Size and structure of hexanuclear plutonium oxo-hydroxo clusters in aqueous solution from synchrotron analysis.

Thomas Dumas,Denis Menut,Sandrine Dourdain,Christelle Tamain,Cyril Micheau,Matthieu Virot,Olivier Diat

doi:10.1107/s1600577521012005

Abstract

The size and shape of a water-soluble hexanuclear plutonium cluster were probed by combining synchrotron small-angle X-ray scattering (SAXS) and extended X-ray absorption fine structure (EXAFS). A specific setup coupling both techniques and dedicated to radioactive samples on the MARS beamline endstation at Synchrotron SOLEIL is described. The plutonium hexanuclear cores are well stabilized by the 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid ligands and this allows a good evaluation of the setup to probe the very small plutonium core. The results show that, in spite of the constrained conditions required to avoid any risk of sample dispersion, the flux and the sample environment are optimized to obtain a very good signal-to-noise ratio, allowing the detection of small plutonium aggregates in an aqueous phase. The structure of the well defined hexanuclear cluster has been confirmed by EXAFS measurements in solution and correlated with SAXS data processing and modelling. An iterative comparison of classical fit models (Guinier or sphere form factor) with the experimental results allowed a better interpretation of the SAXS signal that will be relevant for future work under environmentally relevant conditions.

Highlights

Because plutonium is highly radiotoxic, it represents one of the major long-term risks in nuclear waste management
The intermediate clusters formed through these reactions can further condense to yield colloidal plutonium nanoparticles which might enhance its mobility (Dalodiere et al, 2017; Gerber et al, 2020; Micheau et al, 2020; Neck et al, 2007; Walther et al, 2007; Walther & Denecke, 2013)
Both diagrams exhibit a well defined peak at 4 nmÀ1 which is attributed to the scattering of the four Kapton layers on the sample cell. This peak is still visible after subtraction of the empty cell, as can be seen in Fig. 3(b) where the pure solvent and the sample are compared after normalization of the data according to the sample thickness, transmission, acquisition time and scattering intensity of polyethylene (PE) used as reference

Summary

Introduction

Because plutonium is highly radiotoxic, it represents one of the major long-term risks in nuclear waste management. To predict and control the behaviour of plutonium under various environmental conditions, thermodynamic models have been developed to describe plutonium solubility and surface interactions (Altmaier et al, 2013; Geckeis et al, 2013; Kersting et al, 1999). New experimental inputs are still necessary in order to improve the reliability of these models. Any additional fundamental knowledge of the complex plutonium chemistry at the molecular scale is highly desirable for the safety of long-term nuclear disposal. In this scope, and within the large variety of plutonium chemical and redox states, hydrolyzed forms of tetravalent plutonium are crucial objects to consider. The addition of complexing molecules was considered to limit the growth of these colloids, leading to smaller polynuclear species and

Methods

Results

Conclusion