By Prashant Selvaratnam Department of Earth Sciences University of Cambridge Supervisor: Dr. Ian Farnan

Mineral Based Phosphate Ceramics  Phosphate minerals: - Evidence from nature of long term stability. - Ability to incorporate tri- and tetra- valent actinides and other fission products. - Ability to incorporate halides.  Ceramics: - High durability. - High waste loading. Oklo natural reactor, Gabon Image from DOE Office of Civilian Radioactive Waste Management.

Fluorapatite  Ca 10 (PO 4 ) 6 F 2  Structure and chemistry allow for a multitude of substitutions.  Two distinct cationic sites: Four Ca1 sites, 9-fold co-ordination. Six Ca2 sites, 7-fold co-ordination.  Suitable for waste streams from fluoride- salt extraction: Experimental pyroprocessing techniques. Decommissioning of nuclear weapons. Generation IV nuclear fuels.

Fluorapatite Synthesis  Solid state synthesis.  3Ca 3 (PO 4 ) 2 + CaF 2  Ca 10 (PO 4 ) 6 F 2  Mixture ground together.  Sintered at C for 2 hours.  Re-ground and pressed into ~1g pellets.  Calcinated at 1,000 0 C for 2 hours.  Analysed by powder X-ray diffraction and 31 P Nuclear Magnetic Resonance. } X 2

Powder X-ray Diffraction

31 P NMR  One Phosphorus environment.  Peak at 2.3ppm.  Full width half maximum ~1ppm.

Ce Doping  Ce used as a surrogate for Pu.  Similar electronegativity, ionic radii and oxidation states.  Require Ce 3+ state.  Coupled substitution: Ce 3+ and Na + for 2Ca 2+ 3Ca 3 (PO 4 ) x CeF 3 + (1-20 x )CaF x NaF → (Ca (1-2 x ) Ce x Na x ) 10 (PO 4 ) 6 F 2 Where 0 ≤ x ≥  Problems with melting samples.  Reducing ramp rate from 20 0 C/min to 10 0 C/min helps.

Ce Doping  Use X-ray diffraction and NMR to study phase assemblages, solid solubility, Ce oxidation state and site distribution.

SRIM Calculations  Produce a sample with a uniform damage profile.  Ions must completely penetrate sample.  29MeV/nucleon Pb ion beam, retarded to 11MeV/nucleon.

Xenotime  YPO 4  Empirical potential suitable for molecular dynamics simulations of radiation damage.  Empirically tuned, using GULP, to re-produce: Inter-atomic distances and lattice parameters. Elastic constants.  Mindful of phase separation into P 2 O 5 and Y 2 O 3.  Interatomic potentials: Buckingham Potential: V(r) = Aexp(-Br) – C/r 6 Morse Potential: V(r) = D [1-exp(a(r-r o ))) 2 – 1] Where r is the inter-atomic distance.

Xenotime Potentials Potential 1Potential 2*Potential 3 Charge balanced for P 2 O 5 & Y 2 O 3 X Y-O PotentialBuckingham O-O PotentialBuckingham P-O PotentialMorseBuckinghamMorse C11-0.4%+53%-5.9% C33+0.9%+12.9%+3.6% C44-6.1%+18.4%+3.0% C66+1.1%+52.9%+17.6% C %+5.4%-40.0% C13+2.3%+16.3%-4.6% Lattice constant 10.0%-1.7%-0.2% Lattice constant 20.0%+3.3%+0.2% P-O distance0.0%+1.0%-12.0% Y-O distance 1+0.8%-7.0%+6.8% Y-O distance 2-1.7%+4.2%+4.0% * P-O-P bond angle term used

Preliminary Conclusions Fluorapatite  Pure phase fluorapatite synthesis possible via solid state methods.  31 P NMR peak at 2.3ppm.  Problems with sample melting for Ce-doped sample synthesis in ambient atmosphere.  Sample thickness of < 82µm required to obtain uniform damage profile in 11MeV/nucleon Pb beam. Xenotime  Difficult to get a wholly satisfactory YPO 4 potential that is charge balanced with respect to P 2 O 5 and Y 2 O 3.  Having a Morse potential between P and O improves the output.

Future Work Fluorapatite  Ce-doped sample synthesis under reducing atmosphere.  NMR analysis of Ce-doped fluorapatite samples.  Make and analyse 80µm thick, 1.5cm x 1.5cm samples for ion beam damage. Xenotime  Do one GULP fit for YPO 4, P 2 O 5 and Y 2 O 3.  Run DL_POLY radiation damage simulations using obtained potentials.

Acknowledgements  Ian Farnan, Martin Dove, Clive Brigden, Katie Gunderson, Tony Abraham, Martin Walker (University of Cambridge).  Shirley Fong, Brian Metcalfe, Phillip Mallinson (AWE).  Ram Devanathan (Pacific North West National Lab, US Department of Energy).  Christina Trautmann, (GSI Helmholtz Centre for Heavy Ion Research)  Lou Vance (Australian Nuclear Science and Technology Organisation).