By Prashant Selvaratnam Department of Earth Sciences University of Cambridge Supervisor: Dr. Ian Farnan PHOSPHATE BASED CERAMIC WASTE FORMS
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 • Ca10(PO4)6F2 • 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. • 3Ca3(PO4)2 + CaF2 Ca10(PO4)6F2 • Mixture ground together. • Sintered at 8000C for 2 hours. • Re-ground and pressed into ~1g pellets. • Calcinated at 1,0000C for 2 hours. • Analysed by powder X-ray diffraction and 31P Nuclear Magnetic Resonance. } X 2
31P 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 Ce3+ state. • Coupled substitution: • Ce3+ and Na+ for 2Ca2+ 3Ca3(PO4)6 + 10xCeF3 + (1-20x)CaF2 + 10xNaF → (Ca(1-2x)CexNax)10(PO4)6F2 Where 0 ≤ x ≥ 0.05. • Problems with melting samples. • Reducing ramp rate from 200C/min to 100C/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 • YPO4 • 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 P2O5 and Y2O3. • Interatomic potentials: • Buckingham Potential: V(r) = Aexp(-Br) – C/r6 • Morse Potential: V(r) = D [1-exp(a(r-ro)))2 – 1] Where r is the inter-atomic distance.
Xenotime Potentials * P-O-P bond angle term used
Preliminary Conclusions Fluorapatite • Pure phase fluorapatite synthesis possible via solid state methods. • 31P 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 YPO4 potential that is charge balanced with respect to P2O5 and Y2O3. • 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 YPO4, P2O5 and Y2O3. • 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).