Hello, A yearly allocation for the LCRC cluster has been requested with the following updated information: Submitter/PI: Linyun Liang Project Name: Nuclear_meso Division: NE Project title: Mesoscale model for fission-induced recrystallization in U-7Mo alloy Associated funding: DOE Other Systems: Science: Development of new fuels with low enriched uranium to replace highly enriched uranium fuels used in research nuclear reactors has been extensively researched in order to reduce the nuclear proliferation risks worldwide. U-Mo alloys are selected as a promising candidate fuel because of their excellent irradiation performance. To ensure the safe operation of nuclear fuels, one should well understand the fuel behavior under irradiation. The designed fuel should exhibit mechanical integrity and predictable swelling during irradiation. The fuel swelling increases the risk of a mechanical interaction between fuel and cladding at high fission densities, which can lead to eventual fuel failure. The formation of gas bubbles is the main contribution to fuel swelling. Fission gases, mainly Xe and Kr, resulting from the fission of uranium atoms, tend to be trapped in fabrication pores, on grain boundaries or accumulate on irradiation-induced defects (vacancy clusters, dislocati ons) because of their low solubilities, resulting in the formation of gas bubbles. At high fission densities, the fuel particle exhibits the recrystallization phenomenona, by which the large fuel grains with sizes of several microns are subdivided by small submicron grains, a process also known as grain subdivision. The recrystallized grain structure with increased grain boundary density and reduced gas atom diffusion distance from grain interior to grain boundaries can accelerate the formation and growth of gas bubbles, which can expedite the fuel swelling. Recent experiments shows that the fuel swelling can be different by using different fission rates. Very high fission rate can induce large fuel swelling, which limits the fuel performance at high burnup. Project description: We developed a phase-field model to incorporate the fission rate effect in the nucleation, defect generation, gas atom resolution, and dislocation accumulation processes. We will use our developed model to study the effect of fission rate on the recrystallization and gas swelling in U-7Mo alloy. The fission event introduce fission products in the nuclear fuels, which leads to the accumulation of dislocations. The dislocation density of the fuel increases with the increasing of fission density. The increased dislocation density results in a stored energy in the grains, which provides a driving force for recrystallization. The recrystallized grains start to nucleation and growth. During recrystallization, the irradiation-induced defects such as the Xe gas atoms, vacancies, and SIAs are introduced in the grain structures. These defects evolve with grain structures simultaneously, in which they either interact with the grain boundary or act as nucleation sit es for recrystallization. We will systematically investigate the effect of fission rate, gas atom resolution rate, grain boundary enhancement, and initial grain morphology on the recrystallization kinetics and fuel swelling. These results will be compared to experimental observations. Our study aims to provide insight into understanding of the effect of fission rate on the recrystallization kinetics and fuel swelling in U-Mo materials and provide a way to control the fuel swelling at high burnup. In our multi-phase phase-field model, one phase parameter describes a single grain. For each phase parameter, we have to solve a highly non-linear and coupled partial differential equation, which is called Allen-Cahn equation. For the newly formed recrystallized grain during the irradiation processes, a new phase parameter is assigned for each sub-grain. The Allen-Cahn equation has to be solved for each phase parameter. The fuel particle is around 25.6 µm×25.6 µm. The original average grain size is around 3-15 micrometers, and the newly formed recrystallized grains are around 0.3 micrometers. For the fully recrystallized fuel, the number of recrystallized grains can be thousand. Thus, we have to solve thousands of coupled Allen-Cahn equations simultaneously. In the meaning time, the diffusion and reactions of Xe gas atoms, vacancies, and SIAs are included in the system, which requires to solve the Cahn-Hilliard equations for each time step. They are coupled with the Allen- Cahn equations and can significantly increase the cost of the simulation time. Therefore, to effectively solve this problem, we will need about 200,000 core hours including the 56,000 core-hour to debug and test the algorithm and code and 144,000 core-hours (4 cores/node * 36 nodes * 10 hours of run time * 100 different runs) for recrystallization kinetics calculations. We will use an in-house code to solve the problem. By using the Fusion and Blues computing resources will greatly accelerated the code development and calculations. In conclusion, the total core-hours requested for this project throughout the FY2018 will be 200,000 core-hours. Industry partnership: Project URL: Current FY Hours Used: undetermined amount New FY Requested allocation: 200000 Q1: 50000 Q2: 50000 Q3: 50000 Q4: 50000 Justification: Storage requirements: Thank You, The LCRC Accounts System