Reduction of Np(v) to Np(iv) using oxalic acid: rethinking decades of nuclear processing conventions

Actinide oxalate precipitation has been ubiquitous in nuclear processing since the 1950s due to its simplicity at laboratory and production scales as both a means of actinide purification and as a key intermediate solid phase towards oxide production. Historical processing information has emphasized the need to reduce actinides, including neptunium (Np), to the tetravalent or trivalent state with chemical reducing agents prior to oxalate precipitation. This work revisits the established method and reveals that oxalic acid has an unexpected role as a reducing agent for Np(v/vi). Also, the study uncovers insights into the direct precipitation of Np(iv) oxalate from neptunyl solutions, as identified by spectroscopic and X-ray diffraction analyses. These findings challenge the long-standing notion that redox adjustment of actinides is necessary for oxalate processing.