Reactive Sintering of the Thorium Phosphate-Diphosphate. Study of Physical, Thermal, and Thermomechanical Properties and Chemical Durability during Leaching Tests

Abstract

Taking into account some attractive properties of the thorium phosphate−diphosphate (Th4(PO4)4P2O7), called TPD, we focused our research on its potential use as a ceramic for the long term storage of some radionuclides including trivalent and tetravalent actinides. In this objective, we already investigated with success several ways of preparation of the pure TPD and solid solutions with tetravalent actinides either by wet or dry chemical methods. In the field of nuclear applications and considering handling operations on the radioactive materials, the preparation of this compound in the ceramic form was developed. Therefore, a precursor of TPD was especially synthesized in order to get sintered samples after performing an uniaxial pressing at room temperature followed by a heat treatment at high temperature. The synthesis, the characterization, and the sintering procedure of the precursor are described in this paper. The influence of the heating temperature on the specific area of the powder is examined and then correlated with the granulometry and the densification of the solid. For all of the samples, the apparent and effective relative densities obtained are in the range of 87−99% of the value calculated from XRD data. Moreover, some physical properties such as thermal diffusivity, thermal conductivity, and specific heat were measured from room temperature up to 1000 °C. The main results are given in this paper. Finally, the durability of the sintered TPD samples during leaching tests was studied. The apparent dissolution rates remain low even in very corrosive media (1.25(2) × 10-3 g d-1 in 5 M HNO3 at 25 °C) which shows that the TPD can be considered as a promising candidate for the immobilization of actinides and some fission products for a long-term storage.

Synthesis and characterization of low-temperature precursors of thorium–uranium (IV) phosphate–diphosphate solid solutions

“…Sintered pellets were prepared using a two-step procedure (involving an uniaxial pressing at room temperature followed by a heat treatment at 1250°C). For all the samples, the apparent and effective relative densities reached 90-95% and 94-99% of the value calculated from XRD data, respectively [24]. Leaching tests achieved in various solutions demonstrated that both powdered and sintered samples doped, or not, with tetravalent actinides, exhibit a strong resistance to aqueous corrosion [21,25,26].…”

“…b-TPD appears as a promising material for the immobilization of tetravalent actinides [14,22,24]. Actually, it was demonstrated that b-TPD can incorporate large amounts of tetravalent uranium (up to 47.6 wt%), neptunium (33.2 wt%) or plutonium (26.1 wt%) by substitution of thorium [17][18][19][20]23].…”

Sorption of Eu(III) on Th4(PO4)4P2O7: effects of pH, complexing anions and fulvic acid

“…The synthesis and characterization of thorium and uranium phosphate compounds such as U(UO 2 )(PO 4 ) 2 , Th 4 (-PO 4 ) 4 P 2 O 7 (thorium phosphate-diphosphate) and ThO 2 -MO 2 -P 2 O 5 (M = U, Ce, Zr, Pu) was a popular focus of study in the 1990s [9][10][11][12][13]. These phosphate compounds contain uranium or thorium atoms, indicating that phosphate and actinide atoms possess a strong binding ability.…”

Microwave-assisted rapid synthesis of cerium phosphates and their adsorption on uranium(VI)

“…Several phosphates such as Zr 2 O(PO 4 ) 2 , ZrP 2 O 7 and Th 4 (PO 4 ) 4 P 2 O 7 have been studied as potential substitutes for immobilization and long-term storage of uranium [18]. Phosphates, especially rare earth phosphates, have poor solubility, strong irradiation resistance, excellent thermal stability, good binding ability with actinide elements, and high adsorption capability on actinide elements and heavy metals [15][16][17][18][19][20][21][22][23][24][25][26][27][28]. For this reason, rare earth phosphates as adsorbents are expected to play a significant role in the safety of underground radwaste repositories due their potential use for an engineered containment barrier.…”