Unlocking the potential of thorium for nuclear power
Fundamental non-aqueous solution chemistry is providing data that can be transferable to thorium nuclear fuel cycles, as David Mills, Fabrizio Ortu
and Alasdair Formanuik explain.
The recent implementation of the Paris Agreement, the landmark international legislation for reduced greenhouse gas emissions, heralds a key role for nuclear power for the rest of the 21st Century and beyond. While uranium is expected to continue its dominance as the primary fuel in civil nuclear power generation, the use of thorium is also predicted to rise. This is because thorium-uranium nuclear fuel cycles theoretically provide a number of tangible benefits over uranium-plutonium systems, provided that reprocessing, engineering and cost issues can be overcome.
Among the advantages of thorium-uranium nuclear fuel cycles are the potential for greater efficiency and low amount of long-term nuclear waste. Uranium-plutonium reactors, which are currently far more common, rely on the fission of U-235 to generate a significant amount of highly radioactive transuranic elements – mainly plutonium.
Thorium-uranium nuclear cycles are somewhat different as Th-232, the main isotope, is not spontaneously fissile compared to U-235, and transuranic elements are not formed. Th-232 requires the capture of a neutron to give Th-233, and subsequent nuclear decay to eventually yield U-233, which is fissile. This type of process is known as breeding.
If thermal breeder reactors can be developed then natural reserves will last much longer, as these reactors release a greater proportion of neutrons than traditional fast breeders. This would perhaps be best incorporated in molten salt reactors (MSRs), which operate at high temperatures to produce chloride or fluoride uranium/thorium salts in the liquid phase. However, MSRs are currently underdeveloped compared with solid fuel reactors and some technological issues, such as corrosion, are yet to be fully addressed. Given the expense of developing new technologies, most attention is currently given to adapting existing solid fuel nuclear cycles to accommodate thorium.
Nuclear and chemistry
While the engineering and materials challenges of thorium-uranium nuclear fuel cycles are being studied further, complementary research is being performed in actinide solution chemistry. This has been the case ever since the Manhattan Project more than 70 years ago, as deepening our fundamental understanding of actinide chemistry has implications for nuclear fuel manufacture, reprocessing and dealing with legacy nuclear waste. Specifically, synthetic chemists in this field are often interested in subtle differences in actinide-element bonding regimes and redox chemistry, which is defined as the formal transfer of electrons between elements to attain different oxidation states. One of the major achievements of fundamental actinide radiochemistry research has been the development of fuel reprocessing strategies. The plutonium uranium redox extraction (PUREX) process, which involves the extraction and separation of uranium and plutonium from spent fuels via redox chemistry, is the most famous industrially pertinent example. A crucial aspect of such processes is the ability of tailored extracting agents such as ligands to selectively interact with actinide ions, forming new molecular compounds with different physical properties, thus enabling their chemical separation. Uranium redox chemistry is relatively well studied compared to thorium and the rest of the actinide elements, and this could contribute to a potential barrier to the future use of thorium nuclear fuels.
Thorium has a limited range of oxidation states, with the most stable +4 state predominating. This is seen in simple thorium dioxide (ThO2), and thorium tetraiodide (ThI4). Binary materials containing thorium in a formal +3 oxidation state have been reported since the 1950s. However, these compounds are typically infinite reticular structures, whereas solution chemists deal with discrete molecular entities. The use of non-aqueous chemistry, performed under the strict exclusion of water and oxygen, provides unique opportunities for more exotic thorium compounds to be made with unusual oxidation states and bonding regimes. Comprehensive analysis of such compounds generates important data that may be transferable to real systems.
The first structurally authenticated molecular compound containing thorium in the +3 oxidation state was reported in 1986. This compound contains a single thorium atom surrounded by three bulky carbon-based ligands, which are each assigned a formal -1 charge. Such compounds are particularly challenging to handle because of their extreme air- and moisture-sensitivity and high reactivity. As a result, it is only in the last two years that this compound has been explored more thoroughly, as research worldwide has pushed these investigations forward.
In a proof-of-concept study, focus was directed on the first reaction of white phosphorus with a +3 oxidation state thorium compound. Much interest surrounds the direct activation of white phosphorus, as a protracted chlorination step is currently used industrially for the synthesis of organophosphorus reagents from this starting material – designing the +3 oxidation state thorium compound activated white phosphorus to give a dimeric product containing unusual thorium-phosphorus bonds. Other research groups around the world are also preparing such compounds, providing a library of novel thorium-phosphorus bonds. Given that actinides interact with a wide range of elements during nuclear cycles, an improved understanding of thorium-phosphorus bonding is timely and relevant.
Recently, the first activation of carbon dioxide by a thorium complex in the unusual +3 oxidation state was reported. Studying the mechanisms of carbon dioxide activation and functionalisation is of topical interest, as such greenhouse gases are cited as a major contributor to global warming. In this example, carbon dioxide was converted to an oxalate and carboxylate in a single concomitant process to give a dimeric thorium product. This unprecedented reaction pathway contrasts markedly with better-understood uranium carbon dioxide activation, where reduction to form carbon monoxide, molecular uranium-oxides and carbonates dominates.
Such divergence from uranium +3 oxidation state chemistry is quickly becoming a hallmark of analogous thorium reactivity studies, producing results that complement and contrast with molecular uranium, lanthanide and d-transition metal chemistry. It is predicted that the unique reactivity profiles and interesting physical properties of +3 thorium complexes will continue to provide unusual results and reveal more data. It is this data that will give insights into the fundamentals of thorium chemical bonding and redox chemistry that could be transferable to more applied systems in proposed thorium nuclear fuel cycles in future.
David Mills is a lecturer in the School of Chemistry in the University of Manchester, UK. His research interests include studying the fundamental chemistry of thorium.
Fabrizio Ortu is a post-doctoral researcher in the Mills group at the University of Manchester, synthesising f-element compounds with unusual structures and bonding regimes.
Alasdair Formanuik works in nuclear consultancy, having completed his PhD in low oxidation state thorium chemistry in the Mills group in 2016.