Post irradiation examination of nuclear fuel: Toward a complete analysis

A Comprehensive Review of High Burn-Up Structure Formation in UO2: Mechanisms, Interactions, and Future Directions

In the rim zone of UO2 nuclear fuel pellets, high burn-up and low temperatures drive changes in the microstructure, leading to the formation of high burn-up structures (HBS). This review focuses on the formation of HBS, beginning with a description of the two contentious mechanisms-recrystallization and polygonization-that are believed to be the primary controlling factors. We discuss experimental and simulation studies that support both mechanisms, emphasizing that although each mechanism can explain certain aspects of HBS formation, neither recrystallization nor polygonization alone is sufficient to fully explain the observed phenomena. Furthermore, we emphasize the intrinsic relationship between these two mechanisms, suggesting that they represent different manifestations of the same underlying process under varying conditions, and we reference relevant studies that support this perspective. Lastly, we underline the significance of investigating the formation processes of HBS and provide an outlook on future research directions based on the current state of knowledge.

Hyperspectral imaging suggests potential for rapid quantification of fission products in spent nuclear fuel

An analysis of sintered uranium dioxide has been conducted using a hyperspectral camera sensitive to short-wave infrared wavelengths in the range 949-2472 nm. Three groups of sintered UO2 nuclear fuel pellets were prepared and analysed, with stable sub-group surrogates introduced at the preparation stage to emulate the presence of fission product elements. Results show a clear, consistent, and reproducible spectral response across the pellet groups for pure UO2. Furthermore, the addition of fission product elements is observed to affect the shortwave infrared response, causing an overall flattening of the spectra. We have shown that this spectral change is correlated significantly with the presence of lanthanides in the fuel matrix. This result could have important potential in post-irradiation examination for quantifying nuclear fuel burn-up and radiotoxicity at discharge, as the hyperspectral imaging setup allows multiple (> 20) samples to be analysed in a single image, captured in under 30 s.

Chemical and elemental mapping of spent nuclear fuel sections by soft X-ray spectromicroscopy

Soft X-ray spectromicroscopy at the O K-edge, U N_4,5-edges and Ce M_4,5-edges has been performed on focused ion beam sections of spent nuclear fuel for the first time, yielding chemical information on the sub-micrometer scale. To analyze these data, a modification to non-negative matrix factorization (NMF) was developed, in which the data are no longer required to be non-negative, but the non-negativity of the spectral components and fit coefficients is largely preserved. The modified NMF method was utilized at the O K-edge to distinguish between two components, one present in the bulk of the sample similar to UO₂ and one present at the interface of the sample which is a hyperstoichiometric UO_2+x species. The species maps are consistent with a model of a thin layer of UO_2+x over the entire sample, which is likely explained by oxidation after focused ion beam (FIB) sectioning. In addition to the uranium oxide bulk of the sample, Ce measurements were also performed to investigate the oxidation state of that fission product, which is the subject of considerable interest. Analysis of the Ce spectra shows that Ce is in a predominantly trivalent state, with a possible contribution from tetravalent Ce. Atom probe analysis was performed to provide confirmation of the presence and localization of Ce in the spent fuel.

Fifteen Years of Radionuclide Research at the KIT Synchrotron Source in the Context of the Nuclear Waste Disposal Safety Case

For more than 120 years, systematic studies of X-ray interaction with matter have been the basis for our understanding of materials—both of natural or man-made origin—and their structure-function relationships. Beginning with simple radiographic imaging at the end of the 19th century, X-ray based analytical tools such as X-ray diffraction, X-ray fluorescence and photoemission or X-ray absorption techniques are indispensable in almost any field of chemical and material sciences—including basic and applied actinide and radionuclide studies. The advent of dedicated synchrotron radiation (SR) sources in the second half of the last century has revolutionized the analytical power of X-ray probes, while—with increasing number of SR facilities—beamline instrumentation followed a trend towards increasing specialization and adaption to a major research topic. The INE-Beamline and ACT station at the KIT synchrotron source belong to the exclusive club of a few synchrotron beamline facilities—mostly located in Europe—dedicated to the investigation of highly radioactive materials. Since commissioning of the INE-Beamline in 2005, capabilities for synchrotron-based radionuclide and actinide sciences at KIT have been continuously expanded, driven by in-house research programs and external user needs.