Releases of radioactive particles to the environment arising from various sources in the nuclear energy and defence sectors have been well documented. The analysis of such particles has been seminal to the development of nuclear forensics, which aims to determine the origin, history, and intended use of unknown nuclear or radioactive materials found outside of regulatory control. Radioactive particles are also of concern for radiological impact assessments and remediation efforts, as particles are expected to show different environmental behaviour to other types of radioactive contamination. The weathering of radioactive particles may also result in the liberation and migration of radionuclides through the geosphere. This study explores aspects of the characterisation and environmental behaviour of uranium particles. The findings from a mock nuclear forensic investigation (Collaborative Materials Exercise 5) are presented, examining particles from two different nuclear fuel pellets. Using NanoSIMS, a high-resolution Secondary Ion Mass Spectrometry technique, nano- and micro-scale variations in the 235U content were observed in individual particles. This analysis also identified features of the source materials that were used in the manufacture of these pellets. Also presented in this thesis is the characterisation of five UÃ¢ÂÂbearing particles isolated from the Ravenglass saltmarsh (UK), using a combination of NanoSIMS and synchrotron radiation analysis. Isotopic, elemental, and speciation assessments of these particles reveal that they are derived from MAGNOX spent fuel from Sellafield. In order to investigate the long-term impact and stability of UÃ¢ÂÂbearing particles in the environment, a series of field-scale lysimeter experiments were performed using two different particle types: metaschoepite and uranium dioxide, in systems representative of the Savannah River Site (USA) where uranium particulates have been released into the environment. These particles were reacted in the lysimeters under vadose zone conditions for one year, providing a realistic assessment of uranium particle stability. Characterisation of these systems was performed by XAS (bulk/micro-focus), autoradiography, and traditional geochemical techniques (e.g. sequential extractions). The UO2 particles were shown to undergo oxidative dissolution, with liberated uranium being retarded by mineral interactions. The metaschoepite particles also showed significant dissolution, with evidence of mobile uranyl phases binding to Fe at reactive mineral surfaces.