Behavior of radionuclides in high temperature combustion processes: A radiological hazard?

High temperature combustion processes, such as combustion of fossil fuels or biomass for energy production, tobacco smoking, organic waste incineration, or wildfire events in forests, are governed by the same physical, chemical, and thermal mechanisms that control the partitioning, enrichment, migration, volatilization, and condensation of radionuclides, either anthropogenic or naturally occurring, which may pose a significant radiological hazard to the environment and to human health. A review was carried out up to the end of 2025, using a PRISMA-style workflow. In this review, those combustion scenarios are presented within a common mechanistic framework to better understand the behavior of primordial and anthropogenic radionuclides under these conditions. Modeling and empirical results under controlled conditions show that radionuclide volatility drives the different activity concentrations when comparing heavy bottom ashes, lighter fly ashes, and flue gases (smoke). Enrichment factors can vary by orders of magnitude depending on temperature profiles, redox conditions, chlorine, sulfur and calcium content, and particle size distributions. Radioisotopes of volatile elements and compounds, such as C137s under chlorine-rich atmosphere conditions, or P210o, or P210b exhibit preferential condensation on fine particulate matter (< 2.5 μ m), whereas refractory elements and compounds, including isotopes of uranium, thorium, plutonium, or americium, concentrate in heavier fractions. In particular, C137s, P210o and P210b can show enrichment factors of up to 1000 times in fine particles due to vapor and condensation mechanisms, following an inverse correlation between radioactivity and particle diameter. This fact is relevant as the inhalation of submicron particles (< 1 μ m) may constitute one of the primary exposure pathways, and thus a radiological hazard, with combined-exposure implications with other co-contaminants, like heavy metals or organic toxics, which are often present in the gases of combustion processes. A structured synthesis was developed, highlighting where this framework is applicable, and providing advice on future research on radionuclide speciation in high-temperature, ultrafine-particle characterization, weathering of aged ash, small and dispersed combustion sources, and the effect of modern ultra-low-emission retrofits and biomass co-firing on radionuclide partitioning.