Intergranular Gas Bubbles Research Articles

Effect of grain size on gas bubble evolution in nuclear fuel: Phase-field investigations

Numerous irradiation-induced gas bubbles are created in the nuclear fuel during irradiation, leading to the change of microstructure and the degradation of mechanical and thermal properties. The grain size of fuel is one of the important factors affecting bubble evolution. In current study, we first predict the thermodynamic behaviors of point defects as well as the interplay between vacancy and gas atom in both UO2 and U3Si2 according to ab initio approach. Then, we establish the irradiation-induced bubble phase-field model to investigate the formation and evolution of intra- and inter-granular gas bubbles. The effects of fission rate and temperature on the evolutions of bubble morphologies in UO2 and U3Si2 have been revealed. Especially, a comparison of porosities under different grain sizes is examined and analyzed. To understand the thermal conductivity as functions of grain size and porosity, the heat transfer capability of U3Si2 is evaluated.

Effects of porous fuel structure on the irradiation-induced thermo-mechanical coupling behavior in monolithic fuel plates

A UMo/Al monolithic fuel plate is composed of UMo fuel meat and Al cladding. These fuel plates with a high uranium density have a promising prospect to be used in advanced research and test reactors. Under the neutron irradiation environments, nuclear fissions occur to result in heat generation in the fuel meat; accumulation of fission-induced solid and gaseous products will lead to irradiation swelling; irradiation creep of UMo will affect the mechanical interactions between the fuel meat and the cladding. Especially, intragranular and intergranular gas bubbles will make the fuel meat evolve into a porous structure, and the fuel porosity and pore pressure will change continuously with the irradiation time. As a result, a complex irradiation-induced multi-scale thermo-mechanical coupling behavior needs to be simulated for optimal design and safety evaluation of UMo/Al monolithic fuel plates. In this study, a theoretical model for fuel porosity evolution is developed, based on a fission gas swelling model with the grain recrystallization, the resolution of intergranular gas atoms and the dependence of hydrostatic pressure involved. Moreover, the computation method of microscopic interfacial normal stress is established, with consideration of fuel porosity and pore pressure. The evolution model of fuel porosity is introduced into three-dimensional finite element simulation of the irradiation-induced multi-scale thermo-mechanical coupling behavior, which correlates the current thermal conductivity of UMo fuel with the fuel temperature and porosity. The distribution and evolution disciplines of thermo-mechanical variables in the fuel plate are obtained at different irradiation time. In addition, the effects of fuel porosity on the temperature, the main deformation and microscopic interfacial normal stresses are investigated. Simultaneously, the effects of surface heat transfer coefficient on the thermo-mechanical behavior are analyzed. The research results indicate that the porous fuel structure and fission-gas induced pore pressure become the main fracture mechanisms of monolithic fuel plates.

Fission gas bubbles and recrystallization-induced degradation of the effective thermal conductivity in U-7Mo fuels

We have developed a mesoscale model to calculate the degradation of the effective thermal conductivity in irradiated U-Mo alloys caused by the fission-induced gas bubbles and recrystallization. The phase-field approach is employed to generate the grain microstructures of U-7Mo fuels with intra- and inter-granular gas bubbles. Based on the phase-field microstructures, the thermal conductivities of U-7Mo as a function of the fission density can be predicted by the developed mesoscale model. The predicted values of effective thermal conductivities are consistent with available experimental data although the grain structure and the distribution of gas bubbles were generated from the phase-field simulations that may not exactly correspond to experimental microstructures. Results show that the effective thermal conductivity decreases rapidly with recrystallization compared to the one prior to recrystallization, which can be attributed to the sudden increase of grain boundary densities and corresponding intergranular gas bubbles at high fission densities. Smaller grain size fuel structure has a lower thermal conductivity at the same fission density due to the increased grain boundary density. The current study can provide a better understanding of the fission-induced degradation mechanism of the thermal conductivity in U-Mo fuels.

Three-dimensional phase-field simulations of intragranular gas bubble evolution in irradiated U-Mo fuel

The evolution of fission gas bubbles in irradiated materials plays a critical role in the microstructural processes that leads to dimensional changes of U-Mo alloy fuels, e.g., fuel swelling. Although the intergranular bubbles-induced fuel swelling has been long-discussed for U-Mo fuel, there are very few computational studies of the formation of intragranular gas bubbles and its impact on fuel swelling. To this end, we develop a three-dimensional phase-field model to investigate the evolution of intragranular gas bubbles in U-Mo fuel. Fission induced defect formation and annihilation processes, such as vacancy-interstitial recombination, fission gas atom resolution, and interactions with dislocations and grain boundaries are incorporated in the model. Simulations show that the intragranular gas bubbles can be stabilized to certain sizes due to the balance between the generation and annihilation of defects. The intragranular gas bubbles induced fuel swelling is predicted to be comparable to experimental measurements. The effects of the irradiation and fuel fabrication conditions (i.e., fission rate, fuel grain size, and mechanical work induced deformation) on the bubble evolution and the resultant swelling are investigated. The current simulations provide a better understanding of intragranular gas bubble-induced swelling and a solid foundation for the future study of the nucleation and growth of intergranular gas bubbles and recrystallization in U-Mo fuel.

Fission-induced recrystallization effect on intergranular bubble-driven swelling in U-Mo fuel

We have developed a mesoscale phase-field model for studying the effect of recrystallization on gas-bubble-driven swelling in irradiated U-Mo alloy fuel. The model can simulate the microstructural evolution of the intergranular gas bubbles on the grain boundaries as well as the recrystallization process. Our simulation results show that the intergranular gas-bubble-induced fuel swelling exhibits two stages: slow swelling kinetics before recrystallization and rapid swelling kinetics with recrystallization. We observe that the recrystallization can significantly expedite the formation and growth of gas bubbles at high fission densities. The reason is that the recrystallization process increases the nucleation probability of gas bubbles and reduces the diffusion time of fission gases from grain interior to grain boundaries by increasing the grain boundary area and decreasing the diffusion distance. The simulated gas bubble shape, size distribution, and density on the grain boundaries are consistent with experimental measurements. We investigate the effect of the recrystallization on the gas-bubble-driven fuel swelling in U-Mo through varying the initial grain size and grain aspect ratio. We conclude that the initial microstructure of fuel can be used to effectively control the recrystallization and therefore reduce the swelling in U-Mo fuel.

Assessment of effective thermal conductivity in U–Mo metallic fuels with distributed gas bubbles

This work presents a numerical method to assess the relative impact of various microstructural features including grain sizes, nanometer scale intragranular gas bubbles, and larger intergranular gas bubbles in irradiated U–Mo metallic fuels on the effective thermal conductivity. A phase-field model was employed to construct a three-dimensional polycrystalline U–Mo fuel alloy with a given crystal morphology and gas bubble microstructures. An effective thermal conductivity “concept” was taken to capture the effect of polycrystalline structures and gas bubble microstructures with significant size differences on the thermal conductivity. The thermal conductivity of inhomogeneous materials was calculated by solving the heat transport equation. The obtained results are in reasonably good agreement with experimental measurements made on irradiated U–Mo fuel samples containing similar microstructural features. The developed method can be used to predict the thermal conductivity degradation in operating nuclear fuels if the evolution of microstructures is known during operation of the fuel.

The development of a mechanistic code on fission product behavior in the polycrystalline UO 2 fuel

The current status of a mechanistic code (RTOP) on fission product behavior in the polycrystalline UO 2 fuel is described. Outline of the code and implemented physical models is presented. The general approach to the code validation is discussed. It is exemplified by the results of validation of the models of oxidation and grain growth. The different models of intragranular and intergranular gas bubbles behavior have been tested and the sensitivity of the code in the framework of these models has been analyzed. An analysis of available models of the resolution of grain face bubbles is also presented.