Chris Stanek

Three-Dimensional Characterization of Sintered UO2+x: Effects of Oxygen Content on Microstructure and Its Evolution

The oxygen content during the intermediate and final stages of sintering can have a strong effect on the microstructural evolution of oxide fuels. Two depleted urania (d-UO2.0 and d-UO2.14) samples, sintered up to a theoretical density of 90%, were serial sectioned using a focused ion beam and characterized with electron backscatter diffraction (EBSD). The EBSD data were used to make three-dimensional reconstructions of the microstructures to evaluate their characteristics at an intermediate stage of sintering. The oxygen content was found to affect grain shape and grain boundary (GB) mobility, as curved and elongated grains were observed in UO2.0, as well as stronger pore-GB interactions, which is an indication that microstructure was less evolved in UO2.0. Both samples presented a similar fraction ([approximate]20%) of special, coincident site lattice boundaries, with larger amounts of Σ3n GBs, and a rather large fraction of Σ11 GBs for UO2.14. Crystallographic GB planes were also determined to study the distributions of all GB parameters. The UO2.0 sample had a large fraction of GB planes close to the Σ3 twinning planes, which suggests that lower-energy interfaces are used to minimize energy in this sample, potentially due to lower overall GB mobility as compared to UO2.14.

Microstructurally explicit simulation of intergranular mass transport in oxide nuclear fuels

Transport of fission products (FPs) inside fuel pellets is an important mechanism that affects microstructure evolution as well as fuel performance. To study this phenomenon for low fuel burnups, when solid-state diffusion is likely to be the controlling mechanism that sets the stage for subsequent phenomena, e.g., fission gas bubble formation and linkage, we created a three-dimensional (3-D) finite element model based on the real microstructure of a depleted UO2 sample. The model couples grain bulk, grain boundary (GB), and triple junction (TJ) diffusion by using 3-D elements for grain bulks, two-dimensional elements for GBs, and one-dimensional elements for TJs. Grain boundary percolation theory is applied in one case study, and the result shows that the presence of high-diffusivity TJs reduces the effect of GB percolation. The model is also used with mass generation from grain bulks, and it is found that localized regions with a high concentration of FPs can form in the presence of a dominant GB percolation path. The work introduces an approach to model diffusion through GBs and TJs at a fair computational cost that can be applied to study the effects of microstructure on FP transport.

Carbon-14 decay as a source of non-canonical bases in DNA

BACKGROUNDnSignificant experimental effort has been applied to study radioactive beta-decay in biological systems. Atomic-scale knowledge of this transmutation process is lacking due to the absence of computer simulations. Carbon-14 is an important beta-emitter, being ubiquitous in the environment and an intrinsic part of the genetic code. Over a lifetime, around 50 billion (14)C decays occur within human DNA.nnnMETHODSnWe apply ab initio molecular dynamics to quantify (14)C-induced bond rupture in a variety of organic molecules, including DNA base pairs.nnnRESULTSnWe show that double bonds and ring structures confer radiation resistance. These features, present in the canonical bases of the DNA, enhance their resistance to (14)C-induced bond-breaking. In contrast, the sugar group of the DNA and RNA backbone is vulnerable to single-strand breaking. We also show that Carbon-14 decay provides a mechanism for creating mutagenic wobble-type mispairs.nnnCONCLUSIONSnThe observation that DNA has a resistance to natural radioactivity has not previously been recognized. We show that (14)C decay can be a source for generating non-canonical bases.nnnGENERAL SIGNIFICANCEnOur findings raise questions such as how the genetic apparatus deals with the appearance of an extra nitrogen in the canonical bases. It is not obvious whether or not the DNA repair mechanism detects this modification nor how DNA replication is affected by a non-canonical nucleobase. Accordingly, (14)C may prove to be a source of genetic alteration that is impossible to avoid due to the universal presence of radiocarbon in the environment.

Microstructural Effects on Thermal Conductivity of Uranium Oxide: A 3D Multi-Physics Simulation

Transport mechanisms, such as mass and heat transfer, are critical to the efficiency and the reliability of nuclear fuels such as uranium oxide. These properties can be significantly affected by the microstructure of the material. This paper looks into the effects of grain boundary (GB) Kapitza resistance on the overall heat conductivity of UO2 using a 3-D finite element model with microstructurally explicit information. The model developed is created with a 3-D reconstruction of the microstructure of depleted uranium samples performed using serial sectioning techniques with Focused Ion Beam (FIB) and Electron Backscattering Diffraction (EBSD). The model treats grain bulks, GBs and triple junctions using elements of different dimensionalities, and it is thus capable of incorporating information of all three entities in one model while keeping a manageable computational cost. Furthermore, the properties of these microstructural entities are characterized by misorientation angles and Coincident Site Lattice (CSL) models, which provide a framework to assign spatially dependent thermal and mass transfer properties based on the location and connectivity of these entities in actual microstructures. Coupling between heat transfer and mass transfer of fission products is also taken into account in the study, to make it a multi-physics model capable of following the evolution of thermal performance as fission products are produced. These simulations can provide input and insight into the fuel pellet behaviors at the initial stage of power generation when burnups are low.Copyright