Carbon monoxide equilibrium pressures and phase relations during the carbothermic reduction of uranium dioxide

JOURNAL

OF NUCLEAR

MATERIALS

38 (1971) 286-291. 0

CARBON MONOXIDE EQU~~R~M

PRESS~ES

NORTH-HOLLAND

PUBLISEINO

CO., AMSTERDAM

AND PHASE RELATIONS DREG

THE

CARBOTHERMIC REDUCTION OF URANIUM DIOXIDE * J. F. A. HENNECKE

** and H. L. SCHERFF

Chemistry Department, CCR EURATOM,

Iqwa, Italy

Received 23 August 1970

The csxbothermic reduction of uranium dioxide has been re-investigated and the CO-equilibrium pressures of the differentreactions,in&uliig equilibriainvolving the sesquic~rbidephase, were determined. From these measurements it results that the direct form&ion of the sesquicarbide is thermodynamically not possible under the reaction conditions usually chosen for carbide preparation. It follows furthermore, that within the temperature range investigated (16001800 OK) the sesquicarbide in equilibrium with free carbon is more stable than the dicarbide phase. Dana le cadre d’une nouvelle etude de la reduction carbothermique du bioxyde d’uranium, on 8 mesun la. pression partielle d’equilibre du CO forme au oours des dif?&entes r&actions possibles, y compris celles au tours desquelles se forme du sesquicarbure. Des resultats obtenus, il ressort que la form&ion de sesquioarbure est thermodynamiquement impossible

1.

Introduction

The carbothermic reduction of uranium dioxide is most commonly used in the preparation of uranium carbides, especially on the technical scale l-4). Because of its importance in the fabrication of nuclear carbide fuels detailed investigations of this reaction were carried out during the last decade, especially with the aim to find out the determining parameters, such as temperature, maximum permissible CO-pressure in the system, reaction kinetics 5-s), etc. in order to obtain a final product of a well defined composition with respect to the uranium/carbon ratio, residual oxygen impurities, etc. Several authors have measured the CO-equilibrium pressures during the reaction as a function of temperature and * **

dans les conditions habituelles de preparation du carbure d’uranium. On en conclut Bgalementque deans la zone de temperatures &udi&s (160~1800 “K) la sesquicarbure en equilibre aver du carbone libre est plus stable que le dicarbure. Im Rahmen einer erneuten Untersuchung der ka;rbothermischen Reduktionsreaktionen des Urandioxids wurden die CO-Gleichgewichtspartialdruokeder emzelnen Reaktionen gemessen, darunter auoh solcher, bei denen das Sesquikarbid eine Rolle spielt. Aus den Messergebnissen kann der Schluss gezogen werden, dass eine direkte Bildung des Sesquikarbids unter den iiblichen Reaktionsbedingungen fiir die Karbiddarstelhmg the~od~&~sch nioht mijglich ist. Es folgt daraus weiter, dass innerhalb des untersuohten Tem~r&t~bereiohes (160~1800 “K) Sesquikarbidim Gleichgewioht mit freiem Kohlenstoff stabiler ist ds Diearbid.

phases present in the equilibrium system 7-1s). As a general result of these studies it turned out that the reduction of uranium dioxide with carbon occurs essentially in two successive steps, each of which corresponds to a monovariant temperature-press~ equilibrium the following equations : u&+4 uos+3

C=Uca+2 uc2=4

UC+2

according

Co, co.

to

(1) (2)

These reduction reactions in practice usually being carried out at temperatures below 1800 “C, the “UCz” mentioned in the above equations actually corresponds to the tetragonal oc-UC&phase with a composition of about UC1.s. The different investigators agree that uranium mono-

Part of Ph.D. thesis of J. Hennecke, TU Br&~schweig, 1970. Present address: Gesellschaft fiir Kernforschung, Kaslsruhe, Germany. 285

286

carbide

J.

and uranium

A.

F.

AWD

are the only

H.

L.

SCHERFF

the disappearance

of free carbon

according

carbide phases being formed during the carbo-

reaction

sesquicarbide

would

thermic

according

reduction

diearbide

HENNECKE

of UOZ. The sesquicarbide,

U&S, is formed only in presence of excess carbon -- after the oxygen has been completely removed from the system 13)-according to the reaction 2 UC+C=U&3. These

observations

are

(3)

surprising

at

first

sight, as one would expect that the sesquicarbide would be formed as an intermediate phase between the dicarbide and the monocarbide during the carbothermic reduction reaction according to the equation: uo2+7

U&=4

U&,-t2

co.

(4)

(1)

the

to equation

to

form

(4).

In order to clarify the discrepancy between the results of equilibrium measurements in uranium oxide/carbon/carbide systems T-12) and the results of thermochemical general conclusions diagram,

calculations

and

drawn from the UC phase

the following

work was carried

out

with the aim to measure, at least indirect-ly, the equilibrium CO pressure corresponding to reaction (4). 2.

Experimental Our experimental

arrangement

was similar

to that described by Piazza and Sinnott 8). A schematic drawing of the reaction furnace used is shown in fig. 1. In order to ensure the complete removal of gases adsorbed at the inner surfaces of the furnace, in particular those of graphit‘e,

Knacke et al. 14) in a thorough theoretical thermochemical treatment of the uraniumsystem have calculated the oxygen-carbon equilibrium CO-pressures of all of the abovementioned reactions (1, 21 4), using tabulated thermodynamic data of tahe phases involved. According to these results, reaction (4) should indeed take place as second step in a sequence of reduction reactions finally leading to the

the furnace was thoroug~y outgassed at temperatures above 1500 “C under vacuum, the reaction cell being suspended above the heated zone from a carbon cord. After outgassing the furnace its temperature was regulated to the

monocarbide at temperatures between 1500 “C and 1780 “C, the upper stability limit of the sesquicarbide. At temperatures below 1500 “C, the results suggest the direct formation of the

desired value nation. The vacuum line manometer,

sesquicarbide equation :

The

according

for the following pressure determifurnace was separated from the and connected to a compression and the reaction cell was then

to the following

2 uoa+7

c=u~Ca+4

discrepancy

between

CO. this

(5)

Carbon

theoretical

treatment and the experimental results of Piazza and Sinnott 8) -e.g. that no sesquicarbide was observed upon structural determinations of the solid phases present after the reaction -was explained assuming supersaturation and kinetic reasons. With regard to the U-C phase diagram, the formation of U&a as an intermediate appears to be possible. Sears and Ferris 15) have shown that, within the carbon rioh region of the phase diagram (1.5
cord

Pyrex Quartz A’,% Graphitepowder Graphitecylinder -High

frequency

Graphite

coii

crucible

AL2o3

Fig. 1.

Schematic drawing of the reaction furnace.

THE

CARBOTHERMIC

REDUCTION

OF

URANIUM.

287

DIOXIDE

TABLE 1 Starting materials Substance uoz

*

Source

Lattice parameter

’ E. Merck AC ,,~&lysenrein”

C

RingsdorfPWerke ,,spektr&ein”

uca

NWKEM

U&3

Own

UC

NUKEM

5.4663 i_ 0.0003 .&

a0 3.517 f 0.001 A CO6.016 f 0.001 A

preparation

8.72

8.0899 f 0.0002 kp=)

east material

/

4.9611 + 0.0002 A

5.02

Material contains varying amounts of UC of lattice parrameter: 4.9617 & 0.0002.

lowered down into the hot zone, which oonsis~d of an inductively heated graphite cylinder (see fig. 1). Table 1 gives the characteristics of the raw materials used in our experiments, Samples were prepared by mixing appropriate proportions of the respective starting materials and pressing them to pellets. The CO pressures developing in the system after the sample had been brought into the heated zone were followed as a function of time, and the final reading was taken after the pressure remained constant for at least + h. In order to check whether the pressure thus obtained was identical to the equilibrium value, the system was partially evacuated and the re-establishment of the equilibrium pressure observed once more. As a

The staging as well as the reaction products were subjected to a thorough structural analysis by the Debye-Scherrer method using a Philips goniometer PW 1050/25-PW 1049 and Nifiltered CuK,_, -radiation. Lattice parameters were calculated from the different reflexions observed using the Nelson-Riley extrapolation function. By comparison to standard mixtures it was also possible to determine the relative amounts of phases present in the mixture. 3.

Results and discussion

to

Starting with mixtures of uranium dioxide and carbon our results for reaction (1) essentially confirm those of Piazza and Sinnott *), and we therefore limited our measurements to temperatures between 1500 “C and 1350 “C, a region

a somewhat higher temperature and cooled down to the starting temperature in order to check whether the same pressure was obtained again. The temperature was measured pyrometrically through the hole of the reaction cell. The composition of the gas phase was checked by mass spectrometric analysis of small samples of gas which were obtained sealing off small glass capsules of ca. 10 ml volume of which several were connected in parallel to a side line. It could be thus demonstrated that in all oases the gas phase consisted of > 99.9% of CO, the residue being due to hydrocarbon impurities from the pumping system of the mass spectrometer.

not covered by the above-mentioned authors. As it can be seen from fig. 2, a straight line can be drawn through our results and those of Piazza and Sinnott. When the reaction was continued until all free carbon had disappeared from the mixture, a second equilibrium was obtained corresponding to reaction (2). In this case too, the results of our measurements continue the straight line drawn through the experimental points of Piazza and Sinnott 8). Identical results were obtained starting with mixtures of UOa, UC2 and UC. Structural analysis of the reaction mixtures from UOa and carbon did not reveal any trace of the sesquicarbide.

further

check the system

was then heated

288

J.

I+“. A.

HENNECKE

AND

H.

carbon

L.

SCHERFF

on the reaction

were prepared obtained uranium into

equilibrium.

from the reaction

Mixtures

product

thus

and additional monocarbide and dioxide. The mixtures were pressed

disks

of

determination

& inch

dia and

used

of the equilibrium

for

pressure

the of

the reaction : uozi-3

I

0.52

0.94

0.56

0.56

0.60

Fig. 2.

Measured equilibrium pressures as function of temperature. n @ Piazza and Sinnott 8) 0 0 A own results. l 0 W&+3.9 c=uc1.9+2 co. W q 3 uo9+ 10 uc1.9= 13 UC+6 co. A U02+3 U&&.=7 UC-t-2 co. We therefore prepared U&s reacting uranium monocarbide with carbon, according to reaction (3). Pellets of mixtures containing an excess of uranium monocarbide were heated under vacuum for 15 h at 1450 “C. An excess of monocarbide was used in order to ensure complete reaction of the carbon thus avoiding t,he influence of eventually present excess free

CO-equilibrium

co.

(6)

as shown in the following: Rewriting eqs. (2) and (4) taking into account the generally assumed composition of the UC& phase within the temperature region of interest, * Note, that the former U-C phase diagram assumed 17) the coexistence of U&3 and free carbon at temperatures below 1500 “C. See also below.

TABLE

Phases in equilibrium

UC+2

The results of these measurements are reproduced in fig. 2 together with those of the two preceding ones. In table 2 the equations describing the temperature dependence of the CO equilibrium pressures are summarized for the three reactions investigated. To determine the equilibrium pressures of reaction (4), we have used a sesquicarbide prepared from the monocarbide as described above, but with an excess of carbon and hence containing some dioarbide. The results were not very much different from those obtained by reaction (1). We therefore did not continue these experiments, because we were not able to make sure the absence of free carbon in our reaction mixture which was undetectable by structural analysis *. Instead of continuing these direct measurements we calculated the equilibrium CO pressures of reaction (4) from the data obtained for reactions (2) and (6), which is easily possible

id, I

U&s=7

2

pressure (atm) I.600 “K < T < 1800 OK

uo~/c~ucl.s~~o

log pco=(9.88

+ O.ZS)-(21

UOa/UC1.9/UC/CO

log pco=(S.OO & 0.23)-(18

050 i

u02/u&/uc/c0

log pco=(6.55

800 & 200) ;

f O.lO)-(15

000 f 600) ; 550) ;

THE

CARBOTHERMIC

REDUCTION

OF

URANIUM

289

DIOXIDE

e.g. U&.9 : uo2 + 7op UC ~*~=39/8U~Cs+ZCO-dGa,

(4a)

uoz + IO/3 UC I.s=13/3UCf2CO-dGz

(2at

and combining

eqs. (2a), (4a) and (6) we find :

dG2= 8/21464+

13/214G6

from which in turn the following the equilibrium

(7)

equation

CO-pressure of reaction

for

(4) can

1&-

be obtained : logpco(4)

= 21/8

~ogpcoc2,

-

1313 1ogPcoW

(3)

Using the respective values from table 2, the following relation is obtained : log pcow = = 10.35 (+ 0.6)-21

700 (f

1500) l/T.

(Sa)

In fig. 3 are summarized the equations from table 2, together with eq. (8a). It can be seen from this plot that at, temperatures above 1300 “C sesquicarbide formation is thermodynamically not possible during the carbothermic reduction of uranium dioxide, since the free energy of reaction (4) is smaller than that of all of the other reactions possible. Hence, the sequence of carbide formation in the carbothermic reduction reaction at temperatures above 1300 “C proceeds as :

as

has always been observed At temperatures

fig. 3 the sequence

hitherto.

below 1300 “C according would

to

be:

UOZ + “UC2” -+ U&s which means that at these low temperatures no monocarbide formation will occur. Comparing our results to those of Knacke et al. id), large discrepancies are observed concerning the equilibria plotted in fig. 3, with the exception of that corresponding to reaction (1). From this it can be concluded that uncorrect thermodynamic data have been used, especially as far as the free energies of formation of uranium monocarbide and sesquioarbide are concerned.

d-

lo*-

$ O,M

Fig. 3.

0%

a58

0.62

[OK“]OS6

Comparison of the temperature pressure equilibria for different reactions.

In table 3 the free energies of formation calculated from our experimental data for a mean temperature of 1700 “K are compared to the respective values given by other authors. As it can be seen from this table, our values correspond quite well to the most recent data given by Storms 19). Indeed, Knacke’s 14) values can be seen to be significantly too low, as far as the mono- and sesquicarbide are concerned. A possible source of error in measurements of this kind is the eventual presence of oxygen dissolved in the lattice of the respective carbide phases 173 19). As mentioned above, the lattice parameters of our reaction products were determined after each run and compared to the lattice parameters of the phases present in the starting mixtures {see table 1). In neither case a significant change of lattice parameter could be observed for the UC as well as the W&-phase.

290

J.

F.

A.

HENNECKE

AND TABLE

H.

L.

SCHERFF

3

l?ree energies of formation of nranium carbides at 1700 OK Carbide phase

! /

-dG

(kcal~mole)

Reference

! !

26.5

a-UC1.9

’ i

27.9

UZC3

/

j

/ /

MacLeod 20)

26.6

Piazza,

26.5

Grieveson 9)

Sinnott 8)

26.2

Storms IQ)

25.0

Knacke 14)

21.5

Rand,

Kubaschewski 18)

53.2

/ This work *)

52.6

storms IQ)

44.9

Knacke

42.4

Rand,

’

14)

Kubaschewski

1s)

This work *)

26.0

Piazza,

25.5

I Storms Is)

21.4

j

Leitnaker 21)

27.4

26.0

UC

This work *)

Sinnott 8)

Leitnaker 21)

21.0

j

19.1 18.3

Knacke I*) Grieveson 9)

~ Rand, Kubasohewski 18)

I *

Calculations

based on free energy

functions

for UOz as given by Aokermann

and Thorn 22) and for

CO taken from ref. s3).

Only when the reaction was continued until all the higher carbide present (UC2 or U&a, respectively) had completely disappeared the HO-equilib~um pressures became irreprodu~ible

impossible. From this it can be concluded that within the temperature rage investigated (160~1800 OK) the dicarbide is somewhat less stable than a mixture of the sesquicarbide and

and at the same time the lattice parameters of the monocarbide phase decreased considerably owing to the pick-up of oxygen into its lattice. This means that besides eventual traces of in the starting material oxygen present (according to chemical analysis being of the order of several 100 ppm) no further oxygen did dissolve in the carbide phases under equilibrium conditions.

carbon, which agrees well with the U-C phase diagram as adopted up to now 17) and observations of Nickel and Saeger 24), but being in contrast to more recent findings by Sears and Ferris 15) who, on the contrary, were unable to observe the disproportionation reaction :

4.

Conclusions

It has been shown in the present work that during the carbothermic reduction of uranium dioxide at temperatures above 1300 “C only the dicarbide and the monocarbide phases are formed successively, the intermediate formation of the sesquicarbide being thermodynamically

2 UCz=U&s+C.

(9)

It should be noted, however, that the free energy of reaction (9) according to our values results as low as -0.2 kcaljmole of UzC3 at 1700 “K, a value which of course is very small as compared to the limits of error of our determination. The fact, however, that the formation of the sesquicarbide does not occur during the carbothermic reduction of uranium dioxide, as has been shown by ourselves and many other

THE

authors

before,

decomposition

CARBOTHERMIC

unambiguously reaction

REDUCTION

supports

of the

dicarbide

the and

OF

URANIUM

New

Nuclear

(Vienna,

Materials,

1963)

F. Anselin,

291

DIOXIDE Proceedings,

Vol.

1

349

G. Dean

et al., Symp.

Carbides in

hence a definitely negative value of its free energy of rea.ction . * This free energy of reaction

9

being very small might explain the observations of Sears and Ferris 15) who were unable to detect

7) 0. Heusler, Z. Anorg. Allgem. Chemie 154 (1926)

the decomposition

9

upon heat treating dicarbide

samples between 1260 and 1450 “C. It was shown furthermore that working under equilibrium conditions no oxygen dissolution in the lattice of the respective carbide phases, especially the monocarbide, takes place so that measurements of this kind may yield quite reliable values for the free energies of formation

of the carbide

phases involved.

Nuclear 1964)

Energy,

J. R. Piazza and M. J. Sinnott, Data

9

P.

7 (1962)

Grieveson,

to express their gratitude for his steady encouragein this work and to the fiir Bildung und Wissensupport.

This can be shown easily combining

lo)

3 (1966)

11)

P. Himmelstein

H.

reactions

12

)

Materials,

et al., Euratom

IAEA

N. R. Williams;-Proo.

4,

F.

Ainsley,

RBfract.

p. 64

175

T. Henney,

D. T. Livey and N. A. Hill, AERE

(1963) 814

15) M. B. Sears and L. M. Ferris, J. Nucl. Mater. 32 (1969)

9

101

J. F. A. Hennecke Crystal.

EUR

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B.

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Nuclear 1963) 507

et al., IAEA

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Materials,

et al.,

Conf. Pro-

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2 (1969)

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301

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E. K. Storms, The refractory carbides (New York,

9

M. H. Rand and 0. Kubaschewski,

1967) dynamic

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(London,

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of K.

Thermodynamics

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uranium Storms,

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compounds IAEA

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