Non-Equilibrium Actinide Radiation Chemistry and the Nuclear Fuel Cycle

Abstract
Actinides are inherently unstable elements that frequently coexist with other radioisotopes, generating intense ionizing radiation fields that drive the formation of non‑equilibrium oxidation states. These transient species exert a profound mechanistic influence on the radiation response of actinide‑containing systems due to their unique redox chemistry. Despite their importance, they remain poorly understood, yet such insight is essential for advancing actinide science and accurately predicting radiation‑driven behavior. Actinide separations—critical for nuclear energy technologies, strategic deterrence, space exploration, and nuclear medicine—depend on precise control of actinide oxidation states to recover targeted elements from complex matrices such as used nuclear fuel. However, during these processes, actinides, their coordination complexes, and the separation media are all exposed to intense, multicomponent (alpha, beta, gamma, etc.) radiation fields that can alter process efficiency, selectivity, and chemical stability. Understanding, controlling, and mitigating radiation‑induced reactions is therefore key to innovating and optimizing next‑generation separation technologies. This seminar will provide an overview of the nuclear fuel cycle and non‑equilibrium actinide radiation chemistry in the context of recovering actinides from used nuclear fuel, with a particular emphasis on direct‑dissolution–based reprocessing strategies. We will explore time‑resolved electron pulse radiolysis and alphaand gamma dose accumulation studies, integrated with multiscale computational modeling, to elucidate the molecular‑level roles of radiation‑driven, non‑equilibrium actinide species in process performance and in the radiolytic stability of organic ligands used for actinide recovery. These insights offer new pathways for designing advanced separation methods and next‑generation solvent systems, with broad implications for the future of the nuclear fuel cycle. 

Biography
Distinguished Staff Scientist Gregory Holmbeck (formerly Horne, ORCID: 0000-0003-0596-0660) is the Director of the Idaho National Laboratory (INL) Center for Radiation Chemistry Research (CR2).

Holmbeck earned both his undergraduate and graduate degrees in chemistry from The University of Manchester in 2007 and 2015, respectively. He completed his PhD in experimental and computational PUREX (Plutonium Uranium Reduction Extraction) process radiation chemistry under the supervision of Professor Simon Pimblott. During his doctoral studies, Holmbeck conducted pivotal experiments involving plutonium and americium irradiations at the UK National Nuclear Laboratory on the Sellafield Site, and engaged in time-resolved electron pulse radiolysis experiments with the ELYSE group at the Université Paris-Sud Laboratoire de Chimie Physique in France.

In 2015, following the completion of his PhD, Holmbeck relocated to the United States to undertake a joint postdoctoral research fellowship position at California State University Long Beach and the University of Notre Dame Radiation Research Laboratory working with Professors Stephen Mezyk and Jay LaVerne, respectively. This joint appointment also granted him visiting scientist status at Brookhaven National Laboratory, where he continued his pulse radiolysis studies with the Laser Electron Accelerator Facility (LEAF) group. Holmbeck was awarded a Russell L. Heath Distinguished Postdoctoral Research Associate position at INL in 2017. In 2018, he transitioned to his current role as Director of the CR2. Under his leadership, his center has emerged as a multimillion-dollar international leader in radiation chemistry, with a strong focus on addressing nuclear fuel cycle challenges.

Holmbeck's research centers on elucidating the radiation-induced chemistry of the actinide elements and used nuclear fuel reprocessing strategies, significantly advancing these fields and contributing to the development of innovative solutions for nuclear energy sustainability.