1788
C O M M U N I C A T I O N S T O THE E D I T O R
The Uranium-Uranium Dioxide Phase Diagram at High Temperatures'
Sir: A study of the uranium-uranium dioxide phase diagram is underway at this laboratory. There is a need for a preliminary publication of the main features of this diagram, especially the location of the hypostoichiometric boundary of urania, in view of the large number of current investigations, in the U. S. and elsewhere, of the thermodynamic and transport properties of urania as a function of its stoichiometry. The essential features of the diagram are shown in Figure 1. No solid phases intermediate to urania and liquid uranium in the system are evident. The existence of a liquid miscibility gap of substantial width was established by the metallographic examination of arcmelted alloys of a wide range of composition. The monotectic composition was established as 65 i 5 mole % of UOz (0,KJ atom ratio 1.30 i 0.10). The monotectic temperature was determined as 2500 30' from experiments in which uranium was heated in uranium dioxide crucibles at selected temperatures in the vicinity of the expected monotectic. The liquidus and solidus data below the monotectic temperature were obtained from experiments in which uranium melts were equilibrated with uranium dioxide crucibles in a purified helium atmosphere and then cooled rapidly to room temperature. The liquidus data, which were obtained from the analyses of the uranium ingot products, are consistent with the data obtained , ~ are in sharp disagreebelow 2000' by Gray, et ~ l . but ment with the recent data obtained from 1530 to 2290' by Blum, et aL3 The solidus data, i.e., the hypostoichiometric boundary of urania, were obtained from the analyses of uranium oxide growths which formed between the uranium melts and the crucibles. At room temperature, these porosity-free growths were found to consist of relatively large crystals of uranium dioxide which contained uranium as a particulate phase located at grain boundaries and within the grains. It appeared that at the temperature of the experiments the uranium had been in solid solution in the urania crystals. In a separate s t ~ d y the , ~ hypostoichiometric
The Journal of Physical Chemistry
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Figure 1. The uranium-uranium dioxide phase diagram a t high temperatures.
boundary is being established by the equilibration of uranium dioxide pellets via the vapor phase with mixtures of uranium and urania. Thus far, good agreement has been obtained between the two studies at temperatures below 2100'. Although this appears to be the fmt report of the location of the hypostoichiometric boundary of urania, many investigators, e.g. , Anderson et al. ,6 have reported that urania can exist at hypostoichiometric compositions a t high temperatures. (1) Work performed under the auspices of the U. 5. Atomic Energy Commission. (2) A. G. Gray and J. R. Thompson, N.B.S. Grasselli Laboratories Progress Report on Metallurgy for Dec. 1944,CT-2618,1944. (3) P. L. Blum, P. Guinet, and H. Vaugoyeau, Compt. rend., 257, 3401 (1963). (4) R. J. Ackermann and M. S. Chandrasekhariah, current studies at Argonne National Laboratory. (5) J. 8. Anderson, J. 0. Sawyer, H. W. Worner, G. M. Willis, and M. J. Bannister, Nature, 185,915 (1960).
CHEMICAL ENGINEERINQ DIVISION ARQONNENATIONAL LABORATORY
ALLAN E. MARTIN EDWARDS
RUSSELL K.
ARGONNE,ILLINOIS RECEIVED MARCH 25, 1965
