107HIGHLIGHTS 2020

STABLE PENTAVALENT PLUTONIUM SOLID COMPOUND DISCOVERED

A novel meta-stable pentavalent plutonium solid phase has been discovered on the pathway from aqueous Pu(VI) to PuO2 nanoparticles. For the first time, HERFD experiments at Pu M4 and L3 edges combined with theoretical calculations unambiguously determine the oxidation state and the local structure of this plutonium phase.

Plutonium (Pu) plays a prominent role in nuclear energy production, but is extremely radiotoxic. Countries across the globe are making significant efforts to improve the safety of nuclear waste storage in order to prevent release of radioactive nuclides into the environment. Plutonium can be transported by groundwater from contaminated sites for kilometres in the form of colloids, which are formed by interaction with clay, iron oxides or natural organic matter [1,2]. In the near- field conditions of geological repositories of spent nuclear fuel and other radioactive waste, the formation of intrinsic PuO2 colloids is a key scenario [3]. Therefore, the characterisation of intrinsic colloidal nanoparticles (NPs) in aqueous solution has received much attention. The most debated question is the structural nature of NPs (crystalline versus amorphous), as well as the presence of the Pu(V) and other oxidation states in small NPs (< 3nm) [4].

The oxidation state of Pu determines its chemical behaviour and speciation. Four oxidation states (from III to VI) may co-exist under environmental conditions, while (VII) and even (VIII) states are proposed to be stable under highly alkaline oxidative conditions. Pu oxidation states have been determined using the Pu L3 edge; however, high-energy resolution fluorescence detection (HERFD) experiments at the M4 edge are much more informative on the oxidation state and electronic structure. Figure 89a shows experimental HERFD data recorded at beamline ID26 at the Pu M4 edge for Pu references and PuO2 NPs. HERFD data were also recorded at different stages during the synthesis of PuO2 NPs from the aqueous Pu(VI) precursor.

The kinetics of the Pu(VI) to PuO2 transformation reveal a two-step process: during the first minutes, the formation of an intermediate Pu phase consisting of a yellow sludge (Figure 89) is observed. Later, during the formation of PuO2 NPs, this intermediate phase dissolves and a different equilibrium phase, named final phase , is formed [5]. The Pu M4 HERFD spectrum recorded at the intermediate stage of the reaction is shown by the blue curve in Figure 89a. The spectrum clearly indicates the presence of the Pu(V) oxidation state. This is supported by good correspondence between energy and relative intensities of the main features of the Pu M4 edge spectrum for KPuO2CO3(s) (green curve) and the

Pu intermediate phase (blue curve). The HERFD spectrum of the final product of the reaction, formed after three weeks of the precipitated reaction, shows a profile identical to the one detected for a PuO2 single crystal, confirming that the reaction terminates with the formation of PuO2 NPs with cubic structure and with Pu(IV) oxidation state.

While Pu(V) solid state complexes are always viewed as exotic compounds, a thermodynamically meta-stable Pu(V) solid phase is formed during the reductive precipitation of PuO2 NPs from the Pu(VI) precursor at pH 11.

Fig. 89: a) Experimental HERFD data at the Pu M4 edge from two plutonium phases obtained during the synthesis of PuO2 nanoparticles (NPs) from a Pu(VI) precursor at pH 11. Blue curve: spectrum of the intermediate Pu(V). b) Experimental HERFD spectra of PuO2 and KPuO2CO3(s) compared with the results of Anderson Impurity Model calculations.