Deformation of UO,to opposite edges of the plate and protecting the sapphire and bond area with paraffin during etching. To ensure accurate relocation of the plate in the dilatometer between etchings, the sapphire balls rested in conical indentations on the dilatometer anvils during measurements. Plate thickness was determined either with a micrometer or by measuring the sample weight. A 50/50 solution of 48% HF and H20 was used as the etchant.King' has suggested an alternative procedure for transparent materials in which refractive index changes prevent direct measurement of surface stresses with a polarimeter but in which measurement of central tension is possible. In that case, the strain change produced in etching the plate is reflected as a change in central tension. For the birefringence measurement, the thermal expansion problem involved in the length measurement is eliminated. HV. Stress ProfilesThe general form of the length vs thickness data is shown in Fig. 3. These are raw data for glass" '/8 in. thick which was tempered by quenching it in silicone fluid at room temperature. at High Temperatures 105Slopes of the 1 vs z curves were obtained from these data by least-squares fitting of polynomials up to 11th order and taking first derivatives of the best-fit polynomials. The results of this treatment of the data for the glass and for a glass-ceramic+ plate '/s in. thick stressed by ion-exchange are shown in Fig. 4. References( a ) A. A. Padmos and J. de Vries, "Stresses in Glass and Their Measurement," Philips Tech. Rev., 9 [9] 277-84 (1947-48).The effects of temperature, strain rate, and grain size on the mechanical properties of U02 were investigated using the fourpoint bending technique. Strain rates were varied by two orders of magnitude, and test temperatures up to 1800°C were used. Data are presented on the ultimate tensile stress, yield stress, and plastic strain-to-fracture. Below the brittle-toductile transition temperature, T,, the material fractured in a brittle manner, with no macroscopic plastic deformation. Between T , and a second transition at a higher temperature, T L , a small amount of plastic deformation was measured before fracture. Beyond T t the strength of UO, decreased continuously, and extensive plasticity was observed. This high-temperature plasticity was characterized by a thermally activated rate-controlling process; this behavior is consistent with observations of creep behavior under high stresses. The following phenomenological equations for the strain rate fit the data for the material with 8-pm grain size above T L : P = -u?,' 2 '' exp -( -R~) 82,000 /h T and 2 87,000 8 = u~, , ' " exp -(xF) /h where u,, and o O q , are the proportional limit and steady-state flow stress, respectively, and temperature T is in OK.

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...

A. Cladding Requirements 7B. Pressure Bonding 71. Autoclave Testing 72. Thermocouple Failures 77 3. Bonding Tests 79 VII. Properties of High-Temperature Materials. 8l A. Properties and Materials Considered 82 B. Young' s Modulus of Unalloyed Tungsten 82 C. Young*s Modulus of Tungsten-25w/o Rhenium 85

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.

The crystalline nature and expansion characteristics of compositions in the region surrounding the cordierite area of the system Mg0-A1203-S O z were investigated. The selected compositions were fired to temperatures of 1250°, 1300°, 1350°, and 14OO0C., and the thermal expansion characteristics were determined with the interferometer. Coefficients of linear thermal expansion were determined in the temperature range of 20' to 300°C. and were found to lie between 9.8 X lop7 and 50.5 X The qualitative, as well as the quantitative, determination of crystalline constituents was made utilizing strontium fluoride as an internal standard in the X-ray spectrometer. In the compositions studied crystalline cordierite ranged from 38 to 977& There was a close correlation between the linear thermal expansion coefficient and the crystalline cordierite content of the bodies.series of compositions in the region of the system magnesiaalumina-silica surrounding the cordierite area was investigated.