Dr. Stephen Liddle in the School of Chemistry at The University of Nottingham leads a team that seems to be first to create a stable version of a uranium ‘trophy molecule’, a compound that has eluded scientists for decades.
The research paper has been published in the journal Science. The Nottingham chemistry team has shown that they can prepare a terminal (completed molecules, if you will allow the simplification) uranium nitride compound that is stable at room temperature and can be stored in jars in crystallized or powder form.
The previous attempts to prepare uranium-nitrogen triple bonds have required temperatures as low as 5 Kelvin (-268 °C or –450 °F) – that’s about the temperature of interstellar space, which is difficult to achieve, work with and manipulate, requiring specialized equipment and techniques.
This breakthrough should have important implications for the nuclear energy industry’s future – uranium nitride materials potentially could offer a viable alternative to the current mixed oxide nuclear fuels used in reactors. Nitride compounds exhibit superior high densities, melting points, and thermal conductivities and the process the Nottingham scientists used to make the compound could offer a cleaner, low temperature fuel production process than methods used currently.
Usually uranium nitrides are prepared by mixing dinitrogen or ammonia with uranium under high temperatures and pressures. But the harsh reaction conditions used in the preparation introduce impurities into the compound that are difficult to remove. That has encouraged scientists in to focus their attention on using low temperature, molecular methods. Until now all previous attempts resulted in bridging, rather than the target terminal, nitrides.
The Nottingham team’s method, much of the practical work completed by PhD student David King, involved using a very ‘bulky’ nitrogen ligand (an organic molecule bonded to a metal) to wrap around the uranium center and to create a protective pocket in which the nitride nitrogen can sit. The nitride was stabilized during the synthesis by the presence of a weakly bound sodium cation (positively charged ion), which blocked the nitride from reacting with any other elements. In the final stage, the sodium was gently teased away, removing it from the structure and leaving the final, stable uranium nitride triple bond.
Dr. Liddle simplifies the explanation, “The beauty of this work is its simplicity — by encapsulating the uranium nitride with a very bulky supporting ligand, stabilising the nitride during synthesis with sodium, and then sequestering the sodium under mild conditions we were able to at long last isolate the terminal uranium nitride linkage.”
Liddle adds another important feature that should help encourage the adoption of the process, “A major motivation for doing this work was to help us to understand the nature and extent of the covalency in the chemical bonding of uranium. This is fundamentally interesting and important because it could help in work to extract and separate the 2 to 3 per cent of the highly radioactive material in nuclear waste.”
The new uranium-nitride compound contains an unpaired electron that was found using EPR spectroscopy showing that it behaves differently from similar compounds prepared at Nottingham.
Professor Eric McInnes, from The University of Manchester explores the impact the new look into the molecule with, “EPR spectroscopy can give detailed information about the local environment of unpaired electrons, and this can be used to understand the electronic structure of the uranium ion in this new nitride. It turns out that the new nitride behaves differently from some otherwise analogous materials, and this might have important implications in actinide chemistry which is of vital technological and environmental importance in the nuclear fuel cycle.”
On the down side a new fuel production process would entail years of regulatory review. Still . . .
High densities offer a smaller more energy dense fuel that could offer both full size reactors and the coming small and perhaps now even miniature reactors a physical downsize. Higher melting points offer greater safety putting the popularly fearsome “meltdown” further out of possibility. Higher thermal conductivities permit engineering with faster thermal movement, an advantage in operation and in safety because a smaller more rapidly cooling fuel assembly would cut the time an extensive effort would be needed to handle a reactor in distress.
Further research could very well have a major impact on the dangers of used fuels. If Liddle is right and the research works out for extracting the 2-3% of the most radioactive actinides its going to be very useful – as both the radioactive materials can be reprocessed and the useable uranium recycled – more energy from less uranium – a good thing.
Yet if the Nottingham discovery is commercial the state of the regulated and approved engineering is going to be challenged offering the U.S. and other hysteria based regulatory authorities a chance to stall and delay progress.
Decades have passed waiting for this news. It’s reasonable to expect that years more will go by until the industry, consumers and environmentalists get the benefits.
It would be wise for citizens and consumers to swift kick the regulators. The Brits deserve a big congrats and thanks for a major improvement that’s been decades in the search.
Lets not let the opportunity to put this news to work pass us by.