Synthesis, characterization, sintering, and leaching of beta-TUPD/monazite radwaste matrices.

To study the simultaneous incorporation of both tri- and tetravalent actinides in phosphate ceramics, we prepared several beta-TUPD/monazite-based radwaste matrices through two different chemical routes (called dry and wet routes) involving the initial precipitation of crystallized precursors of each phase, i.e., TUPHPH solid solutions on the one hand, and rhabdophane (LnPO(4).xH(2)O) on the other. The final material was obtained after heating above 1000 degrees C, and no additional phase was detected from elementary analyses and XRD. Moreover, the complete segregation of tri- and tetravalent cations was evidenced when using dry chemical processes. This method also allows for the preparation of dense pellets (90% < d(exp)/d(calc) < 95%) after only 10-20 h of heat treatment at 1250 degrees C. Finally, the chemical durability of the pellets was examined through several leaching experiments in acidic media. The normalized dissolution rate determined from the uranium release in the leachate varies from (8.2 +/- 0.7) x 10(-6) to (2.7 +/- 0.4) x 10(-2) g m(-2) day(-1) between 25 and 120 degrees C in 10(-1) M HNO(3). Near equilibrium, thorium and lanthanide ions were found to quickly precipitate as phosphate-based neoformed phases in the back end of the initial dissolution process. These phases were identified as uranium-depleted T(U)PHPH and as rhabdophane or monazite.