Theoretical Study of Pu and Cs Incorporation in a Mono-silicate Neodymium Fluoroapatite Ca9Nd(SiO4)(PO4)5F2

In the context of nuclear waste disposal, the possibility of incorporating selectively plutonium and cesium in a mono-silicate neodymium-fluoroapatite Ca 9 Nd(SiO 4 )(PO 4 ) 5 F 2 (britholite) is investigated. For this purpose, a force field for the modeling of fluoroapatite Ca 10 (PO 4 ) 6 F 2 is established based upon the experimental data of crystallographic parameters and elastic constants. To test further the validity of the potentials, the constant pressure specific heat for Ca 10 (PO 4 ) 6 F 2 is measured and compared to the calculated values. Neodymium ions and silicate groups are introduced in the fluoroapatite structure using transferable potentials to obtain the neodymium-britholites Ca 10 - y Nd y (SiO 4 ) y (PO 4 ) 6 - y F 2 . The force field accuracy is tested by comparing the calculated and experimental values for the lattice parameters and volumes with respect to y (1 ≤ y ≤ 6). Plutonium and cesium are introduced separately in Ca 9 Nd(SiO 4 )(PO 4 ) 5 F 2 also using transferable potentials. The bulk modulus B and the specific heats, C P and C V, of the resulting structures are calculated. The cases of trivalent and tetravalent plutonium have been studied by considering, respectively, the following compositions: Ca 9 Nd 1 - x (Pu 3+ ) x (SiO 4 )(PO 4 ) 5 F 2 and Ca 9.25 (Pu 4+ ) 0.25 Nd 0.5 (SiO 4 )(PO 4 ) 5 F 2 . In both cases plutonium is found to substitute for calcium Ca(2), which is seven coordinated, by six oxygen ions and a fluorine. Conversely, cesium, which is introduced in the following structure Ca 9.5 Cs 0.5 Nd 0.5 (SiO 4 )(PO 4 ) 5 F 2, will substitute for Ca(1) as well as for Ca(2), with a slight preference for Ca(1) which is nine oxygen coordinated. The activation energies for the lattice migration of Pu and Cs are also calculated in the presence of calcium vacancies.