The diffusion and solubility of hydrogen in uranium dioxide single crystals

40 (1971)

JOURNALOFNUCLEARMATERIALS

THE DIFFUSION

189-194.

0 NORTH-HOLLANDPUBLISHINGCO.,AMSTERDAM

AND SOLUBILITY

IN URANIUM

DIOXIDE

OF HYDROGEN

SINGLE CRYSTALS

V. J. WHEELER UKAEA,

Applied

Chemistry

Division,

Atomic

Energy

Received

16 March

tritium tracer measurements. the equation D=O.O37 The

Establishment,

“C, (ii)

La

The data are fitted by

solubilite

O,4 pug Hz/g defauts

exp - (14300 & 900 calories/RT)cm2/s. of hydrogen

varies between

Die

0.03-

Didcot,

Beds.,

UK

Les donnees obtenues satisfont a l’equation:

D = 0,037 exp - (14300 *

:

solubility

Harwell,

1971

tritium.

The diffusion of hydrogen has been measured in UOz single crystals in the temperature range 500-1000 (i) vacuum-extraction, using two techniques:

Research

de l’hydrogene

UOz,

d&pendant

du monocristal

Diffusion

900 calories/RT)cm2/s. varie entre

0,03

de la structure

et des

d’UO2.

von Wasserstoff

in UOz-Einkristallen

wurde zwischen 500 und 1000 “C nach zwei Methoden,

0.4 ,ug Hz/g UOZ, depending upon the defect structure

durch Vakuumextraktion

of the UOz crystal.

mit

Tritium

gemessen.

und durch Tracermessungen Aus

den Daten

ergibt

sich

die Gleichung La diffusion de l’hydrogene monocristaux

de bi-oxyde

de temperatures 500-1000 1) l’extraction

1.

D=

a ete mesuree dans des d’UOz,

dans

Introduction

900 cal/RT)cm2/s.

Die Loslichkeit von Wasserstoff

l’intervalle

schwankt zwischen

0,03 und 0,4 pg Hz/g UO2 und hiingt von der Defekt-

“C, en utilisant 2 techniques:

sous vide, 2) des mesures par le traceur

0,037 exp - (14300 f

struktur

des UOz-Kristalls

ab.

The present paper describes an experimental study to remedy this gap in our knowledge of an increasingly important nuclear fuel.

A considerable volume of work has now been published on diffusion phenomena in uranium dioxide. Both cation and anion self-diffusion have been studied in great detail, and work

2’

on oxygen diffusion in temperature, electrical and chemical potential gradients has also been described. Fission product migration has been the subject of a number of investigations. A somewhat surprising omission is the absence of any investigation of the solubility and diffusion of hydrogen in UOs. The final stage in the preparation of UOs fuel pellets often involves sintering in hydrogen at - 1650 “C, yet despite some evidence for hydrogen solubility reported by Roberts I), no attempt has previously been made to measure the solubility, nor the rate of diffusion through UOs, although hydrogen solubility and diffusion in a number of other oxides (for example SiOs, TiOs and ZnO) have been reported.

2.1. MATERIALS UOs crystals were obtained as grown from the vapour phase at Mol, Belgium 7, the impurities determined spectrographically being Cr < 50, TiZn< 20, FeCo < 10, Ni 5, all others < 5 ,ug/g. Experiments were carried out on four separate crystals ground to the form of spheres (about 1 mm radius) in order to simplify the mathematical treatment. It was found that the crystals were undamaged during the experiments, and so could be used for a number of separate runs. The gas used for the vacuum extraction experiments was British Oxygen Company “high purity” (99.99%) cylinder hydrogen. For the tritium tracer experiments, B.O.C. 189

Experimental

190

V. J. WHEELER

“Grade

X”

hydrogen

with 1 curie of tritium

was

the Radiochemical Centre, to each 1 litre flask. 2.2. 2.2.1.

used

(98%),

(99.995’$‘0)

obtained

Amersham,

from added

determination carried out. The sequence of operations

is illustrated

in

fig. 1. Analysis of the gas evolved from the UOs crystal by means of adsorption by a

EXPERIMENTAL METHODS

Hydrogen

UOZ crystal could be calculated. Finally, this gas was pumped off and a final “blank”

vacuum extraction

The selected crystal was prepared by heating in a stream of hydrogen at the required temperature (670-1000 “C) for several hours, and then rapidly cooled by tilting the furnace tube so that the crystal fell into a collector at room temperature. It was then transferred to the analytical apparatus, which consisted of a silica or mullite tube connected to a vacuum system, and incorporated a McLeod gauge (for accurate pressure measurements) and a Pirani gauge, the signal of which was recorded continuously on a Honeywell recorder. The furnace tube was outgassed at the temperature of the experiment, until a constant “blank” leak rate was obtained when the pumps were switched off. The UOs crystal was then dropped into the furnace tube and the hydrogen evolution followed until the recorder trace showed that the “blank” leak rate had again been reached. The pressure in the system was then measured with the McLeod gauge, from which the total quantity of hydrogen in the

palladium wire and by mass spectrometry showed that it consisted entirely of hydrogen. 2.2.2.

Tritium

tracer measurements

The selected crystal was heated in hydrogen/ tritium at the required pressure (0.1-0.9 x 105 N em-s) and temperature (500-750 “C) in a, vacuum system for several hours and then rapidly cooled as before. It was then transferred to the side-arm of a furnace tube, the furnace being maintained at the required temperature (again, 500-750 “C), and the tube swept with flowing argon (99.995% purity). The exit gas was mixed with a stream of carbon dioxide (99.5% purity) to give a mixture A : COs = 95 : 5, with a total flow rate of 100 ml/min, and passed F. 0111 gas-flow proportional into a “Panax” counter, which has a counting efficiency of about 65% for sHs. After measuring the background count rate (usually 3-4 counts per minute), the UOs crystal was tipped from the side-arm into the furnace tube, and the count rate measured.

TIME(S) Fig. 1.

Typical recorder chart plot

for vacuum extraction of hydrogen from UOz single crystal (radius 1.30 mm) at 1000 “C.

DIFFUSION

AND

SOLUBILITY

OF

HYDROGEN

The rate of diffusion of tritium from the crystal was thus determined, and the total number of counts obtained in each experiment gave a measure of the hydrogen content of the crystal, which could be compared with the pressure and

IN

3.

volume

SINGLE

191

CRYSTALS

stirred fluid of infinite volume:

&Cl--

,“z>y -$exp -

- F

(I)

where ~~=fraction

temperature of treatment. The absolute correlation of gas count rate with total quantity of hydrogen was determined by counting a measured

u&

of

diffusion

pleted at time t(s) D = diffusion coefficient

process

com-

(cm2 . s-1)

a = radius of sphere (cm).

of gas.

Results

Experimental values of 01 (fraction of the diffusion process completed) as a function of time, for various crystals at various temperatures, are shown in fig. 2. These curves have, of course, all been corrected for the “blank” or background corresponding to each run. There are no individual experimental points for the vacuum extraction curves, since these have been derived from continuous recorder chart plots. The experimental data were analysed by the mathematical treatment described by Crank s), assuming diffusion from the crystal into a well-

A calibration plot of 01 against Dt/a2 may be constructed from this equation (see ref. 3, figure 6.4), leading to a plot of Dt/a2 against time, the required values of Dt/a2 being taken from the corresponding values of oc at the appropriate experimental time t. Figure 3 shows some plots of Dt/a2 against time. If eq. (1) is obeyed, these should be straight lines through the origin, of slope Dla2, from which D can be calculated. Figure 3 shows that the equation is, in fact, fairly closely followed - deviations may be due to (i) imperfect roundness of the spheres (ii) the difficulty of determining t = 0 exactly, since the crystals take a finite time to reach the temperature of the furnace, and- in the case of the tritium experiments-the sweep gas takes tracer N 10 set to pass from the furnace to the counter. In practice, errors introduced by these effects must be very small. (iii) A larger error in the 1rlacuum extraction experiments is

0.0, ,

-

0

CRYSTAL RADIUS

0 A

I~3Ornrn

.......,......

,.,Prnrr, 0.04

1..omm VACUUM

EXTRAcrlON

TRlTlw.4

MEAS”REULNTS

2, %

0.01

Fig. as

c

2. a

Fraction of diffusion process completed (a) function of time, for various crystds and temperetures.

Fig.

3.

Dt/az as &function

of time, derived from fig. 2.

192

V.

introduced

J.

WHEELER

by the response of the Pirani gauge

and the comparatively high “blank”. In the tritium tracer experiments the recording of N lo4 counts in a typical experimental run, and the very low background obtained with the counter, gives reliable values for LY, and correspondingly more accurate values for D. Within experimental error, the diffusion coefficient was found to be independent of the temperature and pressure of saturation of the

Values

for

the

hydrogen

solubility

varied

widely between limits of about 0.03-0.4 ,ug Hz/g UOs, with no simple correlation with the temperature and pressure of treatment. No systematic measurement of temperature or pressure-dependence was possible therefore, the reasons for which are suggested below. The surface area of the crystals used was N 2 x 10-4 m2.g-l, so that surface adsorption of hydrogen

crystal, and to depend only upon the temperature of hydrogen/tritium removal. Figure 4 shows values of D as a function of temperature, where it is seen that agreement between the two experimental techniques, and from crystal to crystal, is extremely good. From least squares analysis of the data the diffusion coefficient is given by:

must have contributed only a Trerysmall amount to the total. When at the completion of a diffusion experiment, the crystal was heated to a higher temperature, a further release of hydrogen was sometimes observed. This too did not appear to depend in any regular manner on the preparative conditions, but was usually N 5-15o/o of the “normal” hydrogen release.

D = 0.037 exp

4.

- (14300 & 900 calories)/M’

1°CI

TEMPERATURE

1000 1

700 I

800

900 I

I

cm2 .s-l.

600 I

500 1

0

lo-6~ 9

9

IO

II

I2

13

!+-I) Fig. 4. Diffusion of hydrogen in UOz single crystals. Open symbols : vacuum extraction. Closed symbols : tlitium

tracer

measurements.

The

different

represent different crystals.

shapes

Discussion

The results show a pattern of very rapid diffusion of hydrogen through the UOs structure, but very low - and irreproducible - solubility. The explanation of these facts lies in a detailed consideration of the crystal structure. UOs has the face-centred cubic (fluorite type) lattice, with a=O.547 nm. It has a wide range of composition over the single phase region (UOs+%) in which oxygen dissolves interstitially, up to a limit ICN 0.2 at 1000 “C. At higher temperatures (> 1500 “C) oxygen may be lost from the structure (to give U02_z), while maintaining the single phase. Neutron diffraction measurements have shown that nearly stoichiometric UOs contains a balance between vacancies on normal anion sites and interstitial anions, the two types of defect ordering into “clusters” in different regions of the crystal 4). The interstitial oxygen is not accommodated in the 8 i + position in the unit cell, but at two sites slightly displaced from it. The thermodynamic properties of UC2 change very rapidly near to the stoichiometric composition, and discrepancies between different sets of measurements have been ascribed to differences in the balance between anion vacancies and interstitials from sample to

DIFFUSION

AND

SOLUBILITY

OF

HYDRO#EN

IN

u&

SINGLE

CRYSTALS

193

of

using a galvanic cell technique have shown that

specimens etc. 5). Now the radius of the hydrogen molecule (0.12 nm) is slightly smaller than that of an oxygen ion (0.14 nm), and it is reasonable to

even with such small differences in composition there are detectable differences in the thermodynamic properties 12). This factor is probably the cause of the variable hydrogen solubility

suppose that the hydrogen molecule ean enter a vacant or unoccupied interstitial anion site,

results : small, undetectable variations O/U ratio and the vacancy/interstitial

without altering the uranium valency, and diffuse through the structure by a vacancy/ interstitial mechanism. An analogous mechanism has been proposed to explain the diffusion of hydrogen through zirconia 6). ESR examination of UOs crystals, heated in hydrogen and then irradiated by y-rays at 78 K, shows no signal due to hydrogen atoms 7), suggesting that the hydrogen is present as molecules (hy~ogen atoms have been detected by this technique in CaFa 8) and MgO9)). The additional release of hydrogen on further heating of the crystal is evidence for more than one type of site, which may he related to the choice between vacancies and interstitials. It is conceivable that - OH groups may be formed, under circumstances involving a valency change of the uranium, but unfortunately it is not possible to test this directly by infrared s~etroscopy due to the high absorption of UOa, although this technique has been successfully applied to SiOa 10) and TiO:! ii) at this level of hydrogen content, and in these cases the -OH group is usually associated with a lower valency cation imp~ity, such as Ala-t or Fea+. In the present experiments, the low

balance, arising from the preparative conditions, will influence the number of sites available in the structure. The very low saturation concentration of hydrogen suggests that only a few sites are available in any ease, but the mobility of the hydrogen molecule is sufficiently high that its diffusion is not significantly hindered. It is somewhat surprising that hydrogen can move so readily from site to site, yet more hydrogen apparently cannot dissolve in the structure. This behaviour is also observed for SiOs and TiOs. Further evidence for this explanation is that the hydrogen content of UOZ crystals heated at 1000 “C in dry hydrogen is always greater than that of crystals heated in hydrogen which has first been bubbled through water at room temperature, a phenomenon which has also been observed for zirconia”). Evidently the slightly higher oxygen potential of the watersaturated hydrogen facilitates an increase in the interstitial oxygen ion concentration of the UOa, although this is not detectable by the

activation energy for diffusion, and the variable hydrogen content from experiment to experiment, suggest that impurities are not significantly involved in the case of UOa. It should be noted that even the upper limit for hydrogen solubility (0.4 pg/g UOa) only corresponds to 1.4 x lOr*Ha molecules/cma, or about 1 hydrogen molecule in every 5000 UOa unit cells. If this number of interstitial oxygen ions were added to stoichiometric UOz, the O/U ratio would only be increased to 2.00006: this is beyond the limits of control or detection by present preparative or analytical techniques (i 0.0005), but careful coulometric titrations

in hydrogen gas may contain up to 2-3 ,ugHaj g UOz ; some of this will be present as “dissolved” hydrogen, but most will be trapped in closed pores or at grain boundaries. For a pellet with 5% closed porosity, if these pores are filled with hydrogen this will correspond to N 0.4 pg/g UOZ -a level about the same as the upper limit for the “lattice” solubility already discussed. It is diRicult to estimate the quantity trapped at grain boundaries, and one can only speculate as to the mechanism here, but the high defect concentration may allow comparatively high hydrogen solubility : the present results suggest l-2 E*lg/g.

sample, depending

on the thermal treatment

in the anion

available techniques. These results have some significance for the commercial production of UOz. Pellets sintered

V.

194

The diffusion

coefficient

J.

WHEELER

is such that most

of

this gas will be released on heating to “C in vacuum or in an inert gas stream, 20% may be retained, and is likely to but be released subsequently during irradiation. Our N 500

experiments suggest that if O/U> 2.00 the hydrogen content will be reduced; on the other hand, if O/U < 2.00, uranium metal will probably be precipitated on cooling, leading to much higher hydrogen contents, owing to its high solubilitv in uranium or. conceivablv. to the formation of uranium hydride. Yl

Lierde,

3)

Strumane, Mater.

J. Crank, The Mathematics University

4)

R.

J. Nucl.

E.

Smets

5 (1962)

and

250

of Diffusion (Oxford

Press, 1956) p. 86

B. T. M. Willis,

Nature,

197 (1963)

755

6) C. E. Holley (ed.), Thermodynamic and Transport Properties

of

Uranium

Phases (IAEA,

Vienna

Dioxide

and

Related

1965)

6, T. Smith, J. Nucl. Mater. 18 (1966) 323 7)

A. J. Tenth (Harwell)

and J. F. J. Kibblewhite,

unpublished

AERE

work

8) J. L. Hall and R. T. Schumacher, Phys. Rev. 127 (1962) 9)

P.

W.

1892

Kirklin,

J. Phys.

P.

Anzins

and

J.

Chem. Solids 26 (1965)

E.

Wertz,

1067

and J. M. Stevels, Phys. Chem. Glasses 3 (1962) 69

The author would like to thank Mr. A. Parker and Mr. T. J. Webber for help with the vacuum extraction experiments.

11) G. J. Hill, British J. Applied 1151;

0.

J. Chem. Sot.

(1955)

3939

W.

P. I. Kingsbury, 1s)

Johnson, Phys.

W. Rev.

Phys. D.

4178

(1962)

1 (1968)

Ohlsen

175 (1968)

T. L. Markin and R. J. Bones, UKAEA AERE-R.

l?PfPrn”FmT

L. E. J. Roberts,

van

S. Amelinckx,

10) A. Kats, Thesis, Delft, 1961; A. Kats, Y. Haven

Acknowledgements

1)

2) W.

and

1102 Report