Study of Fission and High-burnup Induced Restructuring of Nuclear Fuel Ceramics - Applying Computer Science to Investigate Kinetic Process

Abstract

To improve understanding of radiation damage and recovery process, especially under condition of high energy and high fluence irradiation, is currently studied at the new cross-over project (NXO). Most severe irradiation is realized by fission fragments in nuclear fuel. A guiding experiment is taken from experience in power generating Light Water Reactor (LWR) fuel. At high burnup around 7%FIMA, fuel ceramics do have restructuring of the grain sub-division where diameter of grains change down to 50 to 200 nm. The NXO investigates the process mechanism crossing over research activities of universities, national and private laboratories. Simulation studies are being performed to find principal and triggering processes, using accelerator irradiation and computational calculations. Accelerator irradiation partly succeeded to reproduce the process outside of the fission reactor using simulation material of CeO₂. High resolution TEM observations, comparing microstructure changes of high burnup reactor fuel and the simulated material, shows that principal process is polygonization and indicates that Oxygen defects and planar structures, of different scales, have key role on the kinetics. Basic research works of computing science, including first principle calculations, molecular dynamics, Monte-Carlo, and meso-scale cellular automata modeling are underway.

Framework of computational study is based on understanding of the repeat of passing of fission-fragment tracks, which provide overheating and quenching cycles in time domain. It throws local atoms into higher energy quasi-stable placement and configurations. Extensive experimental observations, previously obtained and presently gained by accelerator irradiations, enable the analyses to focus into key configurations in search of the target process. Stability of planar precipitation of Xenon atomson [111] plane of the fluorite structure were demonstrated by molecular dynamics calculation. Complex kinetic processes, including stability and collective behavior, of interstitial Oxygen atoms are investigated by first principle calculations, molecular dynamics and Monte-Carlo studies.

Statistical irradiation-dynamics, for studying repeated energy-deposit cycles, may enable to draw kinetic phase diagrams. Discussions were made to have statistical counting of possible repeated traces, in relevant narrowed “investigation range” defined in time domain, configuration space and local energy.