurations of tooling for simultaneous sintering of multiple pellets in each processing cycle. For double-pellet manufacture, the effect of the center punch length (that separates the two pellets) on the temperature distribution in the pellets was investigated. Finally, for the multiple pellet fabrication, the optimum spacing between the pellets as well as the distance between the die cavities and the outer surface of the die wall were determined. A good agreement between the experimental data on the die surface temperature and FE model results was obtained. The current analysis may be utilized for further optimization of advanced tooling concepts to control temperature distribution and obtain uniform microstructure in,fuel pellets in large-scale manufacturing using SPS process.

The viability of spark plasma sintering (SPS) for fabrication of industrial-grade nuclear fuel pellets is explored by utilizing die designs for single-and multiple-pellet manufacturing. Traditional UO2 pellets were also manufactured by systematically varying processing temperature and pressure as needed for single-and multiple-pellet fabrication. The pellets were then qualified against commercial fuel specifications for density, shape, microstructure, and surface flaws. Pellets produced one at a time met all commercial specifications except for grain size. Pellets produced in batches of two, four, and eight pellets showed suboptimal density indicating that further changes to sintering conditions are warranted. Additionally, commonly used graphite tooling for pellet fabrication was shown to be ineffective in producing large numbers of fuel pellets, as the die and punches were shown to undergo severe wear in each run thus decreasing the reliability of the tooling for production of pellets as per the specification. Finally, additional discussion is provided for identifying the avenues for scale-up of SPS to meet the current commercial demand of 400 million pellets/year. These studies are viewed as first step toward assessing the ability of SPS technology to meet the quality specifications and quantity demands of nuclear fuel pellets.

Micro-Raman spectroscopy (MRS) was utilized to investigate the graphitization of diamond particles within a UO2-diamond composite processed by spark plasma sintering (SPS). While pure diamond gives a sharp Raman peak at 1331.6 cm(-1), the graphitized diamond shows broad peaks either at 1350 cm(-1) (G-peak) or 1580 cm(-1) (D-peak). The degree of graphitization was quantified by calculating the area beneath the diamond and graphite peaks. It was found that more than 20% of diamond was graphitized on the surface of the UO2-diamond pellet and only around 10% diamond was graphitized in the interior regions of the pellet. This current study highlights the necessity to review the implications of these results carefully while implementing UO2-diamond composite nuclear fuel. (C) 2016 Elsevier B.V. All rights reserved.

A novel uranium dioxide (UO2)-carbon nanotube (CNT) ceramic matrix composite fuel concept has been proposed for a nuclear fuel with increased thermal conductivity. Investigations were performed to analyze the dispersion of CNTs in a UO2 matrix utilizing homogenization and sonication techniques. Ethanol and ortho-dichlorobenzene (ODCB) were utilized as solvents during the mixing process. Distributions of both multi-walled carbon nanotubes and single-walled carbon nanotubes (SWNTs) were analyzed. It has been determined that CNTs can be homogeneously distributed into a UO2 matrix using mechanical processes, sonication, and homogenization in the organic solvent ODCB. The powder mixture of UO2 and CNTs was sintered at 1300 degrees C with a hold time of 5 min and 40-MP a pressure in a spark plasma sintering furnace, and the resulting grain size distribution was analyzed. It was observed that where the distribution of CNTs was not well controlled, significant grain growth of UO2 occurred. However, where the CNT distribution is well controlled, the grain growth is limited by the pinning effect of the CNTs along the grain boundaries. The resulting pellet thermal conductivity was improved by 29.7% with the addition of 5 vol % SWNT, relative to pure UO2 values. Raman spectroscopy in conjunction with scanning electron microscopy shows that most CNTs survive both the mixing and sintering processes.

In an effort to increase the thermal conductivity of UO2 fuel pellets, high density UO2-5 vol% diamond composite pellets were fabricated using the spark plasma sintering (SPS) technique. Diamond particles with nano-size (0.25 mu m) and several micro-sizes (3 mu m, 12 mu m and 25 mu m) were mixed with UO2 powder and sintered using SPS at 1300-1600 degrees C with a hold time of 5 min. The resultant density, chemical reaction, microstructure, thermal conductivity and Young's modulus of the sintered pellets were investigated. The pellets with 3 mu m diamond particles had a uniform distribution of particles as well as better thermal and mechanical properties compared to others. An increase in thermal conductivity of up to 41.6%, 38.3% and 34.2% at 100 degrees C, 500 degrees C and 900 degrees C, respectively, was measured in the UO2-diamond composite pellets compared to the pure UO2 fuel pellets. (C) 2015 Elsevier B.V. All rights reserved.

The nuclear fuel cycle utilizes either uranium or thorium, which are relatively plentiful materials. Both materials must undergo several processing steps in order to be converted into a useful fuel for nuclear reactors. For uranium, this involves conversion to UF6, enrichment, and final processing into fuel elements. Thorium is more complex and must first be irradiated in a reactor before a useful fuel is formed.

Uranium dioxide (UO2) is the most common fuel material in commercial nuclear power reactors. Despite its numerous advantages such as high melting point, good high-temperature stability, good chemical compatibility with cladding and coolant, and resistance to radiation, it suffers from low thermal conductivity that can result in large temperature gradients within the UO2 fuel pellet, causing it to crack and release fission gases. Thermal swelling of the pellets also limits the lifetime of UO2 fuel in the reactor. To mitigate these problems, we propose to develop novel UO2 fuel with uniformly distributed carbon nanotubes (CNTs) that can provide high-conductivity thermal pathways and can eliminate fuel cracking and fission gas release due to high temperatures. CNTs have been investigated extensively for the past decade to explore their unique physical properties and many potential applications. CNTs have high thermal conductivity (6600 W/mK for an individual single- walled CNT and >3000 W/mK for an individual multi-walled CNT) and high temperature stability up to 2800°C in vacuum and about 750°C in air. These properties make them attractive candidates in preparing nano-composites with new functional properties. The objective of the proposed research is to develop high thermal conductivity of UO2–CNT composites without affecting the neutronic property of UO2 significantly. The concept of this goal is to utilize a rapid sintering method (5–15 min) called spark plasma sintering (SPS) in which a mixture of CNTs and UO2 powder are used to make composites with different volume fractions of CNTs. Incorporation of these nanoscale materials plays a fundamentally critical role in controlling the performance and stability of UO2 fuel. We will use a novel in situ growth process to grow CNTs on UO2 particles for rapid sintering and develop UO2-CNT composites. This method is expected to provide a uniform distribution of CNTs at various volume fractions so that a high thermally conductive UO2-CNT composite is obtained with a minimal volume fraction of CNTs. The mixtures are sintered in the SPS facility at a range of temperatures, pressures, and time durations so as to identify the optimal processing conditions to obtain the desired microstructure of sintered UO2-CNT pellets. The second objective of the proposed work is to identify the optimal volume fraction of CNTs in the microstructure of the composites that provides the desired high thermal conductivity yet retaining the mechanical strength required for efficient function as a reactor fuel. We will systematically study the resulting microstructure (grain size, porosity, distribution of CNTs, etc.) obtained at various SPS processing conditions using optical microscopy, scanning electron microscopy (SEM), and transmission electron microscope (TEM). We will conduct indentation hardness measurements and uniaxial strength measurements as a function of volume fraction of CNTs to determine the mechanical strength and compare them to the properties of UO2. The fracture surfaces will be studied to determine the fracture characteristics that may relate to the observed cracking during service. Finally, we will perform thermal conductivity measurements on all the composites up to 1000° C. This study will relate the microstructure, mechanical properties, and thermal properties at various volume fractions of CNTs. The overall intent is to identify optimal processing conditions that will provide a well-consolidated compact with optimal microstructure and thermo-mechanical properties. The deliverables include: (1) fully characterized UO2-CNT composite with optimal CNT volume fraction and high thermal conductivity and (2) processing conditions for production of UO2-CNT composite pellets using SPS method.

Master sintering curve (MSC) theory has been applied successfully to UO2 and UO2-SiC composite ceramics processed using spark plasma sintering (SPS). By applying the constant heating rate method, where the powder compact was heated at a constant rate to a specified maximum temperature and then cooled naturally, the apparent activation energies for sintering have been determined to be 140 kJ/mol for UO2 and 420 kJ/mol for UO2-SiC composite. The ability of the derived MSCs to control and predict final density in the sintered compact was demonstrated by additional experimental runs using the isothermal heating method, in which the powder is held for a specified time at the maximum sintering temperature. It is shown that the master sintering curve is an effective tool to characterize the densification and sinterability behavior of a given nuclear powder compact during spark plasma sintering. The reason for significantly lower activation energy in SPS processed UO2 pellets compared to conventional sintering has been rationalized on the basis of field activation in SPS process. (C) 2014 Elsevier B.V. All rights reserved.

High density uranium dioxide (UO2) pellets with grain sizes between 0.9μm and 9μm were produced by spark plasma sintering (SPS). A systematic study was performed by varying the sintering temperature between 750°C and 1450°C and hold time between 0.5min and 20min to obtain UO2 pellets with a range of theoretical densities (TD) and grain sizes. The microstructure development in terms of grain size, density and porosity distribution was investigated. The oxygen/uranium (O/U) ratio of the resulting pellets was found to decrease after SPS. The thermal conductivity of UO2 pellets increased with the theoretical density but the grain size in the investigated range had no significant influence. The measured thermal conductivity values up to 900°C were consistent with the reported literature for conventionally sintered UO2 pellets. The benefits of using SPS over the conventional sintering of UO2 are summarized.

The mechanical properties of silicon carbide (SiC) inert matrix fuel (IMF) pellets fabricated by a low temperature (1050°C) polymer precursor route were evaluated at room temperature. The Vickers hardness was mainly related to the chemical bonding strength between the amorphous SiC phase and the β-SiC particles. The biaxial fracture strength with pre-notch and fracture toughness were found to be mostly controlled by the pellet density. The maximum Vickers hardness, biaxial fracture strength with pre-notch and fracture toughness achieved were 5.6GPa, 201MPa and 2.9MPam1/2 respectively. These values appear to be superior to the reference MOX or UO2 fuels. Excellent thermal shock resistance for the fabricated SiC IMF was proven and the values were compared to conventional UO2 pellets. XRD studies showed that ceria (PuO2 surrogate) chemically reacted with the polymer precursor during sintering, forming cerium oxysilicate. Whether PuO2 will chemically react in a similar manner remains unclear.

This study examines the influence of Silicon Carbide (SiC) particle addition on thermal conductivity of UO2–SiC composite pellets. UO2 powder and β-SiC particles of different sizes and of different volume fractions were mechanically mixed and sintered at 1350–1450°C for 5min by Spark Plasma Sintering (SPS). The particle size (0.6–55μm diameter) and volume fraction (5–20%) of SiC were systematically varied to investigate their influence on the resulting UO2–SiC composite pellet microstructure and the thermal properties. It was found that SiC particle size less than 16.9μm with larger volume fraction is more effective for improving the thermal conductivity of the fuel pellets. Scanning Electron Microscopy examination revealed micro-cracking and interfacial debonding in the composites containing larger size SiC particles (16.9 and 55μm) which resulted in reduced thermal conductivity. For the UO2–SiC composite pellets containing 1μm diameter SiC particles, the thermal conductivity increased almost linearly with volume fraction of particles. However, the addition of a larger volume fraction of SiC reduces the amount of heavy metal in the composite pellet and therefore a higher U-235 enrichment is necessary to compensate for the heavy metal loss. The experimental thermal conductivity values of the UO2–SiC composite pellets are in good agreement with the theoretical values based on the available model in the literature.

Uranium dioxide (UO2)–10vol% silicon carbide (SiC) composite fuel pellets were produced by oxidative sintering and Spark Plasma Sintering (SPS) at a range of temperatures from 1400 to 1600°C. Both SiC whiskers and SiC powder particles were utilized. Oxidative sintering was employed over 4h and the SPS sintering was employed only for 5min at the highest hold temperature. It was noted that composite pellets sintered by SPS process revealed smaller grain size, reduced formation of chemical products, higher density, and enhanced interfacial contact compared to the pellets made by oxidative sintering. For given volume of SiC, the pellets with powder particles yielded a smaller grain size than pellets with SiC whiskers. Finally thermal conductivity measurements at 100°C, 500°C, and 900°C revealed that SPS sintered UO2–SiC composites exhibited an increase of up to 62% in thermal conductivity compared to UO2 pellets, while the oxidative sintered composite pellets revealed significantly inferior thermal conductivity values. The current study points to the improved processing capabilities of SPS compared to oxidative sintering of UO2–SiC composites.

Enhanced thermal conductivity oxide fuels offer increases in both safety and efficiency of commercial light water reactors. Low-temperature oxidative sintering and Spark Plasma Sintering (SPS) techniques have been used to produce UO 2 -SiC composite pellets. Oxidative sintering performed for 4 hours at 1200∼1600°C and SPS was employed only for 5 mins at the same temperature. While oxidative sintering failed to achieve enhanced thermal conductivity, the SPS sintered pellet obtained promising features such as higher density, better interfacial contact, and reduced chemical reaction. Thermal conductivity measurement at 100°C, 500°C, and 900°C revealed maximum 62% higher thermal conductivity value, when compared to UO 2 pellets, in SPS sintered UO 2 -10vol% SiC composite pellet. The result shows that the SPS technique is required to sinter UO 2 -SiC nuclear fuel pellets with a high value of thermal conductivity.