Role of Electronic Energy Dissipation on Radiation Damage Production and Evolution in Nuclear Ceramics

The interactions of energetic atomic particles with solids results in inelastic energy loss to electrons and elastic energy loss to atomic nuclei. In nuclear materials, these atomic particles can be fission products, radioactive decay products, transmutation products, or primary knock-on atoms (PKAs) from fast neutrons. In all cases, these atomic particles can be considered ions generated within the nuclear materials. In order to develop fundamental understanding and predictive models of the effects of these different energy dissipation pathways on radiation damage production and evolution in nuclear materials, energetic ions from external sources are often employed to study the response of materials to irradiation under well-controlled conditions. At ion energies relevant to radiation damage from fission fragments, alpha-decay, transmutation products, and PKAs in nuclear ceramics, the separate and coupled effects of these energy dissipation pathways on radiation damage production and evolution in nuclear ceramics are complex and not well understood. Spatial and temporal coupling of these energy dissipation processes along an ion trajectory and within a recoil cascade can increase or decrease damage production. With increasing defect concentrations, the dissipation of electronic energy can enhance damage production or induce transient athermal diffusional processes that affect defect evolution. The work presented here will provide updates and recent results from ion irradiation studies on the synergistic, additive and competitive effects of electronic energy dissipation on damage production and evolution in ceramics relevant to nuclear applications. Defect production and damage accumulation have been investigated as functions of electronic energy loss, S_e, nuclear energy loss, S_n, the ratio of electronic-to-nuclear energy loss, S_e/S_n, and the damage energy, E_D. Experimentally, ion mass and energy are controlled to vary electronic and nuclear energy loss, and large-scale atomistic simulations that combine ionization-induced thermal spikes and atomic collision processes have been used to model these effects. The results demonstrate that electronic energy loss, typical of MeV ions, can lead to competitive damage recovery processes or additive damage production effects in many nuclear-relevant ceramics. An important factor in the effectiveness of damage recovery or production processes for a single ion event is the spatial and temporal coupling of electronic energy dissipation (via electron phonon coupling) and damage energy dissipation (via elastic scattering events) along the ion trajectory. As the radiation damage evolves, electronic energy dissipation density to the lattice can increase, which can further anneal pre-existing defects along the ion trajectory or interact synergistically with the pre-existing defects to enhance damage production. These results have significant implications for interpreting and modeling the radiation response of nuclear ceramics in accelerated testing using MeV ion irradiation.