[en] This work presents a mechanism of deformation-twin-induced grain boundary failure, and demonstrates the mechanism using molecular dynamics simulations. Deformation twinning is observed as the dominant mechanism during tensile deformation of columnar nanocrystalline body-centered cubic Mo. As a twin approaches a grain boundary, local stress concentration develops due to the incompatible plastic deformations in the two neighboring grains. The magnitude of the stress concentration increases as the twin widens, leading to grain boundary cracking by nucleation and coalescence of microcracks/voids.

[en] U3Si2 and FeCrAl have been proposed as fuel and cladding concepts, respectively, for accident tolerance fuels with higher tolerance to accident scenarios compared to UO2. However, a lot of key physics and material properties regarding their in-pile performance are yet to be explored. To accelerate the understanding and reduce the cost of experimental studies, multiscale modeling and simulation are used to develop physics-based materials models to assist engineering scale fuel performance modeling. In this report, the lower-length-scale efforts in method and material model development supported by the Accident Tolerance Fuel (ATF) high-impact-problem (HIP) under the NEAMS program are summarized. Significant progresses have been made regarding interatomic potential, phase field models for phase decomposition and gas bubble formation, and thermal conductivity for U3Si2 fuel, and precipitation in FeCrAl cladding. The accomplishments are very useful by providing atomistic and mesoscale tools, improving the current understanding, and delivering engineering scale models for these two ATF concepts.

[en] In the first part of this study, the stability of Cu precipitates, up to 2 nm in diameter, in Fe-1at%Cu-1at%Ni-1at%Mn system was evaluated within the framework of phase-field modeling by utilizing a thermodynamic database. The implanted precipitates either in isolated or in clustered arrangements, were unstable and dissolved into the matrix. The dissolution rate decreases with increasing precipitate size; however, it is strongly influenced by the spatial arrangements of the implants and the overall alloy content. In the second part, the precipitation/segregation behavior at a circular dislocation, and square prismatic loops was parametrically studied. While precipitates formed at the dislocation loop, a significant segregation of Cu was observed at prismatic loops with either vacancy or interstitial character. Although, the both types of prismatic loops provide the spatial evolution of the stress-fields with the same absolute magnitude, the vacancy loops appears to be stronger sinks and their sink strength increases with decreasing loop size. The results clearly show the necessity of inclusion of the underlying lattice defects in the microstructure modeling of materials under the irradiation environments.

[en] Zirconium alloys are the major fuel cladding materials in current reactors. The water-side corrosion is a significant degradation mechanism of these alloys. During corrosion, the transport of oxidizing species in zirconium dioxide (ZrO₂) determines the corrosion kinetics. Previously, it has been argued that the outward diffusion of cations is important for forming protective oxides. In this work, the migration of Zr defects in tetragonal ZrO₂ is studied with temperature accelerated dynamics and molecular dynamics simulations. The results show that Zr interstitials have anisotropic diffusion and migrate preferentially along the [001] or c direction in tetragonal ZrO₂. The compressive stresses can increase the Zr interstitial migration barrier significantly. The migration of Zr interstitials at a grain boundary is much slower than in a bulk oxide. The implications of these atomistic simulation results in the Zr corrosion are discussed. (authors)

[en] We analyze the accumulation of fission gas on grain boundaries in a polycrystalline microstructure with a distribution of grain sizes. The diffusion equation is solved throughout the microstructure to evolve the gas concentration in space and time. Grain boundaries are treated as infinite sinks for the gas concentration, and we monitor the cumulative gas inventory on each grain boundary throughout time. We consider two important cases: first, a uniform initial distribution of gas concentration without gas production (correlating with post-irradiation annealing), and second, a constant gas production rate with no initial gas concentration (correlating with in-reactor conditions). The results show that a single-grain-size model, such as the Booth model, over predicts the gas accumulation on grain boundaries compared with a polycrystal with a grain size distribution. Also, a considerable degree of scatter, or variability, exists in the grain boundary gas accumulation when comparing all of the grain boundaries in the microstructure

[en] Pore migration in a temperature gradient (Soret effect) is investigated by a phase-field model coupled with a heat transfer calculation. Pore migration is observed towards the high temperature domain with velocities that agree with analytical solution. Due to the low thermal conductivity of the pores, the temperature gradient across individual pores is increased, which in turn, accelerates the pore migration. In particular, for pores filled with xenon and helium, the pore velocities are increased by a factor of 2.2 and 2.1, respectively. A quantitative equation is then derived to predict the influence of the low thermal conductivity of pores.

[en] A parametric study has been performed that quantifies the effective change in grain boundary Kapitza resistance due to the presence of intergranular bubbles. The steady-state heat conduction equation was solved in three-dimensional space using INL’s MOOSE finite element software, with which spacial mesh adaptivity was used to resolve interfacial widths down to several nanometers while investigating bubble sizes up to a micrometer. Three critical parameters were systematically varied: the intergranular bubble radius, the fractional grain boundary bubble coverage, and the Kapitza resistance of the intact grain boundary. Using the simulation results, a mathematical model dependent on each of these parameters was developed to describe the effective Kapitza resistance. Furthermore, we illustrate how this model can be implemented in a fuel performance code to predict the temperature profile of a cylindrical fuel pellet

[en] A monolithic fuel design based on a U-Mo alloy has been selected as the fuel type for conversion of the United States High-Performance Research Reactors (HPRRs). A 2015 post-irradiation examination (PIE) report showed accelerated swelling in U-10Mo fuels at fission densities much lower than previously observed. This PIE report showed a large amount of compositional banding, or regions of low Mo content adjacent to regions of high Mo content, with low Mo content typically along grain boundaries. Lower Mo content can lead to phase decomposition from the gamma U-Mo body-centered cubic phase to the alpha U phase as well as an earlier onset of recrystallization. Thus, the phenomenon of Mo depletion at grain boundaries is an important factor in the accelerated swelling behavior of U-Mo fuel. However, the physical origin of Mo depletion at grain boundaries is still unclear. In this work, molecular dynamics simulations have been performed to calculate the grain boundary and surface energies of body-centered cubic (bcc) U, bcc Mo and alloys of U-Mo from 600 K to 1200 K. It is observed that the lower grain boundary energy of bcc U, compared to bcc Mo, provides the driving force for Mo depletion at grain boundaries. This driving force diminishes with increasing temperature, but is not eliminated. This information can be utilized as inputs to higher length scale modeling methodologies and provide specification guidance to fabricators.

[en] The thermal behaviors of a porous calcium silicate (PCS) material prepared from coal-bearing strata kaolinite were investigated by thermogravimetry and derivative thermogravimetry (TG–DTG), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy, and Brunauer–Emmett–Teller method. The XRD results showed that the PCS was determined as calcium silicate hydrates (C–S–H) and a small amount of CaCO₃, which transformed to an orderly crystal structure of wollastonite approximately at 700 °C. The TG–DTG results indicated that the dehydration was observed until 300 °C and the dehydroxylation presented at 728 °C. The PCS exhibited a large number of pores with a fibrous flake network structure, which disappeared at 800 °C. The BET data showed that the specific surface area of material decreased as the temperature increased. The high-temperature phase transformation of PCS underwent the following transformation: active calcium silicate → dehydrated calcium silicate → dehydroxylated calcium silicate → wollastonite.

[en] In this study, the intergranular fracture behavior of UO₂ was studied by molecular dynamics simulations using the Basak potential. In addition, the constitutive traction-separation law was derived from atomistic data using the cohesive-zone model. In the simulations a bicrystal model with the (100) symmetric tilt Σ5 grain boundaries was utilized. Uniaxial tension along the grain boundary normal was applied to simulate Mode-I fracture. The fracture was observed to propagate along the grain boundary by micro-pore nucleation and coalescence, giving an overall intergranular fracture behavior. Phase transformations from the Fluorite to the Rutile and Scrutinyite phases were identified at the propagating crack tips. These new phases are metastable and they transformed back to the Fluorite phase at the wake of crack tips as the local stress concentration was relieved by complete cracking. Such transient behavior observed at atomistic scale was found to substantially increase the energy release rate for fracture. Insertion of Xe gas into the initial notch showed minor effect on the overall fracture behavior. (authors)