Hydrofluorination kinetics of doped uranium dioxide

HYDROFLUORINATION

KINETICS

OF DOPED

URANIUM

DIOXIDE

’

W. P. ELLIS Los

Alamos

Scientifcc

Laboratory,

Received

Los

30 January

Based

on work

performed

under

the auspices

Selo

Mexico,

USA

1965

MgO. With 5 y. MgO, however, more than 90 y. of the Mg++ appears to be in solution in the UOz. By the interference color of the fluoride film, it was observed microscopically that although CaO enters the ent,ire UOS matrix, it does so inhomogeneously in a smoothly delineated dendritic array without sharp boundaries, and that MgO also enters non-uniformly but much more evenly distributed than Ca+-t. Both Y203 and ThOa form homogeneous solid solutions in which t’he nonuniformity, if present, is submicroscopic. These observations were confirmed by electron microprobe examinations of the substrate surfaces on all but the ThO2 doped sample for which no probe analysis was run. For material doped with CaO, t’here are 25 7; variations in coilcentration of Ca++ across the surface as detected by the electron probe. Kinetic parameters were chosen such that films wit,11 second-order interference wavelengths of approximately 5000 B were produced on UO2. Conditions varied slightly from reaction to reaction so that the interference wavelengths, &, are reported as ratios of R- &(doped)/& (UOe) where ;lk(UOz) is t’he wavelength for Ohe simult8aneously reacted reference pure fused UOz and &(doped) is the average of the t’ol) and bottom sections of the fused oxide pellet. To a first approximation, film thickness is proportional to &. The exact relationship is given in ref. “). The dat,a are summarized in fig. 1 in which the rat#io, R, is plotted vs the

Previous studies of the hydrofluorination kinetics of Urania have demonstrated that the reaction is limited by diffusion 1, 2), but the specific rate-limiting mechanism has remained unknown. In the study reported in Ohis note, an effort was made to understand the specific transport processes by attempting to alter the equilibrium concentration of point defects in the product through the introduction of impurity cations into the reactant UOz. It was found, however, that doping in the range 1-5 y. (mole basis) does not alter the kinetics appreciably. Separate mixtures of high-purity UOz with 1, 3 and 5 y0 (cationic mole basis) Y203, MgO, CaO and BaO and 6 o/o ThOz were fused inductively in an argon atmosphere in a tungsten susceptor. The sequence for melting has been described previously 3). Surface preparations, the reactor, procedures and conditions are given in ref. 3). The extent of reaction was determined spectrophotometrically on a Cary 14R by the measurement of interference wavelength 3) and each specimen was routinely examined on a Bausch and Lomb research metallograph t’o judge the quality of the film. ,4 ceramographic examination of the lapped fused oxides showed that in the composition range 1-5 o/O, CaO, YzO3 and ThO2 form single phase solid solutions with UO2, but BaO forms only separate minor phases and many bubbles. Magnesium oxide enters the UO2 lattice but there are minor second-phase segregations at grain-boundaries in all samples doped with t

Alanlos,

of the U.S. 212

Atomic

Energy

Commission.

HYDROFLUORINATION

~~u~t~~gcomposition

KINETICS

of un-fused

powder.

The

OF

DOPED

URANIUM

then in 71 with

both

DIOXIDE

213

YsOa and MgO is too

limits in the ratios indicate the spread between the top and bottom sections. Both MgO and CaO retard the rate of hydro-

slight to account for R # 1. Nor can the effect result from a large deviation in 0 as shown by the finding that R of first-order films is the same

fluorination, YsOs increases the rate, and 5 y0 ThOs has no significant effect. The shallow

within a percent or two of the R of second-order films. If 0 were entirely responsible for R # 1,

curve for BaO agrees with the observation that this dopant either boils off or forms separate

a much greater enhancement with first-order layers would be expected 3). Thus the effect in fig. 1 appears to be a basic modification in the

/

I

I

kinetics rather than an optical effect. The hypothetical value of R for CaO and MgO

I

phases rather than a solid solution and thus is of no particular value in this study. The effects upon the rate of hydrofluorination are very

dopings for uniformly homogeneous material is not, however, necessarily the same as in fig. 1 because of non-uniform concentrations of dopant. The reflectance speetrophotometer averages over most of the surface and does not resolve microscopic heterogeneities. Also, the value for 5 o/o CaO may be high because of loss of CaO by volalitization during melting. The values for YzOa and ThOs are undoubtedly nearly correct. Irrespective of these limitations, the major observation to be drawn from fig. 1 is unaltered : the hydrofluorination kinetics of UOs at 285’ C are not greatly altered by the addition of impurity cations in the range l-5 mole percent. The same was shown to be true for the temperature range 231-285’C. The conclusions regarding reaction mechanisms to be drawn from fig. 1 are limited by the lack of knowledge of the defect structure of pure UF4 vs that doped with di- and trivalent cat’ions. Very little work has been reported in the accessible literature on the binary phase

slight, e.g., R = 1.08 for 5 y. YsOs, 0.94 for 5 % NgO and 0.92 for 5 y0 CaO. The agreement among pure UOZ samples fused separately is typically 4 “/b or better so that ratios not equal to unity cannot be attributed to random fluctuations or to a normal spread among samples. Among the optical factors which affect & for a given film thickness are the refractive index of the film, ql, and the net discontinuous phase shift, 0. By immersion spectrophotometry 4) it was determined that Q (on 5 o/o MgO)= 1.609 + 0.003 at 4721 _L%, ql (on 5 y0 YzOs)== 1.595 f 0.00% at 5235 L% and ~1 (on pure UOa) = 1.61 f 0.02 at 5000 8. The change

diagrams of UF4. It has been observed that BaFz does not dissolve to a detectable extent in UF*, ref. 59 6) and that, ThF4 forms a continuous solid solution 7). It was found in this study that at least 5 “/ CaFz dissolves m UPa to form a single-phased solid solution, but 110 information regarding defect structure was obtained from this or the kinetic results. It thus appears that concerning the rate-limiting transport mecl~anism, with respect to this study, at least one of two alternatives is correct: either the impurity cations did not upset the equilibrium concentration of the relevant point defects in the fluoride product or else the rate

I I

1

1

I

2

3

4

IMPURITY

Fig.

1.

purity 400

Plot

of

cationic

Torr

HF(g)

order int,erference

I

5

CATIONIC

MOLE %

iik (doped)/&(UOz) mole and

%.

vs starting

Reactions

90 min which

6

are

at

produce

layers approxi~natel~

im-

285” C, second-

2500 .& t,hick.

214

of

w.

hydrofluorination

P.

is not limited by diffusion

of ions through a thermally activated defective sublattice in the UPa layer.

point-

ELLIS

References 1) L. Tomilson, S. A. Morrow and 8. (iraves, Tranq. Faraday

Sot.

57 (1961)

1008

“) W. P. Ellis and R. W. Rohert)s, .J. (Ihem. Phys. The author wishes to thank E. A. Hakkila for the electron microprobe analysis, the Analytical Chemistry Group for composition and spectroscopic assistance manuscript.

analyses,

and R. J. Bard

A. D. Mulford

for

for reviewing

the

39 (1963) W.

1176

P. Ellis, J. Chem. Phys.

39 (1963) 1172 zi W. P. Ellis, J. Opt. Sot. Am 53 (1963) 613 5)

R. 11’. D. Eye and I. F. Fcrgueon, .J. Chem. Strc. (1959)

3401

6) J. IV. Starbuck (USA)

Report

and

C. R.

MCW-1469

Net,z,

~I\-Zallinckrodt

(1961)

7) C. F. Weaver, R. E. Thoma, H. Insley and H. 9. Friedman,

J. Am.

&ram.

SW.

43 (1960)

213