The precipitation of uranium metal in uranium dioxide single crystals

The precipitation of uranium metal in uranium dioxide single crystals

JOURNAL OF NUCLEAR MATERIALS7, THE PRECIPITATION NO. 2 (1962) 218-219,NORTH-HOLLAND OF URANIUM METAL IN URANIUM PUBLISHING CO., AMSTERDAM DIOXID...

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JOURNAL OF NUCLEAR MATERIALS7,

THE PRECIPITATION

NO.

2 (1962) 218-219,NORTH-HOLLAND

OF URANIUM METAL IN URANIUM

PUBLISHING CO., AMSTERDAM

DIOXIDE

SINGLE CRYSTALS

R. G. ROBINS

University College, Wollongong, NS W, Australia

Received

25 June

It seems likely that the preparation of uranium dioxide single crystals by any method in which the temperature exceeds 1600” C at low oxygen pressures will result in the p~~ipitation of uranium metal in the crystal because of the presence of an oxygen-deficient phase above this temperature. In crystals produced from molten UOs, there is definite evidence of uranium precipitates i-3) and even the vapour-deposited “single” crystals of van Lierde el: ~1.4) could contain precipitated uranium if the temperature of the crystal or its surface had reached 1600” C. The existence of an oxygen-deficient uranium dioxide phase was first reported by Anderson et a&5), who after arc-melting in an inert gas arc observed a second phase in the dioxide which they identified as /&uranium. The concentration of the second phase indicated an oxygen deficiency, at the melting point, of 2-6 mol %. Other observations

of urani~lm inclusions

1902

1600” C compacts having 0.5 mol y0 of added uranium show no second phase in the quenched specimen, and an extremely fine precipitate of uranium in the slowly cooled specimen. It is not suggested that the oxygen-defieient phase constitutes 0.5 mol o/o at 1600” C, since it appears certain that some of the uranium metal added initially has been lost by diffusion and evaporation. The metallographic detection of the uranium precipitate and the di~culties in mixing such small quantities of uranium metal with stoichiometric UOa powder under oxygen-free conditions limit the accuracy of this method in assessing the extent of the oxygen-deficient phase. Single crystals s) of electrolytic UOJ which have been heated to 1800” C show precipitation of uranium both on the surface and in the interior of the crystal. The precipitation on (I I l> faces appears very similar to the surface features on the vapour-deposited crystals of van Lierde et a2.4) (see fig. 3 of their paper) and it is felt that these markings are in fact uranium metal particles or voids left in the surface where uranium has previously existed. The observation of such voids has been made by the author and it appears that one of the electron micrographs of Colombo and Frigerio a) (fig. 6 of their paper) shows a similar void. The habit of the precipitated uranium particles has been determined by the author as (11 l>flOO>, but X-ray confirmation of the ~-uranium structure claimed by Anderson et n1.5) has been unsuccessful, although it appears from microhardness measurements that the precipitate is

in

arc-melted uranium dioxide have since been reported l-9, and Rothwell 6) has indicated that the oxygen deficient phase extends to as low a temper&ire as 1800"C. Experiments at present’ being carried out by the author 7) show that the oxygen-deficient phase may be retained to room temperature by quenching and that this phase still exists at temperatures as low as 1600” C with low oxygen pressure. These experiments are being carried out by annealing compacts of UOZ containing additions of uranium metal at temperatures up to 6500” C in vacua, and comparing a quenched specimen with a slowly cooled specimen. At 218

THE

significantly reported

PRECIPITATION

harder than or-uranium (this is also

by Colombo

and Frigerio).

It is obvious that the measurement of fundamental physical and chemical properties of single-crystal uranium dioxide would be seriously

affected by the presence of precipitated metal

either within

crystals.

uranium

or at the surface

of the

OF

URANIUM

219

METAL

References 1) A. Briggs, Private communication (1960) 2) J. Melehan et al., Battelle Memorial Institute (USA) Report,

BMI-1324

3) R. Colombo (1962) 41

W.

and

(1959) G.

p. 33

Frigerio,

J.

Nucl.

Mat.

van Lierde et al.. J. Nucl. Mat.

5 (19621 250

5i J. S. Anderson el al:, Nature 185 (19$0) 9;s 6) E. Rothwell, UKAEA (Harwell) Report, AERER 3897

5

259

(1961)

7)

R.

G. Robins,

to be reported

8)

R.

G. Robins,

J. Nucl.

Mat.

3 (1961)

294