1. DIAMOND Decommissioning, Immobilisation and Management of Nuclear Wastes for Disposal Density Functional Theory study of defects in zirconolite Jack Mulroue University College London 11/13 April 2011 Huddersfield

2. DIAMOND Decommissioning, Immobilisation and Management of Nuclear Wastes for Disposal Background for zirconoilite Zirconolite is the proposed ceramic encapsulent for geological disposal. Zirconolite is able to accommodate Pu and U as well as Hf and Gd into its crystal lattice. Actinide bearing phase in SYNROC, glass ceramics and single phase waste forms. Geological disposal options for HLW and spent fuel - NDA 2008 http://www.synrocansto.com/CaseStudies/UKPlutonium/CaseStudies/UKGlassCeramicAdv.html

3. DIAMOND Decommissioning, Immobilisation and Management of Nuclear Wastes for Disposal CaZrTi 2 O 7 Monoclinic crystal structure ( α≠ 90 °, β=γ= 90 ° ) Structure comprises of 4 and 6 coordinated Ti layers separated by layers of alternating Ca and Zr ions.

4. DIAMOND Decommissioning, Immobilisation and Management of Nuclear Wastes for Disposal VASP –Periodic plain wave DFT code PBE exchange and correlation Γ point sampling of the Brillouin zone PAW pseudopotentials 176 ion cell

5. DIAMOND Decommissioning, Immobilisation and Management of Nuclear Wastes for Disposal Calculated bulk properties Band gap of 2.8 eV (3.6 eV experimental) Lattice parameters Lattice parameter DFT (Å) Experimental (Å) % Error a12.0912.452.89 b14.1414.552.82 c11.0811.392.72 Miseki et al. - Chemistry Letters 38, 2009 Rossell – Nature 283, 1980

6. DIAMOND Decommissioning, Immobilisation and Management of Nuclear Wastes for Disposal Effect of stoichiometry Zirconolite is able to exist in a range of stoichiometries. CaZr x Ti 3-x O 7 where 0.80 < x < 1.37 Experiments observe 5 fold coordinated Ti polyhedra instead of 4 fold coordinated Ti found in the perfect structure.

7. DIAMOND Decommissioning, Immobilisation and Management of Nuclear Wastes for Disposal Excess of Ti CaZr 0.875 Ti 2.125 O 7 The substituted Ti is 5 fold coordinated in the Zr site. Has no effect on the coordination of the four 4 fold coordinated Ti polyhedra.

8. DIAMOND Decommissioning, Immobilisation and Management of Nuclear Wastes for Disposal Excess of Zr CaZr 1.125 Ti 1.875 O 7 The two 4 fold coordinated Ti polyhedra in the same row as the substituted Zr become 5 coordinated. Therefore to remove all 4 fold coordinated Ti polyhedra, with x = 1.25, the two Zr must be in different Ti layers.

9. DIAMOND Decommissioning, Immobilisation and Management of Nuclear Wastes for Disposal Point defects in zirconolite The point defects have been studied to gain a fundamental understanding into defect behaviour, to allow for more accurate predictions on the long term durability of the waste form. Each defect has been studied in the various charge states, from 0 to formal charge.

10. DIAMOND Decommissioning, Immobilisation and Management of Nuclear Wastes for Disposal Oxygen vacancies 4 of the 7 oxygen environments have been studied. The 2 excess electrons generated from the neutral oxygen vacancy localised on 2 Ti ions. The singly charged vacancy localises the excess electron on the Ti ion it was coordinated too. Similar behaviour is observed in anatase (TiO 2 ).

11. DIAMOND Decommissioning, Immobilisation and Management of Nuclear Wastes for Disposal Titanium vacancies The defect structure has a significant dependence on the chemical environment of the Ti. Vacancy in the 6 fold coordinated chains, results in the formation of an O 2 molecule in certain charge states and a 5 coordinated Ti in others. A vacancy of the other 6 coordinated Ti, results in the 4 coordinated Ti becoming 6 coordinated. The vacant 4 coordinated Ti resulted in the creation different numbers of 5 coordinated Ti in different charge states.

12. DIAMOND Decommissioning, Immobilisation and Management of Nuclear Wastes for Disposal Ca and Zr vacancies The Ca vacancy results in distortion around the vacancy site. The neutral Zr vacancy results in the formation of an O 2 molecule. The singly charged vacancy results in the creation of 5 coordinated at the expense of a 6 coordinated Ti. The higher charge states results in distortion around the vacancy site.

13. DIAMOND Decommissioning, Immobilisation and Management of Nuclear Wastes for Disposal Locating the interstitials The interstitial sites were studied using random structure analysis, based on the AIRSS method. The 88 ion unit cell was optimised using soft oxygen pseudopotentials, with the cell parameters fixed to the experimental lattice vectors. The interstitial ion was then added with randomly assigned coordinates and all the ions were allowed to relax. This was carried out 100 times for each interstitial. The lowest energy structure was then placed inside the 1x2x1 supercell to obtain the interstitial structure.

14. DIAMOND Decommissioning, Immobilisation and Management of Nuclear Wastes for Disposal Oxygen interstitials The oxygen interstitial can only exist in two charge states, 0 and -1. The neutral interstitial forms an oxygen dumb-bell with a lattice oxygen at high interstitial concentrations. However at lower concentrations the interstitial causes two 7 coordinated Ti and a 5 coordinated Ti. The singly charged interstitial produces a 5 coordinated Ti polyhedron caused by displacement of two lattice oxygen ions on the same polyhedron.

15. DIAMOND Decommissioning, Immobilisation and Management of Nuclear Wastes for Disposal Ca and Ti interstitials The cation interstitials were found only to exist in the +2 charge state. Both Ca and Ti interstitials reside within the channels which run through the crystal.

16. DIAMOND Decommissioning, Immobilisation and Management of Nuclear Wastes for Disposal Zr interstitials The most stable interstitial configuration was the Zr ion substituting for a Ti lattice ion. Bond length increase in the polyhedron due to substitution. This configuration was 0.27 eV more stable than the Zr remaining as the interstitial located in the channel.

17. DIAMOND Decommissioning, Immobilisation and Management of Nuclear Wastes for Disposal Conclusions The presents of non-stoichiometry and vacancies in the experimental samples causes the difference in coordination of the Ti polyhedron. O 2 molecules could form at vacancy sites within the lattice. The substitution caused by the Zr interstitial may have a significant effect on the recovery of the lattice from an irradiation event. Require more fundamental experimental work to allow accuracy of our models to assessed.