A key focus of the Nuclear Materials research group within the Department of Materials Science and Engineering at the University of Sheffield is the safe disposal of radioactive waste materials generated through nuclear power generation and atomic weapon decommissioning.
Research into this area is critical as there is a large volume of radioactive waste in the world that has been treated, but can only be guaranteed to be safe for a limited period of time. Long-term disposal is essential to ensure that these materials are stored safely and securely.
Recent publications from the research team at the University of Sheffield detail the latest progress in this area.
In a paper published in the journal Inorganic Chemistry in December 2020, researchers were investigating how periodate double perovskites could be used in the immobilisation of radioiodine - volatile radionuclides which arise from reprocessing of nuclear fuels, degradation of nuclear fuels during reactor accidents and storage, and nuclear weapons tests.
The isotope I-129 has a half-life of more than 15 million years, while I-131 has a half-life of just 8 days. Should I-129 be released into the environment, its long half-life means that the isotope will contaminate water and food sources for generations to come. However, the latter is the most carcinogenic of the Iodine isotopes, thought to cause the majority of excess thyroid cancers seen after nuclear fission contamination.
The discovery that periodate double perovskites can immobilise has stimulated particular interest due to its ability to incorporate between 25-40wt% of iodine making it a highly efficient immobiliser of radioiodine. Moreover, the perovskite material exhibits thermal stability up to 1050°C, and therefore meets one of the criterion of interest for selection of a waste immobilization matrix - that of fire scenarios in waste management facilities.
Fig 1. Structures of A2NaIO6 compounds modeled from neutron diffraction data. a) Ba2NaIO6 viewed down [110], and b) Sr2NaIO6 and c) Ca2NaIO6 viewed down [010]. Green spheres indicate respective alkali earth cations (Ba, Sr, or Ca), yellow octahedra indicate sodium cations, purple octahedra indicate iodine cations, and red spheres indicate oxygen anions. Inorg. Chem. 2020, 59, 24, 18407-18419
Details of the iodine isotope immobilisation work can be found here: https://doi.org/10.1021/acs.inorgchem.0c03044.
The second area of research, published in Nature Communications in August 2020, examined the mechanism by which different valence states of uranium can be reduced to a safe stable form suitable for storage and disposal. In particular, the reaction with iron oxide (in the form of magnetite) was investigated.
Advanced synchrotron and X-ray photoelectron spectroscopy analysis techniques were used to examine the mechanisms at a nanoscale. It was found that uranium in the U(VI) valence state reduced initially to a mixture of U(V) and U(IV), which remained for some time before fully reducing to U(IV) by forming nanocrystals on and around magnetite nanoparticles. These nanoparticles, with a diameter of 1-2nm, then grow into nanowires by self-assembly before forming a network structure. Eventually, these networks disappear, leaving just nanoclusters of uraninite on the surface of the magnetite.
Fig.2 Magnetite particles are indicated in gray and uranium species in yellow. The reaction starts with rapid adsorption of aqueous U(VI) species onto the magnetite surface. Adsorbed U(VI) is reduced to U(V) and U(IV) species. While U(V) remains on the magnetite surface, U(IV) precipitates out as uraninite nanoparticles that self-assemble into nanowires structures anchored to the magnetite surface. Nanowires continue to grow while reduction proceeds. Eventually, crystal growth and coalescence lead to the collapse of nanowires into UO2 nanoclusters in which nanoparticles display a preferred orientation. From: Nanoscale mechanism of UO2 formation through uranium reduction by magnetite, Nat Commun 11, 4001 (2020). https://doi.org/10.1038/s41467-020-17795-0, reproduced without changes in line with Creative Commons Attribution 4.0 International License.
The understanding of how uranium reacts with other minerals contributes significantly to research into the containment and immobilisation of radioactive materials, as we are able to form a picture of how these materials can be prevented from escaping into the environment through weathering and dissolution processes, and therefore, how stable the wasteforms will be in the future.
The experimental work behind this second investigation is detailed here: https://doi.org/10.1038/s41467-020-17795-0.
Both these studies form part of the ongoing work of the nuclear materials research team at the University of Sheffield to develop safe and sustainable ways to process and store radioactive waste materials all over the world.