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

Boiling water reactor (BWR) fuel consists of UO2 pellets within a zirconium barrier liner and Zircaloy cladding. The liner, oxidized during operation, prevents Zircaloy oxidation and provides impact resistance. A helium gap between fuel and liner closes during operation as fuel expands. Spent nuclear fuel (SNF) undergoes significant chemical and microstructural changes, with some fission products retained in the UO2 matrix while noble gases (Xe, Kr) and 4d group metals (Mo, Tc, Ru, Rh, Pd) form gas bubbles and metallic phases. Understanding radionuclide distribution is crucial for predicting potential release during SNF storage, transportation, or disposal. Changes in the FCI zone can also affect fuel thermomechanical properties. Previous research has shown fission product migration to the cladding, with 98% detected within 10 µm of the inner cladding surface. This study focuses on a detailed microscopic analysis of fission products in the cladding of ATM-109, a high-burnup SNF from a BWR, to gain insights into their formation and migration behavior. ATM-109, irradiated from 1979-1992, had a maximum burnup of 70-80 GWd/MTU, resulting in higher levels of Pd and Ag than lower burnup fuels. Previous studies have investigated metallic particles throughout the fuel; this study concentrates on the FCI region and the oxidized Zr metal liner.

The literature extensively covers fission product behavior in nuclear fuels. Studies have described the metallic phases, variously named white inclusions, fission-product alloy, 65-metal particles, epsilon particles, and noble metal phase. Researchers have investigated the distribution of radionuclides in the fuel matrix and their partitioning into discrete phases, emphasizing the importance of this knowledge for predicting potential release in case of cladding failure. Previous work has also explored the changes to the fuel-cladding interaction (FCI) zone during reactor operation and their impact on fuel thermomechanical properties. Studies using micro-Raman spectroscopy and scanning transmission electron microscopy (STEM) have examined various Zr oxide phases formed in the corrosion rind and their correlation with fission-product radiation. The migration of fission products to the cladding has been investigated using stepwise etching processes, revealing the concentration of fission products near the inner cladding surface. Prior research has characterized ATM-109 fuel, focusing on the nature of metallic particles and aspects of their chemistry, including compositional variability and the occurrence of Ag-Pd halides. However, those studies didn’t investigate the FCI region specifically and the distribution of metallic fission product particles in the cladding liner.

The study used a fragment of cladding and a full cross-section of ATM-109 fuel, focusing on the FCI zone. Initial surface examinations were performed using SEM-energy dispersive x-ray spectroscopy (EDS). To prepare TEM specimens, thin section foils normal to the fuel-cladding liner interface were prepared using FIB-based lift-out techniques. High-resolution STEM characterization was performed using a JEOL JEM ARM200C and a JEOL ARM300F probe-corrected STEM, both equipped with HAADF and BF detectors and Bruker EDS systems. The GrandARM also had dual EDS detectors and a Gatan Image Filter Quantum 663 electron energy-loss spectrometer. Diffraction patterns and electron micrographs were analyzed using Gatan Digital Micrograph™ 3.0. Specific methods and precautions for preparing lift-out specimens from SNF using FIB-SEM were followed, ensuring safe handling and analysis of the samples.

Analysis revealed a cladding including the zirconium metal and corrosion rind 851 ± 6 µm thick, with a 30.0 ± 3.3 µm outer corrosion layer. The zirconium liner was approximately 56 ± 5 µm thick. SEM imaging showed "jets" of uranium penetrating the ZrO2 layer, confirmed by EDS mapping. Noble metal phase particles were identified within the ZrO2 layer, often surrounded by a uranium "cloud" with a tail extending to the fuel-cladding interface. STEM analysis revealed particles ranging from a few tens to several hundreds of nanometers, with smaller particles appearing spherical or ellipsoidal and larger particles showing greater shape variation. The particle size distribution varied with depth from the fuel, showing a bimodal distribution near the fuel and decreasing average size with increasing depth. A high number density of voids was correlated with the larger particles. STEM investigations showed that larger noble metal phase particles were polycrystalline with nanometer-sized grains and stacking faults. EDS analysis confirmed the presence of 4d metals and some uranium within the ZrO2 phase. Significant quantities of tellurium (Te) were consistently associated with the noble metal phase particles, showing a distinct Pd-Te phase separation at the nanoscale. STEM-EELS mapping showed that even small (20 nm) particles were agglomerates. Analysis using a higher energy electron beam revealed xenon (Xe) gas within bubbles associated with noble metal phase particles. Tomographic imaging showed the three-dimensional arrangement of the particles.

The presence of discrete noble metal phase particles in the oxidized zirconium liner is unprecedented direct evidence of these particles outside the fuel matrix. Formation mechanisms may involve fission recoil, radiation-enhanced diffusion (RED), and coalescence along grain boundaries. The Zr oxide phase at the FCI is a high-temperature stabilized tetragonal phase; this transformation could be due to thermal spikes or radiation-induced defects. The observed particle size and void density correlate with the tetragonal ZrO2 grain size. The "jetting" features and uranium "cloud" suggest an alternative pathway, possibly involving the forceful expulsion of uranium and noble metal phase particles from the fuel into the cladding liner due to high temperature and pressure gradients. Pressure build-up in Xe gas bubbles might cause the particles to fracture and move. The consistent association of Te with the noble metal phase particles, particularly Pd, indicates a distinct Pd-Te phase within the particles. The observed Xe gas within bubbles associated with particles could originate from fission gas or decay of elements within or near the particles. These findings highlight the complex interplay of radiation, chemical, and mechanical processes within the fuel and cladding during reactor operation.

This study provides the first direct evidence of discrete noble metal phase particles in the oxidized zirconium liner of spent nuclear fuel. The findings reveal a complex distribution of fission products and phase separation within fission product aggregates, which will impact fuel thermal properties and reprocessing. Further investigation into the formation and transport mechanisms of these particles is needed and could significantly influence our understanding of fuel behavior, thermal properties, and long-term disposal considerations.

The study focused on one specific fuel sample (ATM-109). The sample preparation process for Xe detection required sufficiently thick regions, potentially biasing the observations. While the research shows a clear association of Te with Pd in the noble metal phase particles, additional research is needed to fully characterize the specific phases and their formation mechanism within these particles.