RESUMO
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.
RESUMO
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.
RESUMO
A novel relationship between noble metal phase particles and fission gas bubble production in used nuclear fuel is described. The majority of Te atoms within noble metal phase undergo radioactive decay to form stable Xe within a few hours after particle formation. This results in the production of clusters of Xe atoms contained within the solid metal matrix exhibiting an equivalent gas bubble pressure approaching 1 GPa. These high pressure bubbles are stabilized by the UO2 within the bulk of the fuel. However, when these bubbles form near the fuel/cladding interface, in combination with local and temporal damage caused by fission recoil, they are capable of overcoming the fracture strength of the UO2 and rupturing catastrophically. The force of the resulting bubble rupture is sufficient to eject noble metal phase particles several microns into the cladding. This proposed mechanism explains the observance of noble metal phase in cladding and is consistent with a host of morphological features found near the fuel/cladding interface.
RESUMO
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.
