Review of research on analogs of monazite for the isolation of actinide wastes

The lanthanide orthophosphates (LnPO4) are being investigated as a means of primary containment for high-level actinide wastes. Lanthanum orthophosphate and orthophosphates of the first half of the lanthanide transition series (i.e., Ce through Gd) are analogs of the mineral monazite. The known long-term (109 yr) stability of this mineral under a variety of geological conditions, its ability to incorporate the naturally occurring actinides Th and U, and its apparent resistance to α-particle-induced radiation damage indicate that the analogous lanthanide orthophosphates could well represent ideal hosts for the immobilization of transuranic actinide wastes. A flux technique has been used to grow single crystals of every lanthanide orthophosphate (except PmPO4) as well as ScPO4 and YPO4, and these crystals have been employed in a variety of x-ray, electron paramagnetic resonance, Moessbauer, radiation damage, and chemical stability studied. Single crystals of uranium-doped LaPO4 were grown from an oxide mixture containing 10 wt % 238UO2. The optical absorption spectra show that the uranium impurity is present in these crystals in the tetravalent state. Other actinides, 241Am2O3, 242Pu2O3 and 246Cm2O3, have been studied also. Leach tests of the orthophosphate crystals resulted in IAEA leach indices that were smaller than those reported for grouts or borosilicate glasses. The method of precipitation in molten urea does represent a promising technique for the practical synthesis of actinide-containing rare-earth orthophosphates. Examination of suitably doped precipitated powders using electron paramagnetic resonance has shown that rare-earth impurities occupy identical sites in both the single crystals and the urea-precipitated material