Lanthanide orthophosphate ceramics for the disposal of actinide-contaminated nuclear wastes

Abstract

Since the half lives of some of the hazardous, α-active actinide isotopes contained in nuclear wastes are relatively long. severe stability requirements are imposed on the materials that must function as primary containment media in a permanent actinide waste-disposal system. Such materials must maintain a high degree of physical and chemical integrity under a variety of potentially hostile geological environments, including possible hydrothermal conditions. Investigations by McCarthy et al [1] of the stability of borosilicate glasses under hydrothermal conditions led to reservations regarding their suitability as primary hosts for the disposal of nuclear wastes during the desired 10 4 to 10 5 year immobilization period. These concerns, in turn, resulted in renewed interest in alternative materials for the disposal of radioactive wastes. One approach employed in selecting alternatives to borosilicate glass consisted of a process of examining various mineral systems whose long-term stability was well established geologically. An approach of this type let to the SYNROC concept of Ringwood [2] and also to the decision to investigate the lanthanide orthophosphates that are analogs of the uranium and thorium bearing mineral monazite [3] (La, Ce, ⋯PO 4). Geological evidence indicates that monazites have survived various rock-forming and metamorphic processes in the earth's crust for ∼2 × 10 9 years-a time span that exceeds the required stability period for an actinide waste form by a factor of 10 5 to 10 4. A program consisting of both fundamental and applied studies of the characteristics of lanthanide orthophosphates is currently under way for the purpose of evaluating these materials as primary nuclear waste forms. The investigation carried out in this program range from spectroscopic (EPR, optical, X-ray, etc.) studies of doped synthetic single crystals to practical investigations of the formation, sintering, and leaching properties of prototype lanthanide orthophosphate polycrystalline ceramics. In order to study the solid state properties of mixed lanthanide-actinide orthophosphates, single crystal of lanthanide hosts doped with Cm, Am, Pu, Np, and U were grown using a flux technique. Additionally since most actual nuclear wastes contain elements other than the actinides, orthophosphate crystals doped with iron group (and other) impurities were prepared and studied using a number of techniques. These fundamental investigations of the properties of mixed orthophosphates have yielded information on the coordination chemistry, valence states, structure, and solid state chemical properties of the actinides as well as such diverse impurity ions as Pb 3+ and Zr 3+. A typical result form this phase of the effort is given in Fig. I where the EPR spectra ▪ of 243Cm 3+ and 244Cm 3+ in a single crystal of LuPO 4 are shown [4]. One of the most important criteria in establishing the relative performance of a radioactive waste form is its ability to resist dissolution and corrosion in aqueous media. Accordingly, the `leaching' characteristics of lanthanide orthophosphate nuclear waste forms are being emphasized in the ongoing investigations. An example of some of the results obtained in this phase of the lanthanide orthophosphate research program is illustrated in Fig. 2. Here a conductivity cell was used to determine the ionic release rate into distilled water for LaPO 4 and for borosilicate glass (Frit 21) containing simulated Savannah River Plant wastes and for SYNROC B which did not contain simulated nuclear waste. These results show that after ∼100 h at 90 °C, the ionic release rate of the waste-loaded polycrystalline LaPO 4 ceramic is ∼1000 times lower than that of waste-loaded borosilicate glass and is ∼10 times lower than the SYNROC sample that does not contain any nuclear waste simulant [5]. The results of previous and ongoing basic investigations of the solid state properties of lanthanide orthophosphates containing actinide and other impurities will be reviewed along with the current status of the applied studies of complex simulated waste forms. ▪