Motoyasu Kinoshita

Transmission electron microscopy observation on irradiation-induced microstructural evolution in high burn-up UO2 disk fuel

In order to identify the conditions of the rim structure formation as a function of burn-up and temperature, and to clarify the formation mechanism of this restructuring, UO2 fuel disks were irradiated at four thermal conditions, between 400 and 1300 °C, and at four different burn-ups, between 36 and 96 MWd/kgU, without external mechanical constraint. The microstructural evolutions as a function of the irradiation parameters are observed by high resolution scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The SEM observations reveal the transition from original to sub-divided grains of rim structure and make clear that the burn-up threshold is between 55 and 82 MWd/kgU. The temperature threshold of this restructuring could be 1100±100 °C. Moreover, polyhedral sub-divided-grains with size ranging between 0.5 and 2 μm, not only rounded grains in the size range 150–350 nm, are clearly observed. These configurations are explained by assuming that the grain sub-divisions occurred homogeneously within the original polyhedral grains, while the existence of rounded grains might be due to free surface effects. TEM observations of re-structured samples show that most of sub-grain boundaries are low angle and are heavily decorated by fission gas bubbles in the range 3.5–8 nm. In the non-restructured samples, dislocations and small precipitates are present, and many of the bubbles form “strings” along dislocation lines. In specimens irradiated at high temperature, many dislocations seem to be anchored by fission product precipitates. These results suggest that the formation mechanism of the restructuring is based on polygonization, and the precipitates could have some “pinning effect” on dislocations and defect clusters.

An electron microscopy study of the RIM structure of a UO2 fuel with a high burnup of 7.9% FIMA

Transmission and high resolution scanning electron microscopy were used to analyze the microstructure of the periphery of a UO2 pellet irradiated to a cross-section average burnup of 7.9% FIMA, with a period of increased temperature at about half the final burnup. Local burnups at the pellet surface reached nearly 23% FIMA. In this rim region the original grains of about 10 μm diameter were subdivided into about 104 subgrains of 0.15 to 0.30 μm diameter. The spread in subgrain orientation was small ( 1 mm. The size of the subgrains was found to be largely independent of depth.

Concerning the microstructure changes that occur at the surface of UO2 pellets on irradiation to high burnup

Abstract It is shown that in addition to the precipitation of small gas filled pores, there is a pronounced reduction in the grain size at the surface of UO2 fuel at high burnup. These microstructure changes were first observed when the local burnup exceeded 70–80 MWd/kgM. Generally, the change in microstructure does not penetrate more than 200 μm. However, in a HWR fuel irradiated to 75 MWd/kgM a large part of the pellet cross section was found to have been affected. Temperature predictions for this fuel suggests that it is the restructuring accompanying thermally activated fission gas release at 1100 to 1200°C that limited the distance over which the microstructure changes occur. Apparently, the formation and fission of Pu is not directly responsible for the change in fuel microstructure. The porosity evidently contains part of the fission gas that is lost from the UO2 lattice in the region where the microstructure changes take place.

Polygonization and high burnup structure in nuclear fuels

The rim effect, i.e., the formation of the high burnup structure typical for the outer zones of LWR UO2 fuel pellets at extended burnup, is presently being studied in many laboratories. It is caused by a subdivision of the original as-sintered grains of the UO2 pellets into 104 to 105 new small subgrains. The aim of the activities presently ongoing in different laboratories is to define the conditions (burnup, temperature, pressure, type of fuel, grain size, etc.) for the formation of the rim structure and to understand the mechanism of this subgrain division, or polygonization. The aim of the present paper is to discuss the existing knowledge on such polygonization processes in other materials, to confront this knowledge with the observations on UO2 fuel and to discuss the attempts to model the rim structure formation.

Interplay of defect cluster and the stability of xenon in uranium dioxide from density functional calculations

Self-defect clusters in bulk matrix might affect the thermodynamic behavior of fission gases in nuclear fuel such as uranium dioxide. With first-principles LSDA+U calculations and taking xenon as a prototype, we find that the influence of oxygen defect clusters on the thermodynamics of gas atoms is prominent, which increases the solution energy of xenon by a magnitude of 0.5 eV, about 43% of the energy difference between the two lowest lying states at 700 K. Calculation also reveals a thermodynamic competition between the uranium vacancy and tri-vacancy sites to incorporate xenon in hyper-stoichiometric regime at high temperatures. The results show that in hypo-stoichiometric regime neutral tri-vacancy sites are the most favored position for diluted xenon gas, whereas in hyper-stoichiometric condition they prefer to uranium vacancies even after taking oxygen self-defect clusters into account at low temperatures, which not only confirms previous studies but also extends the conclusion to more realistic fuel operating conditions. The observation that gas atoms are ionized to a charge state of Xe+ when at a uranium vacancy site due to strong Madelung potential implies that one can control temperature to tune the preferred site of gas atoms and then the bubble growth rate. A solution to the notorious meta-stable states difficulty that frequently encountered in DFT+U applications, namely, the quasi-annealing procedure, is also discussed.

Temperature and fission rate effects on the rim structure formation in a UO2 fuel with a burnup of 7.9% FIMA

Abstract A BWR design UO2 fuel irradiated to a burnup of 7.9% FIMA was selected for a careful calculational and experimental analysis because the rod experienced an unusual power history: it had two high power periods at 1.7% FIMA and between 4 and 5% FIMA causing increased fuel temperatures and thus increased gas release and damage recovery. As a consequence, two parameters generally considered to be important for grain subdivision (rim structure formation) were locally different from normal fuel, i.e. fission gas inventory and extent of radiation damage. Histories of temperature, fission rate and fission gas release were calculated at different radial positions. Microstructure observations (TEM, SEM) revealed the typical high burnup grain subdivision process (polygonization) which extended to a maximum of 1.65 mm (r/r0 = 0.73) from the pellet surface inwards. For this radial position, the calculations yielded a local temperature of 1200°C and predicted that more than half of the fission gas was released during the second high power period for this radial position. The results give thus information on the importance of the fission gas inventory for the burnup threshold of restructuring.

Stability mechanism of cuboctahedral clusters inUO2+x: First-principles calculations

The stability mechanism of cuboctahedral clusters in nonstoichiometric uranium dioxide is investigated by first-principles LSDA+U method. Calculations reveal that the structural stability is inherited from U6O12 molecular cluster whereas the energy gain through occupying its center with an additional oxygen makes the cluster win out by competition with point oxygen interstitials. Local displacement of the center oxygen along direction also leads the cluster 8-folded degeneracy and increases relatively the concentration at finite temperatures. But totally, elevation of temperature, i.e., the effect of entropy, favors point interstitial over cuboctahedral clusters.

Constituent Migration Model for U-Pu-Zr Metallic Fast Reactor Fuel

A model for constituent migration behavior in U-Pu-Zr metallic fast reactor fuel is proposed. It is based on diffusion equations for the ternary system under a radial temperature gradient, and it takes into account the alloy phase decomposition, assuming a quasi-binary U-Zr phase diagram with a constant plutonium content. Parametric simulations of Experimental Breeder Reactor II irradiation data with appropriate transport properties of the alloy system showed that the model can predict the experimentally observed radial three-zone structure and zirconium and uranium redistribution, although the predicted radial location of zirconium-depleted middle zone disagreed with the experimental result. Accumulation of basic experimental data on transport properties and a ternary phase diagram of the system are needed for a better understanding of the behavior.

Ab initio investigation on oxygen defect clusters in UO2+x

Oxygen defect clustering in uranium dioxide had been indicated in powder neutron diffraction measurements, and an empirical clustering mechanism had been proposed to explain the data. However, using first-principles LSDA+U calculations, we find that this empirical model, in fact, cannot work. A more physically reasonable model is proposed based on a thermodynamical competition between point defects and cuboctahedral clusters. This mechanism interprets the puzzled origin of the observed asymmetric interstitial O′ and O″ naturally. It also gives a good and consistent agreement with all available experimental data, except the high occupation of the O″ site.

Towards the mathematical model of rim structure formation

The high burnup LWR UO2 fuels show a notable micro-structural change around the pellet outer zone which is called the rim structure. It is observed at temperatures as low as 400°C so that fission track and cascade mixing could be the key mechanism. SEM observation revealed that the structure primarily appears on free surfaces of UO2, indicating that strong sink for point defects may play a big role. And as generic observations, increase of lattice parameter indicates extensive amounts of vacancies are stored in high burnup fuel, which may induce the restructuring interacting with dislocations of high density at high burnup. Considering these observations a model of reaction-diffusion process of defects with irradiation induced transport is proposed. The equations are investigated numerically. The model indicates that an instability starts when the dislocation network starts intensive interaction with vacancy flux which is modified by interstitial diffusion between spatial segments. It appeared to be similar to the Turing type instability which indicates that the rim structure formation is one kind of the self-organizing processes of open reaction-diffusion systems.