Created. - JB On Fri, Sep 4, 2015 at 12:16 PM, Bair, Raymond A. <[email protected]> wrote:

PPS. 750K is way too much to use before 9/30.

Please let t hem know that they will need to submit another request for FY16.

Lets start them at 100K

Ray

On 9/4/15, 12:10 PM, "[email protected] on behalf of Bair, Raymond A." <[email protected] on behalf of [email protected]> wrote:

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P.S.

Category: Physical Sciences

Subcategory: Materials Science

POC: Elia Merzari

On 9/4/15, 12:03 PM, "[email protected] on behalf of Bair, Raymond A." <[email protected] on behalf of [email protected]> wrote:

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This looks fine to me, to start on Blues.

John low - are you part of this project effort?

Ray

On 9/4/15, 8:37 AM, "[email protected] on behalf of [email protected]" <[email protected] on behalf of [email protected]> wrote:

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Hello,

A new project on the LCRC cluster has been requested. Please forward the information on to the LCRC Allocation sub-committee.

Applicant's name: Zhigang Mei Applicant's institution: ANL Applicant's division: NE Project Name: Bubble_superlattice Project title: Atomistic Simulation of Fission Gas Bubble Superlattice in Irradiated Materials Associated funding: NNSA Other Systems: Carbon at CNM Science: The accumulation of gaseous species in nuclear fuel leads to deleterious microstructural changes that are of utmost concern to both fission reactor applications [Trinkaus, J. Nucl. Mater. 323 (2003) 229]. Inert gas elements – such as helium, xenon and krypton - are generated due to fission reactions in nuclear fuel. In general, the precipitation of gaseous species into gas-filled bubbles leads to volumetric swelling in fuels. According to conventional nucleation theory, in the early stages of irradiation, gas atoms diffuse throughout the crystal and begin to form clusters due to their insolubility in the solid. This clustering can initiate either by chance encounters of the wandering gas atoms, or by gas-atom trapping at microstructural segregation sites such as grain boundaries (GBs) [Rest, J. Nucl. Mater. 207 (1993) 192]. The subsequent growth or shrinkage of a gas bubble is governed by the fluxes of vacancies, interstitials, and gas atoms to or away from the bubble surface. If gas bubbles are located on sinks (such as GBs), they may grow indefinitely to the point of interconnection, thus lead to rapid fission gas release to the exterior of fuel. It is found that the formation of micro-sized fission-gas bubbles at GBs is the major reason leading to the fuel swellings at high fission density, while nano-sized intragranular gas bubbles have negligible contribution to the overall fuel swelling. Therefore a major task for fuel development is to control the fission gas bubble microstructures and to retain more fission-gas bubble inside fuel grains at high fission density. Some of the proposed approaches include (i) Enhancing the solubility of fission gas in fuel by modifying the fuel composition and (ii) Increasing the grain size in the fuel by heat treatment. So far, these approaches have not been well tested and their effects on the nucleation and growth of fission gas bubbles are not clear. Recent experiments by Gan et al show that fission gas bubbles form fcc superlattice in the irradiated bcc U-7Mo dispersion fuel [Miller et al., J. Nucl. Mater. 458 (2015) 115]. Typical bubble size and spacing in these bubble superlattices are in the range of 2.75-4.5 nm and 10-12 nm, respectively. Although ordered bubble superlattice has also been seen in ion-irradiated materials, common observations show that superlattice will primarily form in the same structure and crystal orientation as the host material. Meanwhile the formation of bubble superlattice is counterintuitive to the fact that fission gas is randomly generated in lattice and gas bubbles should be nucleated in a random manner initially. Our own TEM characterization of Xe irradiated U-Mo however shows that gas bubbles were randomly distributed in grain instead of forming ordered superlattice. The goal of this proposal is to investigate the potential mechanisms of the formation and collapse of the Xe bubble superlattice in irradiated U-Mo alloys using atomistic simulations. We hope this study will provide a microscopic model of the nucleation and growth of gas bubble inside fuel grains and therefore better control of gas bubble microstructure under radiation.

Project description: We will use molecular dynamics (MD) to investigate the potential mechanism of the formation of Xe bubbles superlattice in U-Mo alloys. The obtained simulation results will be compared to the microstructure characterized by TEM in irradiated U-7Mo fuels. A ternary U-Mo-Xe potential has been previously developed by Smirnova based on embedded atom method (EAM) [Smirnova et al. Modelling Simul. Mater. Sci. Eng. 21 (2013) 035011]. However our test shows that this potential cannot predict the accurate formation energy of Xe defects in U-Mo. Therefore, we will use the so-called force matching method as implemented in potfit [Brommer, Modelling Simul Mater Sci Eng 15 (2007) 295] to develop a new potential for U-Mo-Xe based modified EAM (MEAM). To build a potential suitable for the simulation of a complicated ternary system, we have to prepare an extensive set of reference configurations representing all kinds of possible structures. Each configuration is cor responding to one of the possible states of the U-Mo-Xe system. VASP will be used to calculate energy, force and stress of the reference configurations with atoms up to 150. The total computational time using DFT will be 150000 CPU-hours. To model different configurations of Xe gas bubbles in U-Mo alloys, a 120×120×120 cell (40nm×40nm×40nm) based on bcc structure will be constructed. About 3.5 million atoms will be introduced in the simulation box, which allows adding up to 110 gas bubbles with size around 2.5 nm. We will compare the energetics of different bubble superlattices, such as cubic, fcc, bcc, hcp and disordered bubbles configurations. We will also investigate the effect of Mo concentration on the retention of Xe atoms in the lattice. Two different alloys, i.e., U-7wt%Mo and U-10wt%Mo, will be studied and be compared to pure bcc Mo and U cases. Meanwhile, temperature plays an important role in governing the fluxes of vacancies, interstitials and gas atoms to or away from the bubble surface. The interested temperature range is between 300 K and 1000 K. In experiment, it was observed that the gas bubble superlattice collapses at high fission density. A major effect of fission on the microstructure is creating a large amount of Frenkel pair defects in the lattice. Vacancy can provide efficient diffusion channels for Xe atoms. In this work, we will investigate the effect of five parameters (Xe concentration, Xe bubble configuration, Mo concentration, temperature, and Frenkel pair concentration) on the stability of gas bubble superlattices introduced in U-xMo alloys. By systematic analysis of the bubble superlattice’s microstructure and their energetics, we will be able to correlate the key parameters to the formation of bubble superlattice, and provide a potential mechanism of its formation in U-Mo. Combining all the parameters, we will expect about 50 production jobs. Our initial test shows that a single MD simulation for 1 nano-second takes more than 12000 CPU-hours (256 CPU×24 hours). We therefore request the total computational time of 750000 CPU-hours.

Industry partnership: Project URL: Requested allocation: 750000 Q1: 0 Q2: 0 Q3: 0 Q4: 750000 Justification: The main calculations are performed by high efficient molecular dynamics code LAMMPS. The parallel efficiency test for this code shows that it can be well scaled to up to tens of thousands of CPUs. Other DFT calculations will be performed by VASP, which is one of the widely used DFT code. Storage requirements: 1 TB

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