J. Bertsch

Nuclear fuel in generation II and III reactors: research issues related to high burn-up

Nuclear fuel has a much higher energy density compared to conventional fuels like fossil and allows an almost green house emission free energy production. Despite its inherent advantages, the performance of nuclear fuel has to be amended due to economical as well as ecological reasons, without constricting its safe use. A higher performance of nuclear fuel means extracting more energy from the fuel. The higher energy yield also implies less volume of highly radioactive spent fuel per produced energy unit. The stronger energy extraction from nuclear fuel leads to a higher burn-up, which means pushing the limits, with the safety limitations fixed by the regulators. This has been carried out, for instance, in Switzerland for many years and has reached about 80 MW d kg−1 or about 8% of fission per initial metal atom “FIMA” in generation (GEN) II reactors. Nuclear fuels consist of different constituents which are the matrix, e.g. non-inert or inert, a fissile part, components such as burnable poisons, fertile isotopes or grain enlargers. The fuel matrix is currently described as the first barrier between fission products and the next nuclear barrier towards the environment. It must remain effective over years. The higher the burn-up the higher the concentration of fission products is in the fuel matrix. The fissile part in the fuel directly contributes to the energy production. The typical enrichment of fissile isotopes in a commercial nuclear fuel is around 5%. A higher enrichment followed by a respective higher burn-up may affect the fission product retention capability of the fuel matrix and its interaction with the cladding. The doping with grain enlargers leads to bigger fuel grains, implying longer diffusion paths, retaining gaseous fission products in the matrix under certain temperature conditions. This shall allow a better pressure control at higher burn-up in GEN II and III reactors. To reach very high burn up >100 MW d kg−1 (equivalent), the only possibilities are offered by inert matrix fuels. This is investigated in the last part of this study. The zirconia inert matrix concept could be used in a last cycle e.g. to burn plutonium excess and minor actinides prior to geological disposal. Prior to and after burn-up, the fuel material is characterized by micro- or nano-beam analysis to gain information on the dispersity of the system, the presence of defect and segregated phases as well as to track fission products. These studies can be performed through a multiscale approach. The fuel structure can be revealed at the macro-, micro-, nano- and sub-nano-level.

Microbeam x-ray absorption spectroscopy study of chromium in large-grain uranium dioxide fuel

Synchrotron-based microprobe x-ray absorption spectroscopy (XAS) has been used to study the local atomic structure of chromium in chromia-doped uranium dioxide (UO2) grains. The specimens investigated were a commercial grade chromia-doped UO2 fresh fuel pellet, and materials from a spent fuel pellet of the same batch, irradiated with an average burnup of ~40 MW d kg(-1). Uranium L3-edge and chromium K-edge XAS have been measured, and the structural environments of central uranium and chromium atoms have been elucidated. The Fourier transform of uranium L3-edge extended x-ray absorption fine structure shows two well-defined peaks of U-O and U-U bonds at average distances of 2.36 and 3.83 Å. Their coordination numbers are determined as 8 and 11, respectively. The chromium Fourier transform extended x-ray absorption fine structure of the pristine UO2 matrix shows similar structural features with the corresponding spectrum of the irradiated spent fuel, indicative of analogous chromium environments in the two samples studied. From the chromium XAS experimental data, detectable next neighbor atoms are oxygen and uranium of the cation-substituted UO2 lattice, and two distinct subshells of chromium and oxygen neighbors, possibly because of undissolved chromia particles present in the doped fuels. Curve-fitting analyses using theoretical amplitude and phase-shift functions of the closest Cr-O shell and calculations with ab initio computer code FEFF and atomic clusters generated from the chromium-dissolved UO2 structure have been carried out. There is a prominent reduction in the length of the adjacent Cr-O bond of about 0.3 Å in chromia-doped UO2 compared with the ideal U-O bond length in standard UO2 that would be expected because of the change in effective Coulomb interactions resulting from replacing U(4+) with Cr(3+) and their ionic size differences. The contraction of shortest Cr-U bond is ~0.1 Å relative to the U-U bond length in bulk UO2. The difference in the local chromium environment between fresh and irradiated UO2 is discussed based on the comparison of quantitative structural information obtained from the two chromia-doped fuel samples analyzed.

Polydispersity and EXAFS simulations

EXAFS is an important experimental technique for determining the local atomic structure of nanoclusters embedded in a bulk material. In practical cases, nanocluster samples do not contain homogeneous clusters of just one size, and the average cluster size is strongly influenced by the specific distribution of cluster sizes. Combinations of different cluster sizes might provide very similar results; this issue is called polydispersity. The goal of this study is to understand if there are any principal limitations for EXAFS studies related to polydispersity. Here a new approach based on EXAFS simulations followed by linear combination (LC) on EXAFS spectra is presented. The simulations were performed on pure Cu and binary Cu-Fe clusters. The main result of this study concerns the proof that polydispersity does not affect XAFS studies on nano-clusters within a size of up to 140 atoms.

Interpretation of the U L3-edge EXAFS in uranium dioxide using molecular dynamics and density functional theory simulations

X-ray absorption spectroscopy is employed to study the local structure of pure and Cr-doped UO2 at 300 K. The U L3-edge EXAFS spectrum is interpreted within the multiplescattering (MS) theory using the results of the classical and ab initio molecular dynamics simulations, allowing us to validate the accuracy of theoretical models. The Cr K-edge XANES is simulated within the full-multiple-scattering formalism considering a substitutional model (Cr at U site). It is shown that both unrelaxed and relaxed structures, produced by ab initio density functional theory (DFT) calculations, fail to describe the experiment.