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Loss of fracture resistance due to thermal aging degradation is a potential limiting factor affecting the long-term (80+ year) viability of nuclear reactors. To evaluate the effects of decades of aging in a practical time frame, accelerated aging must be employed prior to mechanical characterization. In this study, a variety of chemically and microstructurally diverse austenitic stainless steels were aged between 0 and 30,000 h at 290–400 °C to simulate 0–80+ years of operation. Over 600 static fracture tests were carried out between room temperature and 400 °C. The results presented include selected J-R curves of each material as well as K0.2mm fracture toughness values mapped against aging condition and ferrite content in order to display any trends related to those variables. Results regarding differences in processing, optimal ferrite content under light aging, and the relationship between test temperature and Mo content were observed. Overall, it was found that both the ferrite volume fraction and molybdenum content had significant effects on thermal degradation susceptibility. It was determined that materials with >25 vol% ferrite are unlikely to be viable for 80 years, particularly if they have high Mo contents (>2 wt%), while materials less than 15 vol% ferrite are viable regardless of Mo content.

We report elemental and isotopic analysis for the noble metal fission product phase found in irradiated nuclear fuel. The noble metal phase was isolated from three commercial irradiated UO2 fuels by chemically dissolving the UO2 fuel matrix, leaving the noble metal phase as the undissolved residue. Macro amounts of this residue were dissolved using a KOH + KNO3 fusion and then chemically separated into individual elements for analysis by mass spectrometry. Though the composition of this phase has been previously reported, this work is the most comprehensive chemical analysis of the isolated noble metal phase to date. We report both elemental and isotopic abundances of the five major components of the noble metal phase (Mo, Tc, Ru, Rh, Pd). In addition, we report a sixth element present in high quantities in this phase, tellurium. Tellurium appears to be an integral component of noble metal particles.

Accurate measurements of 235U enrichment within metallic nuclear fuels are essential for understanding material performance in a neutron irradiation environment, and the origin of secondary phases (e.g. uranium carbides). In this work, we analyse 235U enrichment in matrix and carbide phases in low enriched uranium alloyed with 10 wt% Mo via two chemical imaging modalities-nanoscale secondary ion mass spectrometry (NanoSIMS) and atom probe tomography (APT). Results from NanoSIMS and APT are compared to understand accuracy and utility of both approaches across length scales. NanoSIMS and APT provide consistent results, with no statistically significant difference between nominal enrichment (19.95 ± 0.14 at% 235U) and that measured for metal matrix and carbide inclusions.

peer-reviewed Although the existence of a five-metal (Mo-Tc-Ru-Rh-Pd) phase – as nanoparticles observed in irradiated nuclear fuel – has been known for more than half a century, the chemical and physical mechanisms controlling the formation and behavior of such particles remain stubbornly elusive. We present in this work new evidence for the presence of a separate nonmetallic phase associated with the metallic particles and containing a significant fraction of Te in addition to Pd. While this new phase potentially complicates the thermodynamic picture of a mixed alloy in equilibrium with the surrounding fuel environment, it also provides new clues in the search for a chemical mechanism for Pd migration through the uranium dioxide matrix and the nucleation behavior of the particles. Fractionation between phases may subsequently affect the mechanical performance of fuels during irradiation and their interactions with the surrounding environment during long-term waste storage.

We have made observations of noble metal phase fission-product agglomerates and gaseous xenon within the fuel-cladding interaction (FCI) zone of a high-burnup UO2 fuel. The FCI is the boundary between the UO2 pellet outer surface and the inner wall of the oxidized Zr-liner/cladding of the fuel rod. These fission-product agglomerates are well known to occur within the spent fuel matrix, and although radionuclides have been reported by others, we reveal aspects of their speciation and morphology. That they occur as discrete particles in the oxidized Zr liner, suggests the occurrence of hitherto unknown processes in the FCI zone during reactor operation, and this may have implications for the long-term storage and disposal of these types of materials. As expected, the particle agglomerates, which ranged in size from the nanometer scale to the micrometer scale, contained mainly Mo, Ru, Tc, Rh, and Pd; however, we also found significant quantities of Te associated with Pd. Indeed, we found nanometer scale separation of the distinct Pd/Te phase from the other fission products within the particles. Often associated with the particles was concentrations of uranium, sometimes appearing as a “cloud” with a tail emanating from the fuel into the oxidized cladding liner. Many of the noble metal phase particles appeared as fractured clusters separated by Xe-gas-filled voids. Possible mechanisms of formation or transport in the cladding liner are presented.

Thermal aging of ferrite in duplex stainless steels leads to a complex microstructural evolution that can give way to large deleterious changes in mechanical properties. The microstructural evolution mechanisms that happen concurrently are strongly linked to minor changes in composition. By characterizing duplex stainless steels with various compositions over a range of aging conditions, it was revealed that adding a modest amount of Mo to cast austenitic stainless steel (such as changing from grade CF3 to grade CF3M) not only increases the kinetics of spinodal decomposition and Ni-Si-Mn-rich G-phase precipitation, but it also forms a separate Mo-rich phase. At shorter equivalent aging times, Mo segregates from Fe and towards Cr accelerating spinodal decomposition. At longer aging times, Mo enriches in the Ni-Si-Mn clusters before phase separating into an adjoined Mo-rich cluster. These phase changes and precipitation events result in a rather steep drop in fracture and impact toughness. These results will inform the design of similar alloys for use at high temperatures.

Deployment of additively manufactured materials into nuclear energy systems requires investigation of the full range of the unique environmental effects on these materials. Hydrogen isotopes are common gaseous species in nuclear reactors and exhibit ample high mobility in most materials. While hydrogen isotopes mobility in wrought stainless steel is well understood, this property is not thoroughly studied for its additively manufactured variants. In this study, we investigated the deuterium permeation and retention in 316L stainless steel manufactured by laser powder bed fusion. The results showed that the deuterium permeability in the as-built additively manufactured 316L stainless steel (AM SS316L) is greater than that of the reference wrought 316L stainless steel by a factor of 2.8. However, the stress-relieved and solution-annealed AM SS316L samples exhibit lower deuterium permeability in comparison with the as-built condition. Following the solution annealing, the deuterium permeability of AM SS316L is comparable with that of the reference wrought SS316L. Deuterium retention in the as-built AM SS316L is 45% higher than that of the wrought 316L stainless steel and slightly higher than that of the two thermally annealed AM SS316L materials. Transmission electron microscopy and positron annihilation lifetime spectroscopy were used to obtain microstructural information used to determine deuterium permeation and retention behavior in the studied materials.

The ability to acquire high-quality spatially-resolved mass spectrometry data is sought in many fields of study, but it often comes with high cost of instrumentation and a high level of expertise required. In addition, techniques highly regarded for isotopic analysis applications such as thermal ionization mass spectrometry (TIMS) do not have the ability to acquire spatially-resolved data. Another drawback is that for radioactive materials, which are often of interest for isotopic analysis in geochemistry and nuclear forensics applications, high-end instruments often have restrictions on radioactivity and non-dispersibility requirements. We have applied the use of a traditional microanalysis tool, the focused ion beam/scanning electron microscope (FIB/SEM), for preparation of radioactive materials either for direct analysis by spatially-resolved instruments such as secondary ion mass spectrometry (SIMS) and laser ablation inductively-coupled mass spectrometry (LA-ICP-MS), or similarly to provide some level of spatial resolution to techniques that do not inherently have that ability such as TIMS or quadrupole inductively coupled plasma mass spectrometry (Q-ICP-MS). We applied this preparation technique to various uranium compounds, which was especially useful for reducing sample sizes and ensuring non-dispersibility to allow for entry into non-radiological or ultra-trace facilities. Our results show how this site-specific preparation can provide spatial context for nominally bulk techniques such as TIMS and Q-ICP-MS. In addition, the analysis of samples extracted from a uranium dioxide fuel pellet via all methods, but especially NanoSIMS and LA-ICP-MS, showed enrichment heterogeneities that are important for nuclear forensics and are of interest for fuel performance.

The role of interface structure on the trapping of He in Cu–Nb nanocomposites was investigated by comparing He bubble formation in nano-multilayers grown by PVD, nanolaminates fabricated by accumulative roll bonding (ARB), and 3D nanocomposites obtained by high pressure torsion (HPT). All samples were implanted with 1 MeV He ions at room temperature and characterized by cross section transmission electron microscopy (TEM). The critical He concentration leading to bubble formation was determined by correlating the He bubble depth distribution detected by TEM with the implanted He depth profile obtained by SRIM. The critical He dose per unit interfacial area for bubble formation was largest for the PVD multilayers, lower by a factor of ∼1.4 in the HPT nanocomposites annealed at 500 °C, and lower by a factor of ∼4.6 in the ARB nanolaminates relative to the PVD multilayers. The results indicate that the (111)FCC||(110)BCC Kurdjumov-Sachs (KS) interfaces predominant in PVD and annealed HPT samples provide more effective traps than the (112)KS interfaces predominant in ARB nanolaminates; however, the good trapping efficiency and high interface area of 3D HPT structures make them most attractive for applications.

Thermal degradation of cast stainless steels was studied to provide an extensive knowledgebase for the assessment of structural integrity during extended operations of reactor coolant systems. The CF3 and CF8 series cast stainless steels with relatively low (5–12%) δ-ferrite contents were thermally aged at 290–400 °C for up to 10,000 h and tested to measure changes in tensile and impact properties. The aging treatments caused significant reduction of tensile ductility, but only slight softening or negligible strength change. The thermal aging also caused significant reduction of upper shelf energy and large shift of ductile-brittle transition temperature (ΔDBTT). The most influential factor in thermal degradation was ferrite content because of the major degradation mechanism occurring in the phase, while the nitrogen and carbon contents caused only weak effects. An integrated model is being developed to correlate the mechanical property changes with microstructural and compositional parameters.

Metallic uranium is an important material for many applications, especially nuclear energy for low enriched metallic fuel forms and new reactor concepts. Due to the its high electropositivity, production pathways to the metallic form are limited. One of the historically most common synthesis routes involves the heating of a uranium (IV) halide, in this study UF4, with a highly electropositive metal like calcium. This synthesis is referred to as “bomb reduction” due to the temperatures and pressures released by the resulting exothermic reaction. This synthesis route is important for production, but it has also been shown to separate a decay product, thorium, from the parent uranium. This fractionation could be important for purification, but also presents the opportunity for radiochronometric dating of the reduction date. Unfortunately, little characterization has been performed on the products of this reaction. The present study performed bomb reduction on ∼10 g of thorium-doped (2000 ppm) UF4 followed by scanning electron microscopy and energy dispersive spectroscopy. This characterization revealed morphological features of the metal product and slag, the latter of which displayed a wide range of features that indicates a complex reaction in which many variables are involved. Initial characterizations also identified thorium-rich particles, which were extracted and analyzed via transmission electron microscopy and atom probe tomography. These characterizations identified a new thorium-bearing phase, Al9-xFe7+xTh2Si