The phenomena of constituent redistribution, wherein a previously homogeneous metallic fuel forms discrete, radially concentric compositional zones upon irradiation was investigated by examining an irradiated U-19Pu-14Zr fuel (where numbers represent wt. %) with a burnup of 11.5 at.% with electron probe microanalysis (EPMA) and quadruple inductively coupled plasma mass spectroscopy (Q-ICP-MS).EPMA-generated U, Pu, and Zr compositional data obtained from a diameter traverse of the sample was converted to mass and was used to: 1) compare the overall fuel element analysis results between the two methods, 2) determine the number of compositionally distinct zones forming as a result of constituent redistribution; and 3) quantify the post-irradiation loss or gain of U, Pu, and Zr atoms in each distinct compositional zone.Weight percent concentrations of U, Pu, and Zr for the overall cross section compare favorably between the two analytical methods, suggesting that the spatially resolved EPMA analysis complements bulk chemical analysis.Among the four identified compositional zones, post-irradiation quantification of U, Pu, and Zr elemental atom content changes shows that the quantity of U atoms lost from the innermost zone is slightly less than the quantity of U atoms gained by the middle two zones, and the quantity of Zr atoms lost from the high-U third zone is slightly less than is gained by the two innermost zones. Pu is lost from all four zones, although the innermost zone and the high-U third zone lose a significantly higher percentage (> 22 %) of their initial Pu atoms than the other two zones. For all three elements, EPMA cannot distinguish between atoms lost due to transport to a different zone from atoms lost due to nuclear processes; however, the insight gained from using this process can be used to experiment with new modeling techniques to predict constituent redistribution in U-Pu-Zr fuels.

Minor actinides (MA) significantly contribute to the long-term radiotoxicity of spent nuclear fuel (SNF). Separating MA from SNF and incorporating it into metallic fuels for fast reactor transmutation is a potential method to reduce this radiotoxicity. This study focuses on transmission electron microscopy characterization of two samples from the fuel cladding chemical interaction (FCCI) region of an americium (Am) and neptunium (Np)-bearing (MA-bearing) uranium-plutonium-zirconium (U-Pu-Zr) fuel irradiated in the Phenix fast reactor to 9.5 % FIMA burnup at approximately 550 degrees C cladding temperature. The results show that despite the complex chemical interactions between MA and AIM1 cladding elements, excessive FCCI was not induced, and Am penetration depth in the cladding limited to less than 4 mu m. Np remained mostly inside fuel. The Zr-rich compounds layer effectively limited the accumulation of lanthanide on the inner cladding surface. Overall, the FCCI behavior between investigated MA-bearing U-Pu-Zr fuel and AIM1 cladding is benign.

Two tristructural isotropic (TRISO)-coated nuclear fuel particles were examined by electron probe micro-analysis (EPMA) as part of the Advanced Gas Reactor program. The compacts' average irradiation temper-atures ranged from approximately 1260 to 1290 degrees C. One particle was examined in the as-irradiated con-dition, while the other was subject to 1600 degrees C post-irradiation safety testing. This study was undertaken to test a newly-developed EPMA technique to determine fission product masses in TRISO particles on a layer-by-layer basis, and to compare fission product distributions between an as-irradiated and safety -tested particle. Fission product concentration profiles were collected along two radii in each particle, with measured concentrations used to compute the fission product mass in each TRISO particle layer. These measured masses were then compared to those predicted from ORIGEN modeling calculations. Data col-lected from these measurements show that for these two particles, masses determined via EPMA were within +/- 20% of the calculated masses for the rare-earth elements, Mo, Zr, Cs, I, and Pd. Elements that tend to be less homogeneously distributed include Sr, Te, Eu, Ag, and possibly Ba. Measured Ag masses differed by more than 40% from the calculated mass. Lanthanides other than Eu remain primarily within the fuel kernel in the as-irradiated particle but in the safety-tested particle these element masses were divided approximately equally between the kernel and kernel periphery. In both particles, the majority of Sr and Eu accumulated in the carbon-rich kernel periphery, although in the safety-tested particle, Sr and Eu accumulated farther from the fuel kernel than occurred with irradiation alone. A greater mass fraction of mobile elements, such as Cs and I accumulated in the buffer and IPyC in the safety-tested particle as compared to the as-irradiated particle. When fully developed and tested, this mass balance approach to TRISO particle analysis has the potential to provide insight into fuel behavior.(c) 2021 Elsevier B.V. All rights reserved.

Experimental investigations of the fuel microstructure and volatile fission products along the pellet of a high burnup specimen (local burnup 76 GWd/tHM) have been conducted to support future transient testing. Detailed microscopy examinations have been carried out at different length scales. Transmission electron microscopy has highlighted a significant amount of damage across the entire radius with the formation of networks of dislocations. The optical and scanning electron microscopy determined the for-mation of three zones in the pellet with different characteristics. An intermediate region with no grain subdivision, lower porosity than the central zone porosity and high retained fission gas in nanometric bubbles and in the matrix was present between r/r0 approximate to 0.55 and 0.8. The high retention of gas in this re-gion might suggest that the region will be prone to fine fragmentation, in addition to the HBS. An abrupt transition in the structure was observed at mid radius, with a third region developing from the mid ra-dius to the pellet center. In this part of the pellet, metallic and grey phases with size between hundreds of nanometers and a few micrometers have formed at grain boundaries. The majority of fission gas has been released from the grain matrix and the original grains have polygonised, forming sub-grain domains separated by low-angle grain boundaries. No final explanation can be given for the polygonization occur-ring in the center, but on the basis of the irradiation history and the analysis of all the post irradiation examination (PIE) data, it is postulated that the polygonization within the original grains is an effect of dynamic recovery occurring at high temperature in the fuel center.(c) 2022 Elsevier B.V. All rights reserved.

The influence of a reactor overpower transient on irradiated metallic fuel performance was investigated in this work. Such transient studies support safety and performance optimization of future sodium fast reactors. A ternary metallic fuel alloy (U-19Pu-10Zr) in HT-9 cladding which was irradiated in the Experimental Breeder Reactor II (EBR-II) to a burnup of 11 at. %, and then subjected to a 30% overpower transient was studied. The sample was selected from the top of a fuel pin where failure is expected to first appear in the EBR-II pin design. This can occur when the pin is subjected to conditions beyond its design envelope. Main elemental distribution, phase characterization, fission product behavior, chemical form and fuel-cladding chemical interaction were analyzed in this study. Simulation of the temperature profile during the transients were performed using BISON and show that the moderate transient did not maintain prolonged high temperature. For the first time, the porous structure commonly found at the top of metallic fuel pin (termed "fluff" structure given its appearance) was examined in detail via postirradiation characterization. The fluff contains the major fuel elements (U, Pu, Zr) and presents a highly porous microstructure (over 40% area fraction). The presence of fission products relevant to source term was investigated. However, semi-volatile and volatile elements (namely Cs, I, Xe, Eu), expected in this region, could not be detected probably due to sample preparation. Europium, also expected in this region, was only detected in precipitates along with the other rare earth elements. Finally, no fluff (fuel) fragments detached from the fuel slug were detected in the plenum region, indicating the fluff stability under the tested moderate ramp. These analyses indicate that the moderate transient experienced by the fuel pin did not significantly influence fuel performance under this transient condition. (C) 2020 Elsevier B.V. All rights reserved.

Palladium is being investigated as a fuel additive to bind with and potentially immobilize lanthanide fission products. A primary cause of fuel-cladding chemical interaction (FCCI) is the lanthanide fission products migrating to the fuel periphery and interacting with the cladding. This interaction will lead to wastage of the cladding and eventually to a cladding breach. Palladium has previously been identified as a promising additive used to prevent or decrease FCCI by reacting with the lanthanide fission products. In the current study, an alloy cast from the four highest abundant lanthanides found in irradiated metallic fuel, Nd, Ce, Pr, and La, with and without Pd, has been characterized using neutron diffraction, scanning electron microscopy, and electron probe microanalysis. In the lanthanide-Pd intermetallic compounds, all of the constituent compounds, i.e. Nd-Pd, Ce-Pd, La-Pd and Pr-Pd are known. There is very good agreement, both structurally and compositionally, between the out-of-pile lanthanide alloy and lanthanide fission products characterized in irradiated fuels. In both cases, the lanthanide elements form a solid solution in a hexagonal crystal structure. The out-of-pile lanthanide alloy follows Vegard's Law, with the measured and calculated (weighted average of constituents) lattice parameters being within 1% for both the a and c parameters. Pd bonds with the lanthanides (Ln) forming the phases LnPd and Ln7Pd3. The results indicate the properties of lanthanide compounds in irradiated metallic fuel can be reliably simulated in out-of-pile experiments.

This work utilizes electron microscopy-based techniques to examine the radial behavior of solid fission products in plutonium (Pu) bearing mixed oxide (MOX) fuel irradiated to a burnup of 13.7% fissions per initial metal atom (FIMA). Metallic precipitates primarily consist of five fission products: ruthenium (Ru), rhodium (Rh), technetium (Tc), molybdenum (Mo), and palladium (Pd). The five metal precipitates (FMPs) examined in this work have low concentrations of Pd and Mo, with no major compositional differences along the fuel radius. A secondary Pd–Te metallic phase forms in cooler regions of the pellet, likely due to the diffusion of gaseous species away from the central void. X-ray chemical maps indicate that the Pd–Te phase can nucleate on the surface of FMPs before precipitating into separate particles. These particles were also found to alloy with iron (Fe) near the surface of the fuel pellet due to interdiffusion with the stainless-steel cladding. The insoluble perovskite oxide phase was found to form near the central void and at intermediate radial positions, but not at the fuel edge. These findings suggest that solid fission product phases form at varying counts and compositions along the fuel pellet radius, and thus should be considered when describing the thermal behavior of the fuel.

Electron probe microanalysis (EPMA) of actinide elements plays a vital role in the characterization of nuclear fuel as it is primarily these elements that comprise the fuel. Although EPMA analysis of nuclear fuel has been extant since the early 1970’s it was, and remains, extremely challenging for several reasons. Firstly, when actinides are present in irradiated nuclear fuel, they are accompanied by large β and γ radiation fields, which can cause several orders of magnitude increase in background noise as well as contribute to premature detector failure. Secondly, M-line actinide X-ray lines are used for analysis. These peaks suffer from abundant peak overlaps and a paucity of relevant physical parameters, particularly accurate mass absorption coefficients. Finally, analytical standards such as Np and Pu cannot be purchased. They must be made internally and do not have the robust analytical data and documentation that accompanies many geological standards (e.g. Smithsonian mineral standards).

Journal Article Micro- and Nano-Characterization of Neutron Irradiated TRISO Coated Particles Get access Isabella van Rooyen, Isabella van Rooyen Fuel Design and Development Department, Idaho National Laboratory, Idaho Falls, ID, USANuclear Engineering Program, Materials Science and Engineering Department, University of Florida, Gainesville, FL, USA Corresponding author: Isabella.vanrooyen@inl.gov Search for other works by this author on: Oxford Academic Google Scholar Yong Yang, Yong Yang Nuclear Engineering Program, Materials Science and Engineering Department, University of Florida, Gainesville, FL, USA Search for other works by this author on: Oxford Academic Google Scholar Karen Wright, Karen Wright Post-Irradiation Examination Department, Idaho National Laboratory, Idaho Falls, ID, USA Search for other works by this author on: Oxford Academic Google Scholar Thomas Lillo, Thomas Lillo Materials Science and Engineering Department, Idaho National Laboratory, Idaho Falls, ID, USA Search for other works by this author on: Oxford Academic Google Scholar Subhashish Meher, Subhashish Meher Materials Science and Engineering Department, Idaho National Laboratory, Idaho Falls, ID, USA Search for other works by this author on: Oxford Academic Google Scholar Zhenyu Fu Zhenyu Fu Nuclear Engineering Program, Materials Science and Engineering Department, University of Florida, Gainesville, FL, USA Search for other works by this author on: Oxford Academic Google Scholar Microscopy and Microanalysis, Volume 25, Issue S2, 1 August 2019, Pages 1612–1613, https://doi.org/10.1017/S1431927619008791 Published: 01 August 2019

Transmutation nuclear fuels contain weight percentage quantities of actinide elements, including Pu, Am and Np. Because of the complex spectra presented by actinide elements using electron probe microanalysis (EPMA), it is necessary to have relatively pure actinide element standards to facilitate overlap correction and accurate quantitation. Synthesis of actinide oxide standards is complicated by their multiple oxidation states, which can result in inhomogeneous standards or standards that are not stable at atmospheric conditions. Synthesis of PuPO4 results in a specimen that exhibits stable oxidation-reduction chemistry and is sufficiently homogenous to serve as an EPMA standard. This approach shows promise as a method for producing viable actinide standards for microanalysis.

An abstract is not available for this content so a preview has been provided. As you have access to this content, a full PDF is available via the ‘Save PDF’ action button.