John G. Pepin

Crystal data for rare earth orthophosphates of the monazite structure-type

Abstract Unit cell constants for REPO4 (RE = La to Gd) have been re-determined. Nine-fold coordinated ionic radii for Pu3+, Am3+, and Cm3+ in the monazite structure-type have been calculated.

The crystal chemistry of cerium in the monazite structure-type phase of tailored-ceramic nuclear waste forms

Monazite prepared at 1200°C in air was observed to incorporate cerium as 3+ and not 4+ even when charge compensation mechanisms for Ce4+ inclusion were present. The consequences of this phenomenon for the use of monazite structure-type solid solutions for transuranic nuclide isolation in radioactive waste management are discussed. The use of cerium as a substitute for the actinides is also considered.

Subsolidus phase relations in the systems CeO2RE2O3 (RE2O3 = C-type rare earth sesquioxide)

Abstract The systems CeO 2  RE 2 O 3 ( RE 2 O 3 = C-type rare earth sesquioxide) were studied to: (1) investigate the claims of several workers for the existence of a complete solid solution series between CeO 2 and RE 2 O 3 and (2) to characterize the weak C-type X-ray diffraction peaks reported by others from samples in the single-phase fluorite solid solution region. It is shown that a complete solid solution series does not exist, and an explanation for the observations of others reporting such is tendered. It is also shown that the observation of C-type reflections in the supposed single-phase fluorite field can be attributed to the partial reduction of Ce 4+ to Ce 3+ at the firing temperature, resulting in the movement of the bulk composition into a two-phase field of the CeO 2  RE 2 O 3 Ce 2 O 3 phase diagram, rather than the formation of a domain structure due to ordering.

Crystal Chemistry and Phase Relations in the Synthetic Minerals of Ceramic Waste Forms: I. Fluorite and Monazite Structure Phases

McCarthy (1–4) has suggested that synthetic minerals of the fluorite and monazite types are potentially ideal hosts for the lanthanide*-actinide** elements in nuclear wastes. He has synthesized numerous compositions of these structure types relevant to nuclear waste chemistries (1–5). Their geologic stability, radiation stability and high solid solution capacity for actinides have been previously discussed (1–10). The Ln and An elements are very important constituents of high level nuclear wastes (HLW) and transuranic wastes (TRU), and thus are receiving high priority consideration in the design of ceramic nuclear waste forms. The oxides of the Ln’s and An’s can constitute more than 50 weight percent (wt%) of some HLW formulations. Also included among the actinides are long-lived hazardous radionuclides such as Np-237, Pu-238 and Am-241. We discuss here crystal chemistry and recently determined phase relations data on synthetic fluorites and monazites in the context of designing synthetic minerals for HLW ceramics.

Stem Characterization of the Synthetic Rare Earth Minerals in Nuclear Waste “Supercalcine-Ceramics”

“Supercalcine-ceramics” are a type of nuclear waste form made by modifying the compositions of high-level liquid wastes with selected additives so that after drying and firing, an assemblage of mutually compatible crystalline phases is produced (1,2). Wherever possible, these phases are designed to be synthetic versions of nature’s most stable minerals, of which the sedimentary resistates are the most desirable (3). A class of prototype supercalcine-ceramics are under development for the high-level liquid wastes that would result from reprocessing of nuclear power plant spent fuel. In such wastes, the rare earths, taken as a group, are the major constituents. McCarthy (4) has previously addressed the crystal chemistry of the rare earths in nuclear waste forms. In this paper we describe the behavior of the rare earths in two prototype supercalcine-ceramics.

The System SrMoO 4 -BaMoO 4 -CaMoO 4 : Compatibility Relations, The Implications for Supercalcine Ceramics

The stability of a tailored ceramic waste form under hydrothermal conditions was the subject of several previous papers [1,7–10] from this laboratory. One of the results reported in these studies was the apparent dissolution and reprecipitation of the alkaline-earth molybdate phases (scheelite-structure phase). The composition of the scheelite-structure phase after hydrothermal treatment was different from that before treatment (approximately Ca 95 Sr 5 after and Ca 35 Sr 40 Ba 25 before). A schematic phase diagram was presented at that time to explain the results obtained. This paper is a report of our attempt to experimentally determine the phase relations in the alkaline-earth molybdate (Ca, Sr, and Ba) and verify our previous interpretation. For this work, we prepared compositions throughout the ternary system and heated them at 1200°C for periods of 2 to 24 hours. The experimental products were characterized by x-ray diffraction and SEM/EDX examination. At 1200°C, the two-phase solvus is nearly symmetric and extends from near the pure calcium and barium end-members to about 35 mole percent strontium at the critical point of the solvus. The increase in grain size, uniformity of composition within and between grains of the same phase and the approach to “textural equilibrium” in the 1200°C experiments all strongly suggest that equilibrium was attained. New, calcium-rich compositions are suggested for the scheelite-structure phases in the supercalcine-ceramics based upon the interpretation of data from this study. This observation forces a re-examination of the assumption concerning the partitioning of Sr and Ba amongst the phases in the supercalcine-ceramic.