Nanostructure of metallic particles in light water reactor used nuclear fuel

Abstract

An extraordinary nano-structure has been observed in the metallic (Mo-Tc-Ru-Rh-Pd) particles that are known to form during irradiated in light water nuclear reactor fuels. This structure points possible high catalytic reactivity through the occurrence of a very high surface area as well as defect sites. We have analyzed separated metallic particles from dissolved high burn-up spent nuclear fuel using scanning and transmission electron microscopy. The larger particles vary in diameter between ~10 and ~300 nm and possess a hexagonally close packed epsilon-ruthenium structure. These particles are not always single crystals but often consist of much smaller crystallites on the order of 1-3 nm in diameter with evidence suggesting the occurrence of some amorphous regions. It is possible that neutron irradiation and fission product recoils generated the unusual small crystallite size. The composition of the metallic particles was variable with low levels of uranium present in some of the particles. We hypothesize that the uranium may have induced the formation of the amorphous (or frustrated) metal structure. This unique nanostructure may play an important role in the environmental behavior of nuclear fuels.

Distribution of metallic fission-product particles in the cladding liner of spent nuclear fuel

“…6f with figures (g) and (h) the selected area diffraction at two different tilts determined on the analysis of the diffraction patterns. In agreement of the work by Buck et al 13 and Cui et al, 9 STEM/ TEM investigations here show that the larger noble metal phase particles were not single crystals but instead were polycrystalline with grain sizes on the order of nanometers. Figure 6a shows a SEM image using a STEM detector of a large noble metal phase particle that shows that it is comprised of several crystals.…”

“…[3][4][5][6][7][8] Some fission products are retained within the UO 2 matrix in solid solution, while at the same time the noble gases-xenon (Xe) and krypton (Kr)-and the 4d group metals-molybdenum (Mo), technetium (Tc), ruthenium (Ru), rhodium (Rh), and palladium (Pd)-are trapped as gas bubbles and partitioned into metallic phases, respectively. [9][10][11][12][13][14] This metallic phase has gone by several names in the literature, including white inclusions, 15,16 fission-product alloy, 16 5-metal particles, 17 epsilon particles, 18,19 and noble metal phase. 6,20 Knowledge of the distribution of radionuclides across the fuel matrix or partitioning as discrete phases within or outside the fuel grains is necessary to make predictions about potential release in the event of cladding failure during SNF storage, transportation, or long-term geologic disposal.…”

“…32 We have previously reported on the nature of the metallic particles from these fuels and aspects of their chemistry, including the variability of compositions in the 5-metal particles and the occurrence of Ag-Pd halides. 13,[33][34][35][36] In these earlier studies, we investigated metallic particles throughout the fuel; however, in this investigation, we concentrated on the FCI region and, in particular, the oxidized Zr metal liner nearest to the fuel.…”

Sequestration of radioactive iodine in silver-palladium phases in commercial spent nuclear fuel

“…The large majority of the material was the 5-metal (Mo-Tc-Ru-Rh-Pd), epsilon phase and consisted of agglomerates of much smaller particles. The nature of this material has been reported in detail elsewhere [22]. Fig.…”

Microstructural evolution of UO2 pellets containing metallic particles of Ru, Rh and Pd during dissolution in nitric acid solution: 3D-ESEM monitoring

“…These fission products are generally classified in representative groups presenting similar physicochemical behavior. The noble metals group, which is represented in SNF mainly by Mo, Ru, Rh, Pd and Tc, is not incorporated in the crystal lattice of UO 2 , but found as metallic inclusions trapped in the inter-and intra-granular pores and in the grain boundaries [7][8][9] . In light water reactor UO fuel, the metallic precipitates are spherical particles ranging in size from 10 nm to 300 nm, potentially aggregated in larger particles (of several microns).…”