# Radiation Effects on Uranyl Tetrachloro Coordination Compounds: Impact of Lattice Water

## TL;DR

This study investigates how radiation affects uranyl tetrachloride compounds, focusing on the role of lattice water in their degradation.

## Contribution

The study provides new insights into the formation of radiation-induced radicals and changes in uranium speciation due to water in the crystal lattice.

## Key findings

- Radiation exposure leads to the formation of Cl2–• radicals in uranyl tetrachloride compounds.
- Lattice water promotes the formation of HO2• and alters uranium speciation through radiolysis.
- DFT calculations support the formation of a [UO2Cl3(O2)]3– species in irradiated hydrated compounds.

## Abstract

Nuclear materials, such as uranium-bearing solids, are
exposed
to high levels of ionizing radiation throughout the nuclear fuel cycle;
thus, it is important to develop a molecular-level understanding of
how these materials behave and degrade in the presence of gamma (γ)
irradiation. In the current study, three U(VI) tetrachloride complexes,
M2[UO2Cl4]·xH2O (where M = K+, Rb+, or Cs+ and x = 0 or 2), and their respective chloride
salts were exposed to 1–50 kGy of γ radiation using a 60Co source. Irradiated materials were evaluated by using electron
paramagnetic resonance (EPR) and Raman spectroscopy and were further
explored by using density functional theory (DFT) methods. EPR spectra
of the irradiated materials suggest the formation of a Cl-based radical
for both the alkali salts and the uranyl tetrachloride compounds,
and DFT calculations provide evidence that the Cl2–• radical is formed within these materials.
The presence of water in the K+ and Rb+ compounds
leads to additional spectroscopic signatures that could be traced
back to water radiolysis and the formation of peroxide and superoxide
species. DFT results support the formation of HO2• in the lattice and potentially the formation of a [UO2Cl3(O2)]3– species, highlighting
the impact of water within the hydrated material to alter U(VI) speciation
by radiolysis.

Uranyl tetrachloride solid-state
materials are model systems
to explore their spectroscopic signatures upon ionizing radiation
exposure, aiding in developing an atomistic understanding of the actinide-center
contribution with radiolysis species.