Formation of lanthanide phosphates in molten salts and evaluation for nuclear waste treatment

Abstract

The formation of phosphates of the lighter lanthanides (Ln ¼ La, Ce, Pr, Nd, Sm, Eu, Gd, Tb and Dy) by reaction with alkali metal phosphates has been studied in a LiCl-KCl eutectic melt between 450 and 650 C and in a NaCl-KCl equimolar melt at 750 C and under an inert atmosphere. Alkali metal ortho-, meta-and pyrophosphate precursors have been employed. The reaction results in the formation of single lanthanide or double alkali metal-lanthanide orthophosphates, the former favoured in LiCl-KCl melts and the latter in NaCl-KCl melts. The mean crystallite size of precipitated phosphates was evaluated from X-ray powder diffraction patterns, and found to be within 300-450 A ˚. Increasing the melt temperature results in increasing the crystallite size of phosphate phases. This technique offers an attractive means of removing lanthanide fission products from chloride melts arising from pyrochemical reprocessing of spent nuclear fuels. LiCl-KCl based melts are recommended because the bulk of phosphate waste precipitated is smaller since normal rather than double phosphates are formed therein.

Treatment of a waste salt delivered from an electrorefining process by an oxidative precipitation of the rare earth elements

“…In order to reuse these waste salts, the fission products in a waste salt must be removed. At present, various studies are ongoing to remove fission products from a waste salt; a salt-zeolite ion exchange [5], an oxide precipitation with an oxidant addition such as Li 2 O [6] or V 2 O 5 [7], phosphate precipitation [8,9] and a carbonate precipitation [10].…”

Crystal chemistry of the monazite structure

“…A lot of compositions were synthesized, yielding to the incorporation of various elements in terms of ionic radii and oxidation state in the monazite structure. 94 Within this application, particular studies are dedicated to the fabrication processes [95][96][97][98][99][100][101][102][103][104] or to the sintering of monazite compounds loaded with actinides or actinide surrogates. [105][106][107][108][109][110] In these applications, the elementary incorporation in the structure is the key factor driving the material properties.…”

Synthesis of rare earth phosphates in molten LiCl–KCl eutectic: Application to preliminary treatment of chlorinated waste streams containing fission products

“…The first, systematically used in the literature for this type of investigation [6,7,[10][11][12], consisted in dissolving the solvent in an aqueous solution. In this study, salt blocks of about 10 g were placed in 200 mL of deionized water at room temperature and stirred magnetically for 30 min.…”

“…However, the use of molten chloride media considerably diminishes the temperature of formation of rare earth phosphates (monazite and xenotime). Volkovich et al showed that it was possible to form pure REPO 4 phases (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, and Dy) in LiCl-KCl and NaCl-KCl eutectics by adding lithium orthophosphate (Li 3 PO 4 ) or sodium orthophosphate (Na 3 PO 4 ) at 550 and 750°C, respectively [10][11][12]. The same authors also reported that only a large excess of A 3 PO 4 (A = Na and Li) compared with cerium chloride (PO 3À 4 :Ce 3+ molar ratio % 5) allowed a CeCl 3 ?…”