1. GRACE at UCL

2. 2 www.ucl.ac.uk/research-it-services HPC in action: Uranium Bonding and The Nuclear Waste Problem Poppy Di Pietro PhD Supervisor: Dr Andy Kerridge

3. 3 www.ucl.ac.uk/research-it-services Outline Calculations to simulate the electronic structure of molecules containing uranium. Various molecular properties can be derived from these calculations. Investigating uranium bonding to see if we can work out why some molecules will selectively bind some atoms over others.

4. 4 www.ucl.ac.uk/research-it-services Nuclear power accounts for ~ 11% of energy worldwide (2012 – Nuclear Energy Institute) 2000 – 2300 tons of waste generated per year This is only likely to increase! Extremely long lived, radioactive, toxic Current solutions not ideal Source: worldnuclear.org Source: xkcd.com Context

5. 5 www.ucl.ac.uk/research-it-services

6. 6 The f-block

7. 7 www.ucl.ac.uk/research-it-services Introduction - context U, Pu, Np Fission products Lanthanides Minor actinides (Am, Cm) Am and Cm have half-lives of a few hundred to ~16 million years Lanthanides: Actinides:

8. 8 www.ucl.ac.uk/research-it-services Context In a standard fission reactor: But what do we do with the spent fuel?

9. 9 www.ucl.ac.uk/research-it-services Store it on-site? Bury it? Remember, the solution will need to last millions of years! Reprocessing?  Reuse as much as possible using different reactor types  Reduce the volume of high-level waste to minimise storage problem  Transmutate long-lived species via neutron bombardment into isotopes which can either be used again, or which will decay more quickly Source: gov.uk

10. 10 www.ucl.ac.uk/research-it-services Context If we can separate nuclear waste into constituent parts... Uranium and Plutonium are removed and can go through the cycle again Transmutation: Bombarding actinides with neutrons to induce further decay to shorter-lived species BUT lanthanides easily absorb neutrons, shutting this process down Challenge: MA and Ln are very chemically similar We need to selectively bind actinides! The process behind this is not fully understood, so is difficult to improve

11. 11 www.ucl.ac.uk/research-it-services Context There are lots of reasons to do this computationally! Handling radioactive material can be expensive and difficult, requiring highly specialised facilities We have the option of computationally exploring many systems at the same time and can easily predict properties which are difficult to measure experimentally We can use computational data to identify potential targets for experimental scientists vs

12. 12 www.ucl.ac.uk/research-it-services For selective bonding, we need to control the way atoms bond to one another From high school chemistry: Actually most bonds fall on a spectrum between covalent and ionic Evidence that actinides form bonds with more covalency than lanthanides So can we design a separation agent to maximise covalency? Selective extraction? Covalent Interaction Ionic Interaction

13. 13 www.ucl.ac.uk/research-it-services Electronic structure calculations Molecules have a preferred conformation – this is a consequence of their electronic structure which obeys the laws of quantum mechanics We can predict this structure with calculations We need to solve the Schrodinger equation: HΨ = EΨ which gives us the energy of the system Ψ is a wavefunction which describes the electronic structure This looks simple… water methane ethanol

14. 14 www.ucl.ac.uk/research-it-services But even for simple molecules, this is a hugely complicated problem: But we can make a series of approximations… And predict, for example, the structure of these well-known molecules to a high degree of accuracy: Moving on to more complicated systems… Electronic structure calculations water methane ethanol

15. 15 www.ucl.ac.uk/research-it-services We looked at some simple uranyl (UO 2 2+ ) complexes: Uranium is used as a model for other actinides because we expect that covalency will be easiest to spot CO, CN -, NC -, NCS -, F -, H 2 O, OH - These are ligands: molecules or ions which bind to a central metal atom. Here, the metal is uranium Uranyl complexes with small ligands

16. 16 www.ucl.ac.uk/research-it-services One of the things we can model is the way bonds vibrate when they are exposed to light These vibrations can be compared to experimental values to let us know our methods are correct They also allow us to probe other characteristics of the molecule Uranyl complexes with small ligands

17. 17 www.ucl.ac.uk/research-it-services P. Di Pietro & A. Kerridge, Inorg. Chem. 2015  We can plot the frequency of these vibrations against a measure of covalency in the Uranium-Ligand bonds  And show that they can be used as probes of covalency  This measure of covalency also correlates strongly with the stability of the molecule Uranyl complexes with small ligands

18. 18 www.ucl.ac.uk/research-it-services Comparison of BTP and Isoamethyrin [UO 2 (BTP) 2 ] 2+ UO 2 -isoamethyrin We moved on to some more realistic separation ligands And showed that our ligand of interest, isoamethyrin binds uranyl in a very similar way to the industrial ligand BTP. This is interesting because isoamethyrin comes from a family of molecules called hexaphyrins which can be modified in many different ways An ideal candidate for seeing if we can ‘tweak’ a ligand to increase covalency? P. Di Pietro & A. Kerridge, submitted 2016

19. 19 www.ucl.ac.uk/research-it-services cyclo[6]pyrrole - A isoamethyrin - B rubyrin - C hexaphyrin - D We modify the hexaphyrin ligand by adding carbon atoms… And observe that the greater the Uranium-Ligand covalency, the more stable the molecule P. Di Pietro & A. Kerridge, in preparation, 2016 Comparison of different hexaphyrins

20. 20 www.ucl.ac.uk/research-it-services Conclusions Evidence for a strong relationship between covalent character in Uranium-Ligand bonds and stability Uranyl (UO 2 2+ ) undergoes predictable changes when these bonds form, related to amount of electron sharing in U-L bonds Strong similarities between UO 2 -isoamethyrin and industrially useful [UO 2 (BTP) 2 ] 2+ Evidence that covalent character in U-L bonds of uranyl hexaphyrins can be controlled by modifying ligand size. Some promising results…

21. 21 www.ucl.ac.uk/research-it-services The future… As global energy demands rise we need to fill the gap For many reasons, nuclear is a good candidate for doing so But with increased nuclear power generation comes increased waste Although a long way off, an ideal solution would shrink the volume of high-level waste in need of long term storage to a very small amount Theorists working together with experimental and industrial scientists can take steps towards this ultimate goal Computational facilities allowing us to explore the theory behind, for example, bond covalency and selectivity can help with this

22. 22 www.ucl.ac.uk/research-it-services Thank you for your attention Acknowledgements:  Andy Kerridge  EPSRC for the award of a PhD studentship  UCL Legion High Performance Computing Facility (Legion@UCL)  UCL Grace High Performance Computing Facility (Grace@UCL)  EPSRC UK National Service for Computational Chemistry Software (NSCCS) at Imperial College London

23. Lorem ipsum dolor sit amet, consectetur adipiscing elit. Ut efficitur ipsum vitae tortor accumsan, a pulvinar lorem lacinia. Donec eu arcu justo. Fusce eget consequat risus Proin est lacus, interdum vitae feugiat quis, faucibus vel mi. Vivamus accumsan nisi vel nulla viverra semper. Donec purus enim, sollicitudin vitae porta a, commodo sodales justo. Sed iaculis rutrum molestie. Visit www. ucl.ac.uk/research-it-services/grace to download these slides after the event. What did you think? Join the conversation on Twitter with #GraceAtUCL. Don’t forget to follow us for access to the event video and today’s polling results. Questions?