Ruthenium in the nuclear fuel cycle and the environment: Behavior, release, and hazard relevance.

Ruthenium isotopes constitute a distinctive class of fission products whose radiological relevance is governed less by their inventory than by chemical speciation and release conditions. Under normal reactor operation, ruthenium is largely retained in metallic form together with other platinum group metals and represents a negligible environmental hazard. However, under high-temperature and oxidizing conditions encountered during severe reactor accidents, fuel reprocessing, or loss-of-cooling scenarios in high-level liquid waste storage systems, ruthenium can be converted into volatile oxides, particularly RuO4, enabling efficient atmospheric transport and short-term radiological exposure. This review provides a hazard-oriented synthesis of current knowledge on ruthenium behavior across the nuclear fuel cycle and the environment, integrating experimental studies, operational experience, and evidence from historical accidents. The physicochemical mechanisms controlling ruthenium oxidation, volatilization, transport, and retention are systematically examined, with emphasis on process stages that dominate the risk of release. Environmental observations demonstrate that ruthenium contamination is highly variable and strongly scenario-dependent, with significant releases occurring only under specific oxidizing conditions, while reduced forms are rapidly immobilized in soils, sediments, and engineered barriers. Overall, the review demonstrates that ruthenium represents a conditional radiological hazard whose environmental and health significance is determined primarily by transient chemical conditions rather than by total radionuclide inventory. These findings support a risk-based approach to ruthenium management focused on redox control and high-efficiency trapping of volatile species in accident-relevant scenarios.