Ceramic encapsulation of radioactive waste materials

1. Ceramic encapsulation of radioactive waste materials Cambridge simulation work Personnel: Martin Dove, Emilio Artacho Kostya Trachenko (CMI-funded, classical MD) Miguel Pruneda (BNFL-funded, ab initio) Ilian Todorov (NERC-funded, development of MD code DL_POLY)

2. First simulations Initial work was with recoil energies that are too low by more than a factor of 20 due to computational restrictions, but still showing basic insights

3. Polymerisation in first simulations Nevertheless, we were able to observe rebonding of atoms, leading to polymerisation of silica units consistent with experimental data This result is of great importance with respect to the long-term behaviour of the damaged material

4. Current methodology • Using empirical models, recently retuned with new experimental data to give best available model • Short-range overlap models developed using abinitio methods • Using DL_POLY code; version 3 is being funded by our NERC escience grant, and allows us to use of order of 1 million atoms (largest possible), with computer time on UK national supercomputer • Graphical analysis via Cerius and AtomEye (MIT code)

5. First realistic simulations We can now reach realistic recoil energies, creating local temperature rise to 800,000 K These simulations show the structural damage caused by radioactive decay

6. First realistic simulations Key observation is depleted core and polymerised shell

7. Parallel second event We are now tackling the issue of repeated radioactive decay events Note large deflection of recoil atom and damage Aim is to understand large (20%) volume swelling and potential leaching pathways

8. Application of percolation theory The value of percolation threshold in the continuum percolation pc=0.3 (approx.)

9. Model of volume swelling Ratio (r) of number of damaged regions in surface cluster to the total of damaged regions as function of concentration (p), giving percolation point Volume swelling (f) calculated from percolation model

10. Doped zircon

11. Perovskite

12. Leaching in zircon Leaching experiments show diffusion processes in crystallite rims of damaged samples

13. Leaching in zircon Note changes in diffusion at percolation points

14. Direct collision of second event We need to consider a wide range of different trajectories Overall behaviour for encapsulation will depend on linked damaged regions

15. Direct collision of second event Depleted regions formed in a damaged region connect (do not annihilate) during the close overlap

16. Perovskite, CaTiO3 Similar polymerised regions – we identified some common local configurations, including edge-sharing polyhedra

17. Perovskite, CaTiO3 Edge-sharing O-O bridge Electron density shows directionality consistent with strong covalent bonding

18. Work in progress Investigation into resitance against amorphisation following radiation: • role of polymerisation to stabilise damage region now understood for silicates and titanates, and next task is to investigate phosphates and zirconates • need also to look at systems which do not amorphise, such as simple oxides MgO and spinel, MgAl2O4 • aim to perform simulations of pyrochlores in support of experimental studies • key task in each case is to develop interatomic potentials and short-range interactions

19. Milestones Month +3: well-tested interatomic potentials for phosphates and zirconates (including test for transferability), with calculation of short-range interactions Month +6: large-scale molecular dynamics of selected phosphate and zirconate structures Month +9: large-scale molecular dynamics of MgO and MgAl2O4 (note that potentials are easier to determine in these cases)