Monazite and Other Lanthanide Orthophosphates as Alternate Actinide Waste Forms

Abstract

The recent results of leach tests performed under hydrothermal conditions have shown that borosilicate glasses are physically and chemically unstable at elevated temperatures and pressures (1). These results emphasize the need for investigations of alternate high level waste forms which offer the potential of both increased resistance to leaching and stability under various possible metamorphic geological conditions. Although many of the previous (and present) studies of high level waste forms emphasize the containment of fission products, in fact, the dominant long term (i.e., after 300 to 500 year) radiation hazard is posed by the α-emitting actinides. Accordingly, the object of the present investigation is to evaluate an alternate means of actinide containment which is potentially superior to encapsulation in borosilicate glass. The mineral monazite, which is a mixed lanthanide-actinide orthophosphate [i.e., (Ce, La, Nd, Th, U)PO₄] of varying composition, is a crystalline material with established chemical and physical stability when subjected to natural geological conditions over extremely long time periods. Additionally, since monazite is a natural source of the actinides Th and U, it has been subjected to radiation damage (including α-particle damage) during its natural lifetime, and the continued maintenance of its physical and chemical integrity is evidence of its intrinsic resistance to radiation. In view of these obviously desirable characteristics, a series of investigations of the physical and chemical properties of both natural monazite and the related, synthetically produced lanthanide orthophosphates has been undertaken. These investigations have included the usual applied studies such as leach testing under various conditions and the consolidation of precipitated orthophosphate powders by hot pressing as well as more fundamental determinations of the valence states and site symmetry of the actinide impurities. The details of this work are presented in the following discussion.

Guest Scientist from the Swiss National Fund for Scientific Research.

Operated by Union Carbide Corporation with the U.S. Department of Energy under Contract W-7405-eng-26.