INVESTIGATION OF THE SYSTEM THO2-NPO2-P2O5 - SOLID-SOLUTIONS OF THORIUM-NEPTUNIUM(IV) PHOSPHATE-DIPHOSPHATE

Considering that phosphate matrices could be potential candidates for the immobilization of actinides or for the final disposal of the exces s plutonium from dismantled nuclear weapons, the chemistry of thorium phosphates has been re-examined. In the ThO2-P2O5 system, the thorium phosphate-diphosphate Th-4(PO4)(4)P2O7 (TPD) can be synthesized by wet and dry chemical processes. The substitution of thorium by other tetr avalent actinides like uranium or plutonium can be obtained for 0 < x < 3.0 and 0 < x < 1.63, respectively. In this work, we report the chem ical conditions of synthesis of thorium-neptunium (IV) phosphate-dipho sphate solid solutions Th4-xNPx(PO4)(4)P2O7 (TNPD) with 0 < x < 1.6 fr om a mixture of thorium and neptunium (IV) nitrates and concentrated p hosphoric acid. From the variation of the cell parameters and volume, the maximum substitution of Th4+ by Np4+ in the TPD structure is evalu ated to 2.08 (which corresponds to about 52 mol% of thorium replaced b y neptunium (IV)). The held of existence Of solid solutions Th4-xU-xNp -xPuUxUNpxNpPuxPu(PO4)4P(2)O(7) has been calculated. These solid solut ions should be synthesized for 5(xU) + 7(xNp) + 9(xPu) less than or eq ual to 15. In the NpO2-P2O5 system, the unit cell parameters of Np2O(P O4)(2) were refined by analogy with U2O(PO4)(2) which crystallographic data have been published recently. For Np2O(PO4)(2) the unit cell is orthorhombic with the following cell parameters: a = 7.033(2) Angstrom , b = 9.024(3) Angstrom, c = 12.587(6) Angstrom and V = 799(1) Angstro m(3). The unit cell parameter obtained for alpha-NpP2O7 (a = 8.586(1) Angstrom) is in good agreement with those already reported in literatu re. (C) 1998 Elsevier Science B.V. All rights reserved.