Wigner energy in irradiated graphite: A first-principles study

First-principles calculations were performed to examine the defect-induced energy storage in graphite. The accumulation of energy resulting from inducing defects in graphite is a well-known phenomenon. Given the recent interest in exploiting this process for energy-storing purposes, more careful investigation is necessary. Some of the earliest studies of damaged graphite, and the stored energy associated with that, were motivated by the associated technological issues in nuclear reactor operation. A large number of excited state defects, for example Frenkel pairs, can be generated in graphite through bombardment of high-energy neutrons. The sudden release of this energy (also called Wigner energy) poses a serious concern to the safe operation of nuclear reactors. At the same time, controlled defect generation in graphite using neutron/ion irradiation might represent a potential energy storage mechanism. In recently published papers, the design of an integrated energy system that couples a nuclear reactor with a Wigner effect-based energy storing system was proposed. To accurately estimate the performance that can be achieved in terms of stored energy density through defect generation, density functional theory (DFT) based first-principles calculations were performed. In this work, stored energy accumulation was modeled in two ways - by Frenkel pair accumulation and overlapping collision cascade methods. The former was done with ab initio molecular dynamics (AIMD) simulations, and the latter was done with combined classical molecular dynamics (MD) and AIMD simulations. The agreement between the calculated and experimental results for how stored energy changes with dosage suggests that this model could be useful for the on-going research into damaged graphite as an energy storage medium.