Modeling of Microstructure Evolution in Uranium Dioxide

Different kinds of oxide fuels are used in nuclear power plants, but the most commonly used fuel is Uranium Dioxide (UO2). A solid understanding of the fuel performance is central to control in-service performance, properties and degradation of the fuel as well as for safe handling of it.

UO2 as fuel material

As fuel material, the UO2 is usually sintered into small cylindrical pellets, measuring about 10 mm in diameter and similar in length. These cylinders are stacked in fuel rods inside a Zircalloy cladding. The small radial gap between the pellets and the cladding is usually filled with a pressurized gas such as Helium. A number of such fuel rods are then mounted in fuel assemblies together with control rods with a high capacity for neutron absorption. The fuel assemblies are then used as heat source in fission power plants.

As the fuel pellets are “burnt” in the reactor, fission processes take place and the degree of irradiation of the fuel pellets is usual measured in terms of the “burnup”, that is the fraction of the initial material that has undergone fission.

Microstructure evolution in UO2

Under common in-service conditions, the core of the fuel pellets can be maintained at a temperature of 2000K while the pellet surface is at around 800K (the melting point of UO2 is approximately 3140K). The outside temperature is maintained by a constant flow of coolant through the fuel assemblies. Under these extreme thermal conditions, the fuel material undergoes drastic changes. These changes have a strong influence on fuel performance and properties such as the thermal conductivity and structural rigidity. The grain structure will have different morphology in different regions.

The extreme thermal gradients will also cause so-called “hourglassing” of the fuel pellets along with cracking – both radially and circumferentially – due to thermally induced stresses and swelling due to solid fission products. By the release of fission gasses (e.g. Xe, Kr, I and Cs), gas filled pores or voids will form in the microstructure. The gas bubbles form in the grain interiors and migrate by diffusion to coalesce along the grain boundaries. The presence of gas bubbles can cause swelling and cracking of the fuel pellet and the gas can also be released inside the Zircalloy cladding, lowering the heat conduction capacity of the Helium that surrounds the pellets. In either case, the integrity of the Zircalloy cladding is compromised.

Project

In the project, the stability of grain boundary texture under grain growth in UO2 is studied through level set modeling, taking anisotropic grain boundary properties into account. In addition, the characteristic morphology of faceted voids in UO2, due to heterogeneous interface energies, is studied by 3D phase field simulations.

Postdoctoral fellow: Yaochan Zhu.

Local Laser Annealing of Cold Rolled Steel

Heat treatment by annealing can be used to restore the ductility of a cold worked steel. During annealing, the material is held at an elevated temperature for some time and the cold worked material will undergo a sequence of recovery, recrystallization and grain growth, which eventually reduces the hardness and improves ductility. Annealing is, however, usually performed as a bulk process on entire components, for example by placing them in a furnace. Significantly more precision can be achieved if a laser beam is used as the heat source. The accuracy of this method is high enough to allow annealing of regions of millimeter size and in arbitrary patterns. The material’s microstructure can be modified locally, making laser annealing a useful tool in manufacturing of functionally graded steels.

Processes of cold work and annealing, phenomena occurring before and under local laser annealing of cold rolled steel

Project and collaborators

The project combines laser annealing experiments with level set-based simulations of the microstructure evolution and is a collaboration with researchers at École Nationale Supérieure d’Arts et Métiers in Paris.

Microstructure Mechanics in Metal Foams

Metal foams manifest a unique combination of properties, including light weight, high specific stiffness and excellent energy absorption when deformed. These characteristics make metal foams ideally suited to applications in, for example, the automotive and aerospace industries. The strength of individual foam cells is governed by microstructure features like grains, particles, pores and cracks. The evolution of these features – and the interaction between them – has largely been neglected in prior foam studies, which have focused primarily on changes in the cell structure. The present project aims to remedy this deficit by applying synchrotron-based 3D scattering and imaging techniques to metal foams subjected to thermal and mechanical loading, combined with state-of-the art numerical simulations of the microstructure evolution. The goal is to gain a predictive understanding of the mechanical response of metal foams that spans both length and time scales.

Project and collaborators

The 5-year project is jointly funded by the Swedish Science Council and the German BMBF through a Röntgen-Ångström Cluster Grant and involves partners at Lund University (Sweden), Ulm University (Germany) and Malmö University (Sweden) as well as collaboration with the staff at the DESY synchrotron in Hamburg (Germany).

Phase Field Crystal Modeling of Grain Boundary Mechanics – Focus on Abnormal Grain Growth

Abnormal grain growth (AGG) occurs during the heat treatment of polycrystals as certain crystallites undergo an extreme increase in size at the expense of their neighbors. In some cases, AGG is facilitated by the pinning of grain boundaries by second-phase particles, a process called Zener Pinning (ZP). Although of considerable technological importance, there is still no consensus regarding the mechanisms underlying AGG/ZP, and existing data are conflicting. The project will exploit the recent developments in numerical modeling of crystal microstructures offered by the phase field crystal (PFC) method. The PFC method places itself between the scales reached by atomistic simulations and those of standard mesoscale models (such as phase fields) by providing atomistic spatial resolution at time scales that are relevant for AGG/ZP. In addition, as spatial averaging of the atomic scale is avoided, the PFC method naturally provides an accurate description of features such as grain boundaries, crystal orientations, dislocations and other lattice defects. The numerical model will be used to identify how pinning particles influence the selective abnormal growth of individual grains and will yield fundamental insights into the interplay between AGG and ZP that is responsible for dramatic changes in microstructure. The proposed investigations will also shed new light on the individual nature of AGG and ZP, each of which is an important metallurgical phenomenon in its own right.

Project

The 5-year project is funded by the Swedish Science Council.

PhD student: Kevin Hult Blixt, Division of Solid Mechanics, Lund University.

Multiscale Modeling of Recrystallization

Recrystallization (RX) is one of the main mechanisms involved in grain structure evolution. This might be a relatively slow (quasi-static) process or fast-proceeding dynamic process, driven by plastic deformation of the material. In both scenarios, RX proceeds by growth of new grains of low dislocation density which consume the surrounding cold-worked and high-energy microstructure, effectively lowering the level of stored energy in the material.

In this 5-year project, funded by the Swedish Science Council and the Crafoord foundation, a multiscale simulation is established, combining crystal plasticity modeling – to trace evolution of plasticity and texture – with a vertex model, tracing the grain structure evolution. The model is applied to, for example, large scale simulations of sheet metal rolling. Recrystallization, grain growth, particle (Zener) drag on migrating grain boundaries, plastic slip activity and texture evolution are examples of features which are all included in the model.

Grain structure evolution during recrystallization

Project

PhD student: Ylva Mellbin, Division of Solid Mechanics, Lund University.