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 tetravalent actinides and other fission
products.
- Ability to incorporate halides.
Oklo natural reactor, Gabon

Ceramics:
- High durability.
- High waste loading.
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 fluoridesalt 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.

}

X2
Analysed by powder X-ray diffraction and 31P Nuclear
Magnetic Resonance.
Powder X-ray Diffraction
31P
One Phosphorus environment.
 Peak at 2.3ppm.
 Full width half maximum ~1ppm.

NMR
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.


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.

Empirically tuned, using GULP, to re-produce:
 Inter-atomic distances and lattice parameters.
 Elastic constants.

Mindful of phase separation into P2O5 and Y2O3.
Xenotime Potentials
Potential 1
Potential 2* Potential 3
X


Y-O Potential
Buckingham
Buckingham
Buckingham
O-O Potential
Buckingham
Buckingham
Buckingham
P-O Potential
Morse
Buckingham
Morse
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%
C12
-47.2%
+5.4%
-40.0%
C13
+2.3%
+16.3%
-4.6%
Lattice constant 1
0.0%
-1.7%
-0.2%
Lattice constant 2
0.0%
+3.3%
+0.2%
P-O distance
0.0%
+1.0%
-12.0%
Y-O distance 1
+0.8%
-7.0%
+6.8%
Y-O distance 2
-1.7%
+4.2%
+4.0%
Charge balanced
for P2O5 & Y2O3
* 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).