No abstract
The extent and nature of solid solution formation in the system urania-gadolinia were investigated. Compositions sintered in hydrogen, in argon, and in air were analyzed by chemical and X-ray methods. Extensive solid solution exists between uranium dioxide and gadolinium sesquioxide. Some anomalies in the lattice parameters of the solid solutions are interpreted. I. I n t r o d u c t i o nHE nuclear properties of the rare-earth oxides have stim-T ulated interest in the reactions which occur between them and urania. Their use as ceramic burnable poisons, which are designed to burn out at the same rate as the fuel, helps maintain a constant reactivity o€ the core, simplifies control of the reactor, and lengthens thc life of the core. The most promising rare earths for this use are gadolinia, dysprosia, samaria, and curopia.At elevated temperatures, UOz is oxidized to nonstoichiometric U&, with a resultant disruptive increase in volume, an increase in vapor pressure, and a lowering of fission product retention and irradiation stability.' The addition of a material which could form extensive solid solutions with urania and also act as a control material, or burnable poison, would be doubly beneficial in certain applications.Solid solutions should form readily between gadolinia and urania. Gadolinium sesquioxide, as precipitated, is bodycentered cubic. Further, materials whose ionic radii are within a 20y0 variation of the ionic radii of the rare earths are reported to form solid solutions.2 The ionic radius3 of U4+ is 0.97 A, of U6+ is 0.80 A, and of Gd3+ is 0.97 A.The purpose of this investigation was to study the nature and extent of the reactions between urania and gadolinia. Lattice parameters of mixtures sintered a t elevated temperatures in environments of hydrogen, argon, and air were determined. Literature ReviewGoldschmidt and his co-workers4 recognized three crystallographic forms of the rare-earth oxides. They reported form C, which is body-centered cubic with 16 molecules to the unit cell, to be stable from room temperature to 750°C for gadolinia. Form B, whose structure was not identified, but which appeared to be two modifications, B1 and Bz, was stable from 750" to 13OO0C. Form A was hexagonal and optically ncgativc with basal cleavage planes. This form was stable to the melting point of gadolinia, which is 2330' =k 20°C. They concluded that the A and B forms could transform reversibly but that the transformation between B and C was more complex. Brauer and Gradingers stated that gadolinia was form C cubic a t 750°C and at 1400°C with a one-half unit cell dimension of 5.385 A. Curtis and Tharp6 listed the unit cell dimension as 10.79 A with a Tlz03-type structure a t room temperature and a theoretical density of 7.66 g/cm3. The crystal structure of gadolinia heated above 1300°C was undetermined. Guentert and Mozzi7 prepared gadolinia in form B by heating a t 1400" t o 1500°C for several hours The crystal form was monoclinic with lattice values of a = 14.OG1 + 0.013 A, b = 3.566 f 0.006 A, c = 8.700 f 0...
Uranium dioxide becomes hypostoichiometric when heated above 160OOC in vacuum or in a reducing atmosphere. Upon cooling, free metallic uranium is rejected. The role and the effect of rare-earth oxide additives on the stabilization of urania at high temperatures in a hydrogen environment are evaluated. A tentative phase diagram for the system uraniagadolinia is proposed.
Data on the effect of calcination temperature on surface area, apparent crystallite size, compactibility, sintered density, and volume shrinkage are presented for thoria prepared from the oxycarbonate, chloride, nitrate, and oxalate. Surface area and volume shrinkage decreased with rising calcination temperature. Thoria obtained from the oxycarbonate exhibited the greatest sinterability ; material derived from the nitrate showed the least. Maximum bulk densities were achieved using material resulting from the calcination of the oxycarbonate between 600' and 100O'C. Densities of 95 to 98% of the theoretical value of thoria were attained by compacting these powders at pressures above 20 tsi and firing at 1500'C for 24 hours. Uraniathoria solid solutions incorporating thoria obtained by calcination of the chloride exhibited the highest fired densities.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.