RECEIVED BY 3. J ^ L / ^ 1968 ^ ^ ^ ,

A REPORT TO THE VIENNA PANEL (1968)

THE URANIUM-CARBON AND PLUTONIUM-CARBON SYSTEMS

by

Edmund K. Storms

Los Alamos Sc ien t i f i c Laboratory University of California

Los Alamos, New Mexico > , 8 7 5 ^

^ *

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A Makes any warranty or representaUon expressed or implied, with respect to the accu-" racy, completeness, or usefulness of the information contained in this report, or that the use t of any information, apparatus, method, or process disclosed in this report may not Infringe

privately owned rights, or

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with the Commission, or his employment with such contractor

Work done under the auspices of the V, S. Atomic Energy Commission.

-- •• •*-•• • — ••'^- — - - -•• • - - < « ' » ^ . I « ! * - ' " - S M J

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URANIUM-CARBON SYSTEM

1. Phase Relationship

I. Fhase Diagram

A considerable effort has been made in recent years to detezmine the

phase relationship in the U-C system. It is clear that this is a very

difficult system to study and, as a result, there are areas of disagreement.

Two problems are particularly troublesome; the proclivity to oxycarbide

and carbonitrlde formation with the important influence these impurities

have on the measured properties, and the unexpectedly slow reaction

kinetics.

A discussion of observations in this system which were reported before

early 1967 has been published by Storms (1967). A phase diagram, shown as

Figure 1.1,1s based upon this evaluation, and it summarizes much of the

current knowledge of the system. However, there still are several areas

of uncertainty which will be discussed in the following sections.

It is generally agreed that the formation of U^Cs is quite slow under

normal pressures, even in the absence of oxygen and nitrogen impurity*

For this reason, the conditions for its formation and its decomposition

temperature are still in some doubt. Henney et al. (1963) argue that the

presence of oxygen in concentrations above '>'1000 ppm inhibits the

formation of UgCs and that in the presence of UQg its decomposition

temperature is reduced to l650"c (Henney, 1966). Henry et al. (196?) find U2C3,

UC 0 , and u(C0)2 in equilibrium at 17OO* (Figure 1.2) in agreement with the X y

general observations of Anselin et al. (l96^) and Brett et al. il96k). It

would appear that an xmderstanding of this compound will have to depend

on more extensive studies of the oxycarbide system.

c/u/noMwmo

Figure 1*1 Phase Diagram of the Utanlum-Carbon System

sot to PHASES

a uc,o,'«-u,C|

0 oc,o, ••ue," • UC,0, 4^U0|

* UC,0,'tU,C,f'UCt* • UCiOy-fUCi'+UOi

# UC,0,«U4U0, ISM*I">*" nambcrt IIMIMII

OXYGEN, •tofflie HKtnt

Figure 1.2 Phase Relations in the U-C-0 System at 1700*G.

^2a.

Once formed, U2C3 appears to be equally difficult to decompose.

Neutron diffraction studies (Bowman, 1966) which have been carried into

the miscibility gap (Figure l.l) indicate that times over one hour are

needed to produce partial decomposition. This is supported by anneal

and quench experiments (Witteman and Bovnnan, 1965) so long as the

temperature is not carried above the misicibility gap.

A range of temperatures for the decomposition reaction 2UC2 » U2C3 + C

has been observed. Oxygen stabilization of UC2 should lead to a lower value.

Thus, Henney et al. (1963) argue that this temperature is 1T50* in the pure

system rather than ~1500* as has been observed by others. The value chosen

becomes rather important when the above reaction is used to add to the meager free

energy values for U2C3 (Figure 2.10). Studies by Witteman (196?) and by

Wallace et al. (1968) using material containing less than 100 pian oxygen

have given an average decomposition temperature of 1^20 ±10**. If this

small amount of oxygen is responsible for a 2 0** lowering of this tempera­

ture, then there Is no hope for an accurate measurement* However, there

has been no indication in any other direct measurement of this reaction

that oxygen could have such a pronounced effect*

In the absence of UgCs, the decomposition of UC2 into UC + C occurs

below 1520". On the basis of data given in Figures 2.4 and >

2.12, this should happen near 15T7°C. It is this reaction which accounts

for the frequent observation of UC in slowly cooled UC2 + C. Further

studies of this metastable region would be very useful.

It is clear from the above comment's that a further understanding of

the pure U-C system will have to depend on studies of the ternary systems,

U-C-0 and U-C-N*

II. Crystal Structure:

The structures of the various phases in this system have been

determined on a number of occasions and the excellent agreement

leaves little doubt about the results. A svmnnary of the structure

and lattice parameters is listed in !l!able 1.1.

Table 1.1

STRUCTURE AND LATTICK PARAMETERS OF a-U, UC, AND a-UCj

Phases Composition Lattice parameter Structure Investigator present oT first pliase (A)

a-U Pure A - 2.8539 ± 0.0001 Cmcm D'jl IVlucllcrand 6 « 5.8691 ± 0.0001 Ortliorliombic Hitlcrman(I960) c - 4.9554 ± 0.0001

UC + U UCo.,j a - 4.9563 ± 0.0007 fee Magnicr and Accary (1964)

UC + UCj UC,.o a m 4.9605 ± 0.0002 fee Figure 1 ' 3 U,Cj UjCj a - 8.0889 ± 0.0009 bcc Witteman and

Bowman (1963) a-UCj+C UC,.,, o - 3.519 ±0.001 Tetragonal Witteman and

c = 5.979 ± 0.002 Bowman (1963) a-UC, UC,.,4 a •« 3.5241 ± 0.0005 Tetragonal Witteman and

c = 5.9962 ± 0.0008 Bowman (1963)

Uranium metal has three crystal forms: orthorhombic, tetragonal,

and body-centered cubic. Only the first is stable at room temperature

and is seen in quenched material.

UC is fee and has a very small composition range at low temperatures.

However, at higher temperatures the lattice can accommodate not only

vacancies in the carbon sublattice but C2 groups can replace the single

carbon atoms. In this way, the cubic lattice occurs between UCo.es ancl

UCj.g at 2125*0. If the quenching rate is sufficiently rapid, the lattice

can retain most of the carbon vacancies and a small fraction of the C2

groups, depending of course on the stoichiometry of the sample. The

lattice parameter of UC in U + UC or UC + UC2 is consequently quite sensitive

to the quenching rate. Whether vacancies and C2 groups exist together at

high temperatures near UCI.Q is uncertain, but probable.

- 4 _

Figure l.J shows the variation of lattice parameter and the effect of

4.9640

4.9620

4.9600

« 4.9580

S 4.9560

4.9540

4.9520

I r

o As melted * Annealed IZhr ot 1300'

- A Annealed 88 hf al 1300' o At melied BurHey (I'Jfil)

_ • As melted Wiltemon (1963) ^ Magnier ond Accary (1964) Xh i Storms (1966a) "^ "-

- S Wilson (I960) 0 Witlemon and Bowman

(1963) ^ . - • Forr et al (1959)

arc melted UC + U a Witteman et al (1958) A

~ arc melted

Williams el ol (I960)

4.9500 I l-o—h I ^ 1 I 1 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 U 1.3

C/U, atom ratio

Figure l.J. Lattice parameter of UC as a function of composition.

various annealing times. This behavior is further complicated by the effect

of dissolved oxygen. When oxygen replaces carbon in the lattice the gross

effect is to lower the parameter. Small amounts of oxygen, in the presence

of uranium, however, have been found to produce a maximum (Magnier et al.,

1961*J Anselin et al., 196^). This effect, shown in Figure 1.4 is

apparently due to the presence of carbon atom vacancies in the lattice.

The vacancy concentration would depend on the ambient CO pressure and the

heating temperature, and this would account for the rather scattered results

in this region. Since the lattice parameter is occasionally used to indicate

the stoichiometry and/or purity, a better understanding of this behavior

should be sought*

*^*o

t»??

11**0

r^ -

' » 0 I

^

01

\

\

oa

vv

. - ot

'

*

s. » 4

1

AMfl« f i M . j ^

F i g . Xk The variation of lattice parameter of UCtHiO. with , pxygen content.

U2C3 was found to be bcc (l^jd) frcan x-ray (Mftllett et al., 1952) as

well as from neutron diffraction studies (Austin, 1959)* A lattice

parameter of 8.088A IS usuall y found regardless of how It is prepared.

The compound has a very narrow composition range as well as a low

solubility for oxygen and nitrogen. Its formation has been suggested

to result from a slij bit shift in the uranium positions (Glllam, 1962)

with the movement of the carbon atoms by interstitial diffusion accounting

for the slowness of formation. Magnier et al* (l964a) observed the phase

to fozm from UC2 by nucleation and growth. Its formation is facilitated

by the application of stress and, indeed, the use of pressure caja lead to

a rapid conversion (Witteman, I966).

The structure of a-UCa has been determined on several occasions by

neutron diffraction (Atoji and Medrud, 1959j Austin, 1959; A. L* Bowman

et al., 1966; Atoji, 1967)* Each assigned the CaCs-type structure {ik/mam).

A. L. Bowman et al. (1966) examined the structure at 1700*C and Atoji (1967)

extended the measurements from room temperature to 5*K* No structure change

MliiBAiMMillilUiaMUaimiiMta^HiafiWUI

or change in the magnetic properties was observed* Although no super-

lattice due to vacancy ordering was observed, the sensitivity may not have been

sufficient to see it* It is quite important to the thermodynamics of this

X^ase to determine if such ordering occurs and a further study at low

temperatures should be made using more sensitive techniques* The lattice

parameters of this phase increase as the carbon content increeuses, but

oxygen apparently has the opposite effect (Henney, 2$66). As yet, a

definite study of this behavior has not been reported.

Two conflicting stztictures for P-\JCs had been suggested by several workers

from indirect evidence, but until a neutron diffraction was used, this problem

could not be resolved. A* L. Bowman et al* (1966) have made such a study at

hig^ temperatures and found a structure similar to RCN with the C2 groups

either as free rotators or randomly oriented along the (ill) plane*

.7-

I. UC:

A. Low Temperature Heat Capacity

A number of low temperature heat capacity studies have been made

but with some differences. Westrum et al. (1965) used massive pieces

broken from an arc-melted button. Provided a surface oxide was not formed

during handling, this sample should have been oxygen and nitrogen

free, although no analyses were given for these elements. An average

composition for the three buttons used is UC1.039, which means a small

amount of UC2 must have been present. No correction was apparently made.

The thermal values at 298.15° are listed in Table 2.1. Andon et al. (l96ila)

made a similar measurement using arc-melted UC containing 2.5 mole UC2

for which corrections were made. The analyzed oxygen content of 0.12 wt

is rather high, but its effect on the measurements is not known since its

chemical form is unknown. These data are also listed in Table 2.1. The

rather limited values given by deCombarieu et al. (1963) are significantly

higher compared to the preceding studies, while the material was apparently

much purer. For subsequent calculations, an average between the values

reported by Andon et al* (l96^a) and those of Westrum et al* (1965) will

be used. Table 2.1

A COMPARISON OF MEASURED LOW TEMPERATURE THERMAL VALUES FOR UC. UjCj, AND UCi AT 298.15''K

C; S° H«8.is - Ho Assumed (cal/deg-mole) (cal/deg-mole) (cal/mole) composition investigator

UC n i l 14.28 2193 UC,.o Westrum er ol.

(1965) 11.84 14.03 2159 97.5 UC + 2.5 UC,., Andon et al.

(1964a)

U,C, 25.66 32.93 4829 UjCj Farr I't a/. (1965) 26.55 32.91 4836 75.3 U,C, + 10.6 UC Andon et a/.

+ 4.1 U C , . , + 9.9C (IV64b)

a-UC, 14.46 16.30' 2513 72.05 UC,.,o + 10.03

UC + 17.9 C 14.52 16.33' 2521 UC, ,4 14.50 16.31' 2522 4.7 UC + 95.3 UC,.„

Westrum et al. (1965)

Furrt-la/. (1965) Andon et al.

(1964a)

* Entropy of mixing between random C, and C, groups is not included.

B. High Temperature Heat Content

Within a short time of one another, two measurements of the heat

content of UC were reported, one at lower temperatures and the other to

near the melting point, both in excellent af^reement.* In the overlap

region there is aji average deviation from the least squares line of

+1$ and 40.^^, respectively, for the two studies as listed in Figure 2.1.

Levinson's values clearly show an increasing slope, i.e., heat capacity,

as the temperature is increased. The usual function which has

been used to fit such data (Maier and Kelley, 1932) is inadequate.

Consequently, Storms (1967) has chosen to use an equation of the form:

H^ - H29a = a + bT +0!^ + dT^ + e/T (l)

and Leitnaker and Godfrey (1967) adopted the fona

H^ - H298 = a + bT + cT^ + dT^^^ + e/T. (2)

Contrary to the experience of Leitnaker and Godfrey (1967), equation (l)

gives the smaller deviation from the data. By combining the data and

forcing C at 298.I5" to agree with the low temperature measurements,

the equation for IL,-H298> as well as thermal functions shown in Tables 2.2

and 2.3 were obteiined. The data and equation (l) are compared in Figure 2.1 "

The values obtained using the two equations differ only in the choice of

C and S° at 298.15°. P

Because of this increasing heat capacity, particularly in the case of

VCz, there has been a general reluctance to accept the data of Levinson.

In defense of his data I would like to point out that he has measured the

*To adjust to a common reference temperature (298"*K), 5OO cal were subtracted from the values of Harrington an'l Rowc {l06h) and ITO cal wore added to the values of Levinson (196^ ) . Vncn the former data were reported at the Harwell Conference, this correction was omitted.

- 9 -

heat content of six other carbide systems (XiC, ZrC, HfC, KbC, TaC and WC),

and in each case, where compaxison could be made with other measurements,

the agreement has been excellent. Most of the carbide systems, however,

show this rising C^ (TiC, ZrC, TaC) but not all (nfC, MhC, Wc). If this

effect were due to the apparatus used by Levinson, it is surprising that

each of his studies do not show the same behavior. Indeed, if other heat

content measurements which have been made by other workers in this tempera­

ture range (Nb, Mo, Ta) are fit by equation (l), a similar increase in C P

is found at high temperatures. The use of the Kelley equation in the

past has hidden this behavior (storms, 196?).

Table 2.2

Till RMAt, l-UNCMION.n OP UC,,„*

r CK)

298.15 300 400 500 600

700 800 900 1000 1100

1200 1300 1400 1500 1600

1700 IROO 1900 2000 2100

2200 2300 2400 2500 2600

2700 2800 2823

HT — !ll<tt (cal/molc)

0.0 22.18

1282 2618 3995

5402 ; • 6832

8285 9760 11260

12780 14320 15890 17490 19120

20780 22470 24200 25970 27780

29620 31520 33450 35440 • 37470

39560 41700 42200

c (cal/mole-deg)

11.98 12.00 13.06 13.59 13.94

14.19 14.42 14.64 14.86 15.08

15.32 15.58 15.85 16.13 16.44

16.77 17.11 17.48 17.86 18.27

18.69 19.14 19.61 20.10 20.60

21.13 21.68 21.81

ST (cal/molc-dcg)

14.15 14.22 17.84 20,82 23.33

25.50 • 27.41 29.12 30.67

32.10

33.42 34.66 35.82 36.92 37.98

38.98 39.95 40.88 41.79 42.67

43.53 44.37 45.20 46.01 46.8!

47.59 48.37

48.55

-in - iii->i>yT (cal/mole-deg)

14.15 14.15 14.64 15.58 16.67

17.78 ^ 18.87 • 19.91

20.91 21.87

22.77 23.64 24.47 25.26 26.03

26.76 27.46 28.15 28,81 29.45

30.07 30.67 31.26 • 31.83 32.39

32.94 33.48 33.60

*H'T - H\,n ,5 =- - 4 0624 x 10' + U3\yr - 1.5130 x tO-T* + 3.5038 X ID-'T' + 2.0828 x IOVr{cal/molc,298"-2P23°K, ±0.4%).

10 -

40 [—I—f—,—I I I I—r-T—1—1—7—1—1—I—]—I—1—r—p-1 f—1 r

200 400 600 800 lOOO 1200 l''00 h 00 lUOO 2000 2200 2400 2600 TempLraluro, "K

F i g u r e 2 . 1 , High lempcrature enthalpy of UC, o-

Tdble 2.3 High-tomporaturo tliormiS fi<nctloi)o of UO

Temperature

i'K)

208.15 300.00 400.00 500.00 UOO.Ofl

700.00 800.00 000.00

1000,00 1100.00 12UO.00 1300.00 1400.00 1500.00 1000.00 1700.00 1800.00 1000.00 2000.00 2100.00 2200.00 2300.00 2400.00 2500.00 2000.00 2700.00 2800.00

O'p (otJ/molo-dog)

12,11 12.14 13.08 13.67 13.80 14.15 14.38 i i n i ti Pi

, 15.03 15.33 15.50

•• 15.87 ' 10.17 '•

, 10.47 , 10.80

17.13 17.40 17.85 18.23 18.02 19.03 10.45 10.H8 20.33 20.78 21.25

H-r- / /"« . (cal/molo)

0 22

1200 202-1 3i'!tl! CUM Cn27 8277 0740

• 112H 12705 143U 1588-4 17480 10118

,, 207H1 224 78

• 24200 2.5071} 27770 20022 31005 33428 3r>3!l5 *-?l()5 .T'ifm -Jlf ("

S'T (cal/molo • dog)

l i . 2 8 n . 3 5 17.00 20.07

' 23.47 20.04 27.54 • 20.25 30.80 32.22 33.55 34.78 35.05 37.05 3H.U 3U.12 40.09 41.02 41.t)3 42.81 43.60 44.50 4C.32 40.12 ' f .!> 1

-S7nD 4f<.45

il'-°r-ir,n)IT (c»i/inolo'dog)

- 1 4 . 2 8 - 1 4 . 2 8 - 1 4 . 7 7 - 1 5 . 7 2 - 1 0 . 8 1

• - 1 7 . 0 2 . - 1 0 . 0 1

— 20.05 - 2 1 . 0 5 — 22.(10 - 2 2 . 0 1 - 2 3 . 7 8 - 2 4 . 0 0 - 2 5 . 4 0 - 2 0 . 1 0 -20 .8 (1

, —27.00 — 28.28 - 2 8 . 0 4 - 2 0 . 5 8 - 3 0 . 2 0 — 30.ml - 3 1 , 3 0 — 31.00 - 3 : : . 5 2 - 3 3 . 0 7 - 3 3 . 0 1

208.15"'K-2481''K

WV-/i°»M= U.l.M'T' - !."74 X 10-'7'» ^ 1 RfOx IC/T - f 3.473 V 1') ^T"* t.tJt,) , jo»

Avurego % error -0.81

- 11

In the absence of measurements, a nunber of estimations of the

heat capacity have flourished. Two of the more widely used attempts

are compared to several direct measurements and the results of the

present treatment in Figure 2.2»'

* * l — I — I — I — I — \ — I — I — I — r

rt-t

Ti ri< IAB ifa

-^i^ " ' iOITTCHM • KHMWCKIMMI

• ««iTmMIM«.tlHM

••HUlUMOMiitltM

t> IrtHtlOMMMMI \ t-MOMH AND KHUMH llMTt

ttMKMTUMi'R

Figure 2.2 Heat Capacity of UC

C. Heat of Formation from Ccmibustion Calorimetry

The heat of formation of UC has been measured by ccnbustion

calorimetry on three occasions. Farr et al, (l959) obtained -21.0 ± 1.0 kcal/.

mole after correcting for about ^> free uranim and 850 ppm oxygen. Droege

et al. (1959) gave -20.0 ± 5,0 kcal/mole, after corrections were made for

the l 4 ^ ppm oxygen and the uranium meteO. present. If a common heat of

formation for U3O8 is used, this value becomes -19,7 kcal/mole. A recent

recalculation of the former measurement by Huber et al., (1963) has led to

-21.7 ± 1.0 kcal/mole. This was subsequently accepted by the Vienna Panel

(1963). Based on a better estimation of the stoichiometry, this becomes

-21.1 db 0.9 kcal/mole for UCo*9e (storms and Huber, 1967).

- 12 -

The vaporization study by Storms (1966) s\icgested that the heat of

formation of carbon-deficient UC might be rather composition-sensitive.

Since carbon-deficient material was used in both of the calorimetric studies,

another combustion measurement was undertaken by Storms and Huber (1967)

using UCo.996 and UCi.osa* Values of -23.3 ± O.9 kcal/mole and -25.0 ±

1.0 kcal/mole, respectively, were obtained for A H (298.15°). The effect

of stoichiometry is shown in Figure 2.3.

tfM

t*o-

H.0

o

^ M.0

4

t l .O

t o o

I t o

tl.O

tT.O

••"I""

»-FAnn«toi tJi -. RCCALCULATION T

» - S T 0 f " 3 ( l | RECALCULATION !

• -THIS « o m

/

/ /

/

/

/ 8PECTR0t«ETE«

-UC •

oT 099 10 C/U, ATOM RATIO

-UC-*OC|

Figure 2.3. Heat of Formation of UC as

a Function of Composition.

- 13 -

D. Free Energy Measurements

The various high temperature studies which give free energy values

are compared in Figure 2.4. The shape of the curve is based on the thermal

functions listed in Table 2.2 and its position ha. been adjusted to pass

through the most reliable free energy value (l). Each of these measvire-

ments will be discussed individually in the order listed in the figure.

(1) The heat of formation from combustion calorimetry has been

converted to the free energy of formation by using the following thermal

^*'*^°°^* -fef. cal/mole-deg

UCx.o (Table 2.2) 1^.15

U (Flotow and Lohr, i960) 12.00

C (JMAF Tables I960) I.36

Afef (formation) 0.79

(2) By equilibrating a bismuth melt with a mixture of UC + UCa and

determining the concentration of U in the melt, Craig obtained the

activity of U in the carbide mixttire. The same procedure was used to

obtain the activity of U in UC2 + C. In order to calculate A F „ for

UC, the A F for UCa in equilibrivim with carbon was assumed equal to

A F - for UC2 in equilibrivim with UC. This assumption will create an

unknown error, but one which will make the calculated free energy of

UC numerically too small. Since UCa is unstable at the temperature of

this measurement, a partial decomposition into carbon and either UC or

U2C3 co\xld further increase the uncertainty in the free energy value.

(5) Robinson and Chiotti (1964) studied the equilibrium between

UC and liquid Zn. The disappearance of UZng.s above ~TOO°C was

used to obtain -22.7 kcal/mole (700''c) as the free energy for UC

- 14 -

( i NO. 3 4 1 - M DIETZGEN GRAPH PAPER MILLIMETER o eUBENE DIETZGEN CO.

MADE IN U. S. A. l - i

Z6

E4

:t3: ittt'Ht; I '. Ett B y jHC °UC|,Q

iliiPiiii t strict ;.i>*iiu

rrrrfc

- r t : -

3Mi;:

2 0

19

iffTi-?

/IB

f l iP^it l ;-© iSn,

-iirp

i:;i.;i-

-5:r:-:r-

* S ^ ^ S f t g i i t s^qi

%~\^=^ M i s V r eI12< .,4:

m ~

mm

to i t-- : * -

z i E g

igt;

ffiS$:::t±HH??~|--rfie-:P:ES

i i i M l i i i i l i i l ^ i l i l i C F ^ . ^<^ii^-oa mid Cixlotti (1964) Zn equlllori'.i::

-ittt:

i t l i ! i ^ i | l : ^ ^ ; Eo^5.n3on end CMotti (196^.) S-ff

| | ^ | | | : ' t Y 3 Mclver (1966) E.IF

;:-i^>C

"ll-SSipil^lB •;-.::il--

and Hubsr (1907) CG:;ibu3tion

^::f^, Craig (l9o5) Bi ecruil '"h':'! V

| | p | ^ | ^ / T ) : Piasza and Sirrnott (1962) UOp + CO urnu

y^ Figure 2.6, Vapor pra5.3-jre( .lf = 123.0 kaal

/nole)

:T:rrr

f i t t j i

•TLASL-AEC-0'-:....AI /^o / 6 ^ 0 • asoi /LA^LAHC-C^FFICUL

through the relationship AG (u. Cg) + AG°(uZna.5) - 3/J G°(uc) = 0. The

absence of UaCa in the system at room temperature was explained by asstiming

an oxygen stabilization of the UC phase which increased as the temperature

was lowered, thereby causing the UsCa to decompose. Since most of this work

was done in a graphite crucible, unit carbon activity could have resulted.

Under this condition, however, UC and U2C3 cannot exist together. In

addition, the formation of U2C3 is so slow at this temperature that an

equilibrium involving this phase would be unlikely. In the absence of

proof that U2C3 actually formed at 700°, there is a serious doubt whether

the claimed reaction actually occurred.

(4) Robinson and Chiotti (l964) have measured the free energy of various

compositions within the oxycarbide system using the emf produced by the cell

U/LiCl-KCl-UCls/UO C ,C. By extrapolating to zero oxygen content, they

obtained AF? x » -l6,9I5 - 1«9T (460''-760''C) as an expression for the

free energy of formation of pure UC. Unit carbon activity was assumed.

Such a low free energy could result if unit carbon activity was not actually

achieved at the oxycarbide-UCI3 interface. According to the phase diagram euid

an extrapolation of vapor press-ure studies, a very small change in the carbon '

content at this temperature can drastically alter the viranium activity. If

this were the case, the above equation would give the partial free energy

of uranium in UCi»x> where x represents a very small but unknown defect in

the carbon lattice.

(5) Mclver (1966) measured the potential created by a cell having an

electrolyte of LiCl/KCl containing UCI3, and electrodes of U and UC. A

nuaber of reasons for the low result can be suggested. The UC sample

analyzed to be UCo.971^ but the lattice parameter (4.9554A) was too low

- 16 -

for this composition. Thus, either the material was impure or the

stoichiometry was actually much lower, i.e., about UCo«90' Since

such a low composition for the UC phase cannot be quenched in, the

former possibility is the more 3.j]'.';ly one. This creates two counter­

acting sovirces of error. The low ;3toich,lometry will raise the uraxtiimi

activity, but oxygen stabilization of the phase miglit produce a lower

value. The net result is bound to be very uncertain. The total free

energy was then calculated assuming unit carbon activity, an impossibility

under the circumstances.

(6) The UOa + UC2 + UC + CO equilitari-um has been studied by several

workers. Piazza and Sinnott (1962) measured the CO pressure between 1761°

and 1953°K. Bazin and Accary (1967) extended the measvirement from 1723° to

2073°K. They observed the same behavior whether the starting material was

pure UCi.o, U(Co.gOo.i), U(Co.r60o'24) or u(Co.70o«3)' Henry et al. (1967)

determined the CO pressure as a function of composition at 1973°• At this

temperature the UC phase contained 5 atomic percent oxygen. The pressures

reported by these studies are in excellent agreement. The resulting free

energy of formation of UC is shown in Figure 2.4. Apparently the dissolution .

of 5 a-t. /S oxygen does not have much effect on the free energy of UC.

(7) A number of vapor pressure measurements have been made in this

system with varying success. Indeed, if viewed on an equal basis, the

measurements would lead to total uncertainty (Figure 2.5)« However, sufficient

experience has been accumulated that one can reasonably eliminate

quite a few of the attempts solely on the basis of the described

experimental procedure. For example, if a powder is used without a

- 17 -

N

O t

o Ul V) •s. E o o I o»

LLT

<

z o < CC

o 0. < > Ul

l/T, "K"* nlO "

Figure 2.5 Evaporation Rote of U^ \ from UC- + C and

of Carbon froiri Grai^hite,

- 18 -

Table 2.5

Thermal Funct.ions of UCi.ao*

a -

P -

Temperature ^1-^293 °K cal/mole

298.15

500

4oo

500

600

700

800

900

1000

1100

1200

1500

l400

1500

1600

1700

1800

1900

2000

2038

2038

2100

2200

2300

2400

2500

2600

2700

2800

a-UCa

p-UCa

0.00

26.92

1607.5

3321.3

5093.4

6900.9

8740.6

10618.

12543.

14527.

16585.

18730.

20978.

233^^.

25844.

28493.

51308.

34304.

37497.

38766.

41264.

43089.

46033.

48976.

51920.

54864.

57808.

60751.

65695.

Hj-H§gB = 8.377 ^

5.487 X 105/T

HJ-H|98 = -1.875

cnl/nr le-de -;

14.52

14.58

16.67

17.49

17.91

18.23

18.57

18.99

19.52

20.187

20.99

21.94

23.05

24.30

25.72

27.29

29.03

30.92

32.98

33.80

29.Ml-

29.44

29.44

29.44

29.44

29.44

29.44

29.44

29.44

: 10^ + 23.4OT -

X 10^ + 29,44T

.-O

en'' /• ".le-clcf;;

16.33

16.42

20.95

24.77

28.00

30.79

33.25

35.46

37.48

59.37

41.16

42.88

44.55

hC.iB

47.79

49.40

51.01

52.62

54.26

54.89

56.32

57.00

58.37

59.68

60.93

62.14

63.29

64.40

65.47

5.752 X 10"3T^

-(F°-Hi98)/T

cal/nole-deg

16.33

16.33

16.93

18.13

19.51

20.93

22.32

23.66

24.94

26.17

27.5^

28.47

29.56

30.62

31.64

32.64

55.61

5^.57

55.51

55.87

55.87

36.48

57.^5

58.59

59.50

40.19

4i.o6

41.90

42.72

+ 2.727 X 10"^^+

•Randomization entropy not included

prolonged purification about 2000°C, the presence of important amoxmts of

evaporating UO/ \ and of serious concentration gradients within the sample

would be a certainty. The grinding of a sample between measurements, even

if this is done in a dry box, is a very -• • procedure. Without berating

each measurement, there are several \?hich I would exjpcct to give trust­

worthy values. The resulting uranium pressures are compared in Figure 2.6,

and the partial molar heats of vaporization are plotted in Figure 2.7.

Given this body of data, there remains the problem of interpretation.

Because most measurements do not fall at ccmpositions for which thermal

data or heats of formation values are available, it is impossible to make

thermodynamic calculations without assuming that the properties are

independent of composition. Furthermore, it has been impossible to

determine if the measured pressures and partial heats at the various

compositions were internally consistent. The mass spectrometric study-

reported by Storms (1966) allows these isolated measurements to be

compared and interpolated to more convenient compositions. Mass

spectrometry is ordinarily given a rather high vmcertainty because of

several unknowns in the machine pressure calibration. However, the

internal consistency of the data within a system shoTild be very good.

Therefore, I propose to adjust the pressure curve given by Storms (1966)

so as to average the various absolute measurements at the various

compositions. This requires an 8; increase in pressure. As a result,

the pressures at all compositions have the benefit of four

absolute measurements and, therefore, should be more certain than a

single measurement. Other mass spectrometric studies at UCx.o4 (Pattoret

et al., 1967) and UCa "** C (iforman and Winchell, 1964) are also in

excellent agreement.

- 20 -

10

10

r 10 -T

(O

iti

5 10"* < cc

10 r»

10 °

- 2 1 0 0 *

- 2 0 0 0 * -

1900

1800

U j j UC

j - - 8 % T

1 1

• LEITNAKER a WITTEMAN(i962j + ANSELIN a POtTREAU (1966) • KRUPKA (1965) xPATTOBETttal. (1967)1

STORMS (1966) J

MASS SPECTROMETER >EICK ttat. (1962)

2100'

—2000*-^

/3UCg+C

1900"

UC4^.UC, *I800»—^

<b

1 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.8

C/U, ATOM RATIO Figure 2*6 Pressure of U

1.8 2.0 2.2

^\ over various Coiqiositions in the Uranium«.Carbon System.

UJ

rx

O 2

< O

e O O o CM

O

UJ

< Q: lit >

180

170

o O O 160

CM

150

140

130

120

STORMS (1966) KRUPKA (1965) VOZZELLA (1965) LEITNAKER a WITTEMAN (1962) EICK et ol. (1962) NORMAN a WINCHELL (1964) FUJISHIRO (1961) RAUH a THORN (1954) DROWART et at. (1965)

ii IDEAL SOLUTION (2nd LAW)

• ANSEUN a POITREAU (1966)

• A + A O

V PATTORET etal. (1967)

$

no

too UjLj+UC

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

C/U, ATOM RATIO

1.6 2.0 2.2 2.4

--/r Figure 2.7 Par t i a l Molar Heat of Vaporization of lh:ani\im as a Function

of Con^os ItioQ.. Act A r / * y ^ m * y: Aci .Acr.rtrcin Al

In order to calculate the total free energy of UC it is also necessary

to know the carbon activity and the heat of vaporization of uranium. These

will be discussed in order.

The carbon activities at several compositions have been obtained by

measuring the UCa/ \/U/ \ intensity ratio in a mass spectrometer (storms,

1966; Pattoret et al, 1967), from the vaporization behavior of the con­

gruent composition (Pattoret et al, 1967; Vozzella, 1965)^ and

from CH4-H2 equilibrium measurements (Wiitc and Juenke, I967). These

results are compared to a Gibbs-Duhcm integration of the uranium pressures

in Figure 2.8, The small extrapolation of the measured carbon activity,

1975' to 2000° in the case of t^itc and Juenke (1967) and 2500 to 2400"

for Pattoret et al, (1967) was made using the tempere.ture coefficients

reported by Storms (1966). For subsequent calculations, the Gibbs-

Duhem values which are listed i.n Table 2.4, will be used.

The heat of vaporization of uranium is another problem, but one to

which this system can make a contribution. There are three areas of

the U-C system from which an estimate of the heat of vaporization of

uranivtm can be obtained. A look at Figure 1.1 will help place these in

context. The regions consist of liq\iid uranium which is saturated with

carbonJ a composition of UCi.o for which thermal data and a heat of

formation value axe known; and UCi.9 which must be treated rather

cautiously. I will discuss the first two here and UC2 in a later

section.

One would expect the dissolution of carbon to lower the pressure

of uranium over the liquid, but by some unlmoira amount. Hence, the

U^ \ pressure over this solution gives a lower limit to that for pure

- 21 -

1.0 S

t 0.1

o <

o CD <

0.01

0.001

2000* K

2400»K

• PATTORET etal. (1967) 2500»K • WHITE AND JUENKE (1967) I973»K • STORMS (1966) 2400« A VOZZELLA et al. (1965) 2400*

1 1 1 09 1.0 l.l 1.2 1.3 1.4 1.5 1.6

C/U, ATOM RATIO 1.7 1.8 1.9

<

H Figure 2.8 Activity of Carbon as a Functlcm of Composition

in the uranium-Carbon System.

, ir

Table 2.4

Activity of U oxid C, ma. t: v'

-t Y^ C/U Uranium* Carbon " f

Atom ratio activity activity kcal/nole 2000°K

1.86

1.80

1.75 1.09

1.07 1.05

1.03

1.02

1.01

1.00

0.99 0.98

0.97 0.96

0.95 0.94

0.93 0.92

0.915

1.67

2.35 5.20 3.20

3.80

4.75 6.40

8.00

1.05

2 .20

7 .00

1.15

1.65 2.10

2.60

3.10

3.60

4.20

4.50

X 10 "3

X 10-3

X 10 "3

X 10"3

X 10"3

X 10"3

X 10 '3

X 10-3

X 10"^

X 10 "2

X 10 "2

X lO"!

X 10 "1

X 10"^

X lO"!

X 10"!

X 10"!

X 10"^

X 10"^

1.0

8.30

6.97

6.97

5.95 4.82

3.62

2 . 9 1

2 .22

1.07

5.55 2 . 0 1

1.59 1.08

8.65 7.18

6.12

5.18

4.81

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

10"^

10-1

10-1

10-1

10-1

10-1

10-1

10-1

10-1

10-2

10-2

10"2

10-2

10"^

10-3

10-3

10"3

10-3

25.41

i^5.39

25.34

24.39 24.36

24.31

24.24

24.19

2i i . l4

24.06

23.95 23.81

23.65

23.47

23.29

23.09

22.89

22.69

22.58

*Heat of vaporization of tu-anium = 128.0 kcal/mole

• :,. 2000" and 2H00"'K ^

c/u Uraniim* Carbon " f Aton rn l io ac t iv i ty ac t i v i t y kcal/mole

piiOO^K

1.90

1.08

1.86

1.84

1.82

1.80

1.76

1.72

1.66

1.60

1.55 1.40

1.55 1.25 1.20

1.10

1.05

1.03

1.01

1.00

0.99

0.98

0.97

0.96

0.95 0.94

0.93

0.92

0.91

0.90

3.35

4.35 5.00

5.70

6.30

6.80

7.80

X

X

X

X

X

X

X

10-3

10-3

10-3

10-3

10-3

10"3

10-3

8.8 X 10"3

9.50

1.00

1.07

1.15 1.20

1.25

1.33

1.75

2.35 2.80

3.60

4.50

7.00

1.00

1.55 1.70 2.10

2.50

2.80

3.20

3.50

5.85

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

10-3

10-2

10-2

10-2

10-2

10-2

10-2

10-2

10-2

10-2

10-2

10-2

10-2

10-1

10-1

10-1

10-1

10-1

10-1

10-1

10-1

10"!

1.0

8.71 8.08

7.53

T.13 6.84

6.55 5.90 5.64

5.47 5.24

4.99 4.84

4.69 4.45

5.50 2.66

2.25 1.76

1.41

9.05

6.29 4.62

5.64

2.92 2.45

2.15 1.86

1.69 1.52

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

10-1

10-1

10-1

10-1

10-1

10-1

10-1

10-1

10-1

10-1

10-1

10-1

10-1

10-1

10-1

10-1

10"!

10-1

10-1

10-2

10-2

10-2

10-2

10-2

10-2

10-2

10-2

10-2

10-2

27.18

27.17

27.15 27.15 27.10

27.06

26.99 26.89 26.74

26.56

26.42

25.94

25.77 25.41

25.25 24.80

24.51

24.57 24.22

• 24.15

24.05

25.91

25.77 25.62

25.^5 25.28,.

25.10 •

22.92 ^

22.72 ^

22.52 %

- 22 -

Tiranium. A farther refinement rcGiilts if the solution is assumed to be

ideal. This provides another lower l?ji;it to the pressure, but one which

would be closer to the truth. Wa bh thir, gsr.umption, a third lavr treat­

ment gives 130.2 kcal/mole as the upper Ijjnit to the heot of vaporization

at 298.15°K. Since the solubility oT cr.rbon bccomop qirito Kmall as the

eutectic temperature is approached, the measured pressure should approach

that for the pure liquid as the temperature is droi ped. As a result,

the slope of the log P vs 1/T curve should be less than that of the pure

liquid, and give a lower limit to the heat of vaporization. A value of

125.2 ±0.5 kcal/mole is obtained from a second law treatment. The

error is due solely to the statistical uncertainty in fitting the

pressures with a straight line. This limit woiild apply to pure \iranium

only if the dissolution of carbon had no effect on its vapor pressure,

a rather unlikely possibility. If an ideal solution is assumed, this

limit becomes 126.4 kcal/mole. These limits encompass the spectrum of

values which are currently being reported as the heat of vaporization,

but the value which was adopted by the last Vienna Panel (1965) is

completely excluded.

The calculation of A F _ for UC from the vaporization behavior present

some problems. Since the activities change very rapidly in this region, it

is difficult to draw a proper curve through the data and pick a value from

it. Nevertheless, the free energy, shovra in Table 2.4, will be used. For

this calculation, 128.0 kcal/mole will be used as the heat of vaporization

of uranitm [AH (298.15°)]. On this basis, the free energy of formation

for UCi.o is shown in Figure 2.4. It is apparent from this figure that

if the heat of ccmbustion value is correct, then the vapor pressures are

- 23 -

too low by 1.5 kcal/mole. In short, there is a discrcpomcy of I.5 kcal/mole

which could be eliminated by chooGing 3?6.5 kcal ns the heat of vaporization

of urani\M. The rather scattered v.ilues which were obtained from other

techniques give no help in resolving this proolrm.

A composition near UCi-i is congt-ucnbly vapori-zing above - 500**K.

This fact has been emply demonstrated by direct evaporation (Vozzella, I965;

Bowman and Krupka, I963; Loitnnker and Wittcmoji, I962) and it is consistent

with the vapor pressures shown in Figure 2.6. This is opparcnt in Figure 2.9

where the molar evaporation rate ratio, c/u is plotted against the solid

composition, C/U atom ratio. The congruent composition occurs where these

two quantities are equal. Vozzella (1965) has used the Lajiginuir technique

to measure the uranium pressure over the congruent composition. However,

the resulting uranium pressures are a factor of O.3 below those obtained

from Knudsen measurements (Figure 2.6). This would suggest a low evapora­

tion coefficient for uranium. On the other hand, Pattoret et al. (1967),

using a mass spectrometer, conclude that the evaporation coefficient for

U/ \ is vinity. These inconsistencies subject the Langmuir measurements

to a very large uncertainty. The apparent agreement with other data

occurs because the meastirements also lead to a very high carbon activity

(Figure 2.8) which compensates for the low uremium pressure when the total

free energy is calculated.

24 -

10 p

0.01

I r 1 r

I

(UC,„) ^ 'i^—— * ^ /cvc (UC.o,)

I 1___L. 0 o.a 0.4 0.6 oa i.o u \A \A I.O .o ^2 ZA

Solid composilion, C/U olom rolio

"Figure 2 . 9 . A comparison between the soUd and gas composition as a function of coiaposition at 2100" and 2300 ' 'K .

I I . UaCa:

A. Low Temperature Heat Capacity

Low temperature data for this compound has been provided by Farr

et al. (1965) and by Andon et al. (l9(S ia).* As is genei'ally the case with

this compound, the data, shovm in Table 2.1, are in excellent agreement.

B. High Temperature Heat Content

No studies have been reported. The tmusual behavior of UC2 and

UC would make any attempt to estimate values very uncertain.

C. Thermochemical Measurements

A heat of formation of -49 ± 4 kcal/mole of U2C3 was obtained by

Huber and Holley (1962) from the heat of combustion. A repeat of this

measurement by the same workers (1968) gave -43.3 O.9 kcal/mole when

purer materials were used. Galvanic measurements bctv;cen 700° and 900*'C

by Behl and Egan (1966) gave the equation FJ = -43,860 - 7T°K (cal/mole).

"The reported values were subsequently emended slightly (Andon et al., 1964b).

- 25 -

Unit carbon activity i/as ascvirirc rTi.hou'"_,M '-l/ro i.-os no experimental

Justification for this reportel. '"ho Cyc cncrc;y con also be calculated

from the equilibrium U,':Co = O-T ' '-n, .e-i -•- I.24CTJCX.Y3 which oec\jrs near

2000''K ajnd the reaction U^Cg + O.TGC = 2UCx.n-' which is found near 1790''K.

If the froG energy valuoo oblnii.vtl from ibr v,u,.nr jn-«sawrc moanurcmcnts

are used (Table 2.4) for the fir^t reaction and Figure 2.12 for the

second, the respective reactions result in -49.96 kcal/mole and -52.0

kcal/mole. These measurements are compared in FiQure 2.10. Because of

the absence of thermal dota, it in impossible to Imow if these measurements

are internally consistent, lloi/cver, the value based on the vapor pressures

is clearly much too low if 128.0 kcal/mole is used as the heat of vaporization

of uranium. Again, the use of 126.5 kcal/mole produces better consistency,

as can be seen in the Figure.

III. UC2:

A. Low Temperature Heat Capacity

UC2 has been studied at two different compositions. For UCi.go,

Andon et al. (l964a) as well as Westrum et al. (1965) obtained essentially

identical results. A study of single-phase UCi,g4 reported by Farr et al.

(1965), also shows no significant difference from the lower composition.

These values are compared in Table 2.1.

B. High Temperature Heat Content

Two measurements of this property have been reported, Macleod and

Hopkins (1967) and Levinson (1963). These ai'e compaxed in Figure 2.11.

When these two studies are combined and fit by equation (l). Section I-B,

two points of interest emerge.

- 26 -

t>

o E

o o

uT <

53

52

51

50

49

48 >

47

4 6 -

45 -,

44 -

300

—

r-"

—

L—

1® —

1 3C +

/

y®

2U =

® ® ®

1 1 Ua C3 1 '26-5

®

i i2ao ®

HUBER etal. (1968) BEHL AND EGAN (1966) TEXT TEXT

1 1

—

—

—

' " '1

—

A

__j

—

—

1000 1500 TEMPERATURE. •K

Figure 2.:^0 Free Energy of Forsatlon of 1X202'

2000 2500

^rr •M^A^. ABT-OFHCIAl

-rr.

35

-o 30 I

? 25

o 20 o

O 15

HEAT CAPACITY OF UC,

/

o £

60

o

5 0 -

40

.•-30

5) 20 (D

OJ 10

o-ua i3-UC2

UC2 METASTABLE-I

HEAT CONTENT OF UC LEVINSON (1963) UC MACLEOD (1968 )UC

200 500 800 1100 1400 1700 2000 2300 2600 2900 -TEMPERATURE. *»K

- V ^^'^^^ ?'H 'fe*' Content and Heat Capacity of DC. g. A A « -AFC-OFFICIAl

i I NO. 3 4 1 - M DIETZGEN GRAPH PAPER

MILLIMETER EUGENE DIETZGEN CD.

MADe IN U. S. A.

- t : : : EETI • • . ; • ( • _ •

: ! • : : t : - : .

I Huba- e t a l . (1953) Ccrsbu^t^on .

f-M(B). ' ^ I v e r (1956) EJ-S"

0 j } Crals (1955) Bi equllibrivn?.

iM(^j). Rica e t a l . (1952) Bi equ i l ib r ium

iE^^:l iM;:^;!M£:: | r^lfe3 Behl and Egan (1935) EM?

| p z | E i . i | l i ^ ( ^ ) Piazza and S innot t (1952) VO^ + CO

j[j Figure 2 , 6 , Vapor p ressure (.6^"= 128.0 k c a l / E o l e )

iinh/% i f f ^ i n Si^*-vf«^ -; .-.. •—y- . -1. - . - . . • ^--.^ . • -• - . . ' ; — . . I •• ' f Z —

1000 ISOO SiOOO asGo yiAsl-AEC OFFICiAL

The two highest data points c, lu'iclco'l .?''i 'lonliina \,1967) suggest a somewhat

lower heat content curve at hi(;her temperatures and, as a result, a

lower heat capacity. This trend ia attracLivc in view of the sharply

rising heat capacity reported by Lcvin.ion {Y)6'^). On the other ha.nd,

this small difference could result because different compositions were

used. The heat content of UCi-o* 3hou3.d be higher than that of UCi .39

and the difference should increase as the temperature is increased.

In the absence of more data, there is little reason to question the

values given by Levinson (1965) below ~1650''K.

Above 1650*, however, some co-ution must be used. Vfnen UC2 + C

is cooled from above 2028''K, the presence of precipitated UC is often

observed. Presumably this occurs between 2058° and approxmately 1650°,

as the metastable phase diagrojn indicates (Figure l.l). Such a precipi­

tation would probably occur during Levinson's measurements. LeitnaJier

and Godfrey (1967) chose to correct for this by calculating the thermal

functions for the mixture O.O55UC + 0.9^5 UCx.91 + O.O7C. In principle

this is perfectly correct, but there is no way of knowing how much UC

actually precipitated dvuring" cooling in Levinson*s calorimeter. Besides,

the correction to the free energy function is very small, although the

heat capacity would be reduced somewhat. Consequently, this refinement

will be ignored in subsequent calculations in this paper.

One factor which cannot be ignored is the randomization entropy in

substoichiometric UCa^ As discussed by Lcitno,kcr and Godfrey (1967),

0.65 e.u. must be added to the measured entropy of UCi.90 if the structure

does not order within the range of the low tcmperatui'e heat capacity measure­

ments. Since there is no evidence that such ordering occurs, this value

should be added to the thcrirml functions li.:;ted in Table 2.5.

Within these I)mitationG, tl.' t'\cn.a<'1 rinictions of UC2 appear to be

essentially correct.

C. Thermochemical Measurcni' ul.s

A number of free cn'- r y r'" forTai.ioti vnlues are compared in

Figure 2.12. The curve is baaod on \,h<z Ihcrauii riuiclioua listed in Table 2.5

and its position was adju.itcci to avcrpge Vr.c ncoKvu-emcnts (2), (5), and (5).

These will be discussed according to the nunbci-ing system on the Figure.

(1) A i-edeterminatj on of the heal of onnbustion of UCi .90

(Huber et al., I965) has yielded -21.1 i 1.J+ Iccal/molc at 296.15°.

The Vienna Panel (I965) odoplcd -25 ± 2 kcd/molc. This has been

converted to the free energy of formation through the thermal functions

in Table 2.5 and Section 2.I.D,

(2) Mclver (1966) mcaRured the cnf produced by the coll described

in the UC section, but with electrodes of U and UC2 containing oxygen, in

one case, and with electrodes of UC and UC2 of various purities in the

other case. An effect of oxygen on the stability of UCa was claimed. In

each case, the arc-melted material was amicalcd at 1500''C for 2h, 26 ajid

72 hr., and the respective saniplcs were each compared to UC in the cell.

A decrease in potential was found when the annealing time was lengthened.

This was attributed to an increased purity. On this basis, the data were

adjusted to minimum oxygen content.

(5) Craig (1966) equilibrated UC2 + C with liquid 3i and, from

the urajiiura concentration in the melt, obtained the luranium activity.

- 29 -

('!•) Hice et al. (iS'-"' ) u.-cd t1u; Gorae technique as described

above.

(5) By measuring the cnif ocvoloped by a cell involving C, UCg,

UF2, and U, Behl and Egan (1966) oi-tai!;cd the cnuotions A F° = -15,820 - 8.2T

(cal/mole, 975"-ll'^5") and />./^ = -10,9^0 - 5.2T (cal/Eioie, 10^+5"-1195')

for the free energy of UC2.

(6) -The CO pressure over an initial mixture of UC2 and graphite

has been measured and attributed bo the cquilibri\jm UO2 + hc = UC2 + 2C0

(Piazza and Sinnott, I962). The values sho'.m in the figure are based on a

re-evaluation of this measixrement by I-jiitnakor and Godfrey (1967). If

oxygen stabilization of UC2, as cl'-'incd by Mclver (1966), is a real

phenomenon, then the good agreement between this measvirement and those

where oxygen was absent must be pm-e coincidence. On the other hand,

it suggests the uno.ttractiye possibility that oxygen stabilization of the

UC2 phase is actually small.

(7) The various vapor pressure measurements vrere treated as

described in Section 2.I.D and were converted to the free energy of

formation using 128.0 kcal/mole as the heat of va.porization of uranium.

Once again, agreement between the vapor pressures and other free energy

data can be obtained if a value of 126.5 kcal/mole is adopted for the

heat of vaporization of uranium.

IV. Some General Comments

During the past 10 years there has been a controversy between 117

kcal/mole and a value near 3.26 kcal/mole for the heat of vaporization

of pure uranium. Two direct measurements, one by mass spectrometry

(Pattoret et al., 1967) and one 1 ; f ir(y.^ colV->'"tjon (Olson, I96O) both

support the higher value as ao nv.-'"'o'yr ic cycles in various uranivim

binary systems. It is inconcebA''n-« 'n l this large and thoroughly

studied body of data could bn wror~ Dy • lirort 10 kcal/mole. However,

even if this problem is loid !o re t ml lau l -_,or viluo i.-? accepted,

there still remain several dl?cr"p"oci c,. Olson (196^) and Pattoret

et al. (1967) observed uranjijm prc-Gurns ••' licU lead to l OJt- and 128.5

kcal/mole, respectively, for the licot of •"•nnorization at 298.15".

Olson (1968) observes that caroon ha j little effect on the vapor pressure.

Consequently, the pressures over U v + UC, as measured by Stonns (1966),

add support. However, the second law heats obtpined from these studies

are consistently lovrer thnn Ihe thiTd lair. TliLs is one uiscreapncy.

Another one occurs when the measured uranium pressures ore combined

with the pressures over the U-C sy&tcm. The resulting free energy

values are much too low as can be seen in a"' urc 2,^ and 2.12. Is there

perhaps a single factor which I'Lll elnTnate each of these differences

as Olson has suggested? One poysjLility is thfit the thermal functions

for uranium (condensed state) are \rror£,. In fact, an increase of 2

entropy units in the free energy function of uranium vrould lead to total

agreement. This calculation is &.ti.''Rarisod in Appendix I, Furthermore,

if this change is made and the values are used to corapare the various

free energy measurements, as 3s done in Figure 2.15, the effect of

oxygen stabilization on the U ^. [hioC now becomes more obvious. Hovrever

the study involving UO^ + CO (6), which snould show the lai'gest effect,

still is apparently unaffected by oxyi en.

A considerable improvement in ihc consistency between various measurements

- 51 -

NO. 341-M DIET2BEN GPJAPH PAPER MILLIMETER

EUGENE DIETZGEN CO. MADE IN U. S. A.

i^tff^^r^l^silBiiniii^ t[i^|r-?2h'entTopyhun'ic.& jaddod, to;! tha:; tS4real

irf~;f unC t ioris H Of 1 .U (o bhdensied; 's tate)i::S::;

\-';ir (1) Kubar e t a l . (1963) Corubustion

'Li:? (2) M-Ivar (1958) EHF

r:^^ (3) Cr- ig (1956) Bi equ i l ib r ium

^V.E; (4) Rica e t a l . (1962) Bi equi l ibr iura

•-^^ (5) Bahl and Egan (1955) ZK?

%

. /

.- .•

. • • - y y

•A -

/ /

/

r.z.'- U.:"L:

MM .;i;:ti:x;-:

SI?ltEt Hr i.:-!": i -^itP$glpl?^iSi|S7 :f"a-;,r-lrL;.j:r:f;bL::i--v'

;T.:Is[S:r; r -rEfa-KlirLtp: :4EMi

a££IIi 300

.'LA^L-AFC-O---., -.Al

1000 1-500 Temperature, *K

2000 2500 /LAS LASI-AEC-OFFICIU

can be ol)taincd i f t h e entropy of U a t ''1'/').'JS) i s I'l- e . u . i n s t e a d of

12 e . u . Vfith th i ,•". ch.Tnge, the hcfit of v-.j-ori r.n t ion of ura.nium would

become 126.5 kca l / ino lc .

One n a t i i r - a l l y a.' 1':R hn\.'' ,';••!'Ii ,-•, '^•'^^-:•<•, ;,..IOT,--I', of r u t r o p y coulfl )iavc

been missed by t h e va r ious low tcr.pcr--:,n.rG horrt caxiacity s tud ios? Alpha,

uranium shows an anoraolj'- in sever, ';! c la : jhi^ Ta-oporties (Flotow a.nd

Osborne, 1966; FivSher, 196 !-) near i!-l°K. Wliilc neutron d i f f r a c t i o n

s t u d i e s f a i l t o r e v e a l a, s t r u c t u r e chon^-:, t he re i s evidence for a

magnetic t r a n s i t i o n (Mueller , e t a l . , r^b'-i-). vrncthor t h i s could con­

t r i b u t e 2 entropy u n i t s i s u n c e r t a i n , but i t docs suggest t he p o s s i b i l i t y

and t h e need for a c l o s e r examination i n t h i s te inperature r ange .

\ , : H x J

2 0 0 0 " K J •" 1 .J C -- UC, . ,50

1 . Assume th<^rmal f u n c t i o n s JM T ' I <•! ?.'' "t-e c o r r e c t

2 . Ascninc /l^ 1\^,{29Q) = -P'i.l / - O S / ' - . M -

5. Assume pressures in Figure 2,6 arc cor->'ect.

h. Assume pressures measured over piu-o iirrni'om by Olson (1968) are correct.

AF^,„ = -25.01 hcnL

dli = -21.1

fef„ = ~~;-i + fef„„ -1.9fcfr. U i UL/p O

fef , ^ -56.11 b' 2

1.9fef^ = -10.)i5

fcf,, = -24.75

measured fef„ ^ -22.25

d- = 2.50 e.u.

2000''K U + C = UCi.o

1. Assume pressures measured by 03 son (1968) are correct, i.e. / -H (298) = 130.4 kcal/mole

2. Assume pressures in Figiu-c 2.6 arc correct.

'3' Assume the heat of formation of UC^.o = -25•3 kcal/mole at 298.15°

h. Calcula.te fef. to give agreement.

iiif = /: F'^-T'' ccf

£: fef = fef., ,-fcf,-fof,.,

• ^ U - — ' J 1-. + pcf -fef T UC C

fef.,, = -23.Ol e.u. dC

fef,. - -5.4}

fef, J = -Sii-.l l

measured fef = -22.25 u

/, = 1.91 e.u.

- 3h -

2000-K U(,) . U(^j

1. Assxime pressures and slope measured by Olson (l963) are correct

AH(2nd) = 126.5 kcal/mole.

2 . Calculate fef,, to give secoml and idijrd law agreement.

log p = -2.:^'ii x 10 'VT + 5.59

A F ^ = +66,9^'7 kcal

A H ° = 126.5

A fef = fef„ -fef„

"(G) V ) A ]]-&• F

fef(^) = -55.S5

fef,, = -24.05 "U)

measured = -22.23

A = 1.82 e.u.

- 35 -

Alcock, C. B. and Grieveson, P. (iS&l) in "Thermodynamics of Nuclear Materials, p. 563, I.A.E.A., Vienna.

Alexander, C. A., Ward, J. J., Ogden, J. S. and Cunningham, G. W. (1964) in "Carbides in Nuclear Energy," Vol. 1 (L. E. Russell, ed.), p. 192, Macmillan, N.y.

Andon, R. J. L., Counsell, J. F., Martin, J. F., and Hcdger, H. J. (l964) Trans. Faraday Soc. 60, 1030.

Anselin, F., Dean, G., Lorenzelli, R., and Pascsrd, R. (l964) in "Carbides in Nuclear Energy, Vol. 1: Fnysical and Chemical Properties/Phase Diagraais" (L. E. Russell, ed,,)p. II5. Macmillan, N.Y.

Anselin, F. and Poitreau, J. (1966) CEA-R-296I

Atoji, M. (1967) J. Chem. Hiys. 47, II88.

Atoji, M. and Medrud, R. C. (191J9), J. Chem. Hiys. , 332.

Austin, A. E. (1959), Acta Cryst. -_2f 59.

Bazin, J. and Accary, A. (1967) Proc. Brit. Ceramic Soc. #8, 175*

Behl, W. K. and Egan, J. J. (1966) J. Elcctrochem. Soc. II3, 376.

Boettcher, A. and Schneider, G. (l958)/ Peaceful Uses At. Energy, Geneva, Vol. 6, p. 561.

Bowman, A. L. (1966), private communicaTiion.

Bowman, A. L., Arnold, G. P., VJit^cman, u. &,, Wallace, T. C. ajnd Nereson, N. G. (1966), Acta Cryst. 21, 670.

Bowman, M. G. and Ki'upka, M. C. (1963), 4th Uranivim Carbide Meeting, East Hartford, Conn. TID-7676.

Brett, N. H., Harper, E. A., 11 cger, H. J. and Pottinger. J. S. (l964) in "Carbides in Nuclear Energy," Vol. 1 (L. E. Russell, ed.) p. l62, Macmillaja, N.Y.

Buckley, S. N. (1961) AERE-R 3872.

Chubb, W., Getz, R. W. and Townley, C. W. (l964) J. Kucl. Mater. 1^, 65.

Craig, J. A. (1966) Ph.D. Thesis, University of Michigan.

Droege, J. ¥,, Lemmon, A. W., Jr. and Filbert, R. B., Jr. (l959) BMr-1513 pp. 58 and A-5, Battelle Memorial institute, Columbus, Ohio.

Drowart, J., Pattoret, A., and Smoes, S. (1965), J. Chem. ?hy3. 42, 2629«

Eick, H., Rauh, E., and Thorn, R. (1962) in "Thermouynaiiu.C5 of Nuclear Materials," p. 549, l.A.E.A., Vienna.

-36 -

(2) F a r r , J , D. , h'uber, E. J . , Jr., ' i - d , r,, L. and Hol ley , C. E . , Jr . ( l 9 5 9 ) , J . n i y s . Chem. 65, 1^05-

:•', r r , J . D. , Witteman, W. G., Stone, P. L. J-TI Westrvim, E. F . , Jr . (1965) in "Thcrmophysical ]'ro-nortie •. '», I^xtrcrac 'i'.n,.;iotal.iirfs and Pressures" (S . Gratch, ed.) p. 162, Am. Soc. Moch. j.i.grs.

Fisher, /.. S. (196*0 ANL-7000, Ar.- onnc Kattonal laboratory.

Flotow, h. E. ana Lohr, H. 1<. (LJCO) J. nrja. Ch<-m. (A, 904.

FloGOw, H. and Osborne, D. (1966), Phys. Rev. 151, 564.

(4) Fujishiro, S. (1961) J. At. Energy 3oc, Japan 3 913»

Gillam, £. (1962), Acta Cryst. I5, II83.

Guinet, P., Vaugoyeau, H., and Blvim, P. L. (1965) Compte Rend. 26l, 1312.

Harrington, L. C. and Rovre, G- II. (l.964) reported in "Carbides in Nuclear Energy," Vol. 1 (L. E. Russell, ed.) p. 542, Macmillan, N.Y.

Henney, J, (1966/ AERE-R4661.

Menney, J., Livey, D. T. and Hill, N. A. (1963) AERE-R 4l76, Groat Britain A'&omic Energy Research Establishment, Harvrell, Berkshire, England. Trans. Brit. Ceram. Soc. 62, 12 (1963).

Henry, J. L., Paulson, D. L., Blickensderfer, R. and Kolley, H. J. (1967) Bureau of I-Iines RI 6968.

huber, E. J., Jr. and Holley, C. E., Jr. (1962) in "Thermodynaxaics of Nuclear Materials," p. 58I, l.A.E.A., Vienna.

Huber, E. J., Jr., Head, E. L. and Holley, C. E. Jr. (1963) J. Phys. Chem. 67, 1730.

Huber, E. J., Jr. and Holley, C. E., Jr. (1968), to be published.

JAKAF Tables (1960), Dow Chemical Co.

Krikorian, 0. H- (1962) UCRL-6785, Univ. of Calif., Lawrence Radiation Lab., Livermore, Calif.

Krupka, M. C. (1965), reported in Storms (1966).

(5) Leitnaker, J. M. and Witteman, W. G. (i;'62), J. Chem. Phys. , l445.

Leitnaker, J. M. and Godfrey, T. G. (1966), J. Chem. Eng. Data, 11, 392.

Leitnalccr, J, M. and Godfrey, T. G. (196T), J. Nucl. Mater. 21, 175«

Lovinson, L. S. (\765), J. Chem. Riys. ^ , 2IO5.

- 37 -

Levinson, L. S. (l964) i- "nrrbi'i r in ?'.v.clcar Energy," Vol. 1 (L. E. Russell, ed.) p. 429, Macmillan, ii.'x.

/gv Lonsdale, H, K. and Graves, J. TJ. .,'1962) in "Thermodynamics of Nuclear Materials, ^ ' p, 601, l.A.E.A., Vienna.

Macleod, A. C. and Hopkins, S.W.J, (1967) Proc, Frit. Ceramic Soc. 8, 15*

Marnier, P, and Accary, A. (n6't) in "Cnrbldes in Nuclear Energy," Vol. 1, (L. E. i.iuGscll, cd.) Macmillau, li.'f.

Magnier, P., Collard, C , Tournarie, M, and Aecary, A. (l964a) in "Carbides in Nuclear Energy," Vol, 1 (L. E, Russell, ed.), p. 4l, Macmillan, N.Y.

Maier, C. G. and Kelley, K. K. (193^) J. Am. Chem. Soc. ^ , 3243.

Mallett, M. W., Gerds, A. F. and Nelson, H. R. (l952) J. Electrochem. Soc. 99, 197.

Mclver, S. J. (1966) AERE-R4983, Atomic Energy Research Establishment, Harwell, Berkshire, England.

itoser, J. B. and Kruger, 0. L. (1967), J. Appl. Hiys. 38, 3215.

I>iueller, M. H., Heaton, L. and Hitterman, R. L. (l964) ANL-7000, Argonne National Laboratory.

Mueller, M. H. and Hitterman, R. L. (l9(''0), USAEC Report NK:0-8O4, p. 49.

Hukaibo, T., Naito, K., Sato, K. 8.nd Uchijiraa, T. (1962) in "Thermodynamics of Nuclear Materials," p. 645, l.A.E.A., Vienna.

(58) Nesmeyanov, An. N. (1963) "Vapor Pressure of the Elements," Academic Press, N.Y.

(8) Roman, J. H. and Winchell, P. (19-34), J, Fnys, Chem. 68, 3802.

Olson, W. (1968), private communication.

Osthagen, K. H. and Bauer, A. A. (l964) BME-I686.

Pattoret, A., Drowart, J. and Smoes, S. (1967), Presented at the 3rd Seminar on New Ceramics, French Institute of Ceramics, Paris, Feb. 8-10, See also WADD-TR-60-782, Part XXXI.

Pattoret, A., Drowart, J. and Smoes, S. (1967), to be published.

Piazza, J. R. and Sinnot, M. J. (1962) J. Chem. Eng. Data, 1, 451.

Rauh, E. G. and Thorn, R. J, (l954), J. Chem. Phys. 22, l4l4.

Rice, P. A., Balzhiser, R. E. an^ P.agone, P. V. (1962) in "Thermodynamics of Nuclear Materials" p. 33I, l.A.E.A., Vienna.

- 38 -

Robinson, W. C, and Chiotti, T'. {'^I'^bG), TJ~106l, Iowa State Univ., Ames, Iowa.

Sears, K. B., Ferris, L. M. and Gr-/, E. J. (1966), J. Electrochem. Soc. 113, 269.

Storms, E. K. (1966) in "Thcrmodynr idc. " Vol. 1, p. 5O9, Intern. At. Energy Agency, Vienna.

Storms, E. K. (1967) The Rcfrartnry C-TJMCV.^, Academic Press, New York, I967.

Storms, E. K. and Huber, E. J., Jr. (1967) J. Kucl. Mater. 25, 19.

Stull, D. R. and Sinke, G. C. (l J'j6) "Thr-ririodynainic Properties of the Elements Am. Chem. Soc, Washington, D. C

Vienna Panel (1963) Technical Geriea rl4, "The Uranium-Carbon and Plutonium-Carbon uyst(;raK" l.A.E.A., Vienna.

Vozzella, P, A. (1965), E'JAC-478, Pratt and vmitney Aircraft Co. Reported in "Carbidcn in Nuclear Energy," Vol. 1 (L. J;. Russell, ed.) p. 342, Macmillcun, N.Y- (1964).

Wallace, T. C , Witteman, W, G., Eodoscvich, C. L. and Bo-*'nnan, M. G. (1968) 6th Plansee Seminar High Temperature Materials, Reuttc/Tyrol, June 24-28.

Westrum, E. F. Jr., Suite, E., and Lonsr'alc, H. K, (1965) in "Thcrmophysical Properties at Extreme Temperatures and Pressures" (S. Gratch, ed.), p. 156, Am. Soc. Mech. Engrs.

White, J. F. and Juenke, E. F. (1967) Gr,?l?~475A, p. 273, General Electric Co.

Williams, J. T,, Sambell, R,A.J. and Wilkinson, D. (1960) AERE-M-625; J. Less-Common i'letals 2, 352.

Wilson, W. B. (19&3), J. Am. Coram. Soc. 43, 77.

Witteman, W. G. (1967), private communication.

Witteman, W, G. and Bowman, M. G, (1963) 4th Uranium Carbide Meeting, East Hartford, Conn. TID-7676, Technical Information Div. USAEC.

Witteman, W. G. and Wallace, T. C. (1966), private communication.

Witteman, W. G., Leitnaker, J. A. .'d Bowman, M. G. (l958), LA-2159, Los Alamos Scientific Laboratory, [.ew Mexico.

Witteman, W. G. (1963), private corasnunication.

- 39 -

PLUTONIUM-CARBON SYSTEM

1. Riase Relationship

It is not surprising that data on the properties of the plutonium carbides

are few and a knowledge of this system is growing rather slowly. Consequently

the review by Storms (196T) is still topical and will not be repeated here. A

Idiase diagram based on this evaluation is shown in Figure 1.1* There have been,

howeve;r, several recent studies which lead to free energy information at hig^

temperatures. These will be discussed in the following sections.

2500

2000 -

ISOO

I fi 1000

T—r 1 — I — I — I — r

.,'-1 I

— I

- I

1 600 — . - « • • • !

Pu 0.4 0.8 1.2 1.6

C/Pu,olom ratio

/ L L -I M«IHna polftli

t { o Mutford M al. (I960t I I * Kruger (1964) ) I f Ddllen (»«4)

Phow boundory a RoMn • ! el. (1963) t Krit9*r (1962) » Kruoar (1963) • Burnhom • ! ol. (1964) -• Quorlorly itoldt rtporl ~ (1962)

Pu^^+C

J I I I L za 2.4

Figure 1.1* Phase diagram of the Pu-<; system*

2. Thermochemical Properties

I. PuaCa + C and PuCg + C

A. Vaporization Behavior

Mulford and co-workers, in a continuing study of the vaporization

behavior, have determined the following least squares equations for the

- 40 -

C "1

N

.

Wo

2 <

a u <

a I 0. <

III

z •-b z u _

t.i.tH

ri umn

\ ill ,j

' id 1

r r

11

1 1,

,H ,

•-

N

IT

^O

t.T\ r-{

v.-" T

-(

*-.' K

)

..''l

( . rj

O

vO

cr

r-l v

-'

T.) P

O

•-W

T

-J

r) « •T

)

c IT)

.' J

n

,-' O

/-\ CM

r-(

v-./ * r

-(

CO

*J H

I

TJ l<

o

u^ r—

t 7t ^^

"'t

Ld

_..

J...

f.H-H-Htf itt'M

i m^ikM^M

Mm

lh I , ,iit,:.

^1

.1 •H

-)-f-

"tilt ('I

,. .J..,i!!!,l!. l^lt.l ,n

inJ.„.l,„'a

.},LM

IliMii.i^Jiil

^

o o m

CM

O

o o CM

o o o o o v.

00

CM

^O

C

M

<t.

CM

C

M

CM

o C

M

1

31

om

/T[B

o>

i ';a

CO

o

indicated temperature and compo-iiuion rmccs.

log P = -17920/T + 2.779 (2000-2500°K) FaCa + C Mulford et al. (1962)

log P = -(20550 ± lH0)/T + ()i.39 ± O.OO) (I5lif-1989''K) PuaCa + C

Olson and Mulford (1968)

r' C-j/ \ war; found to bo .1 minor r.^-'rh^ r.vrr rn,-.C-, + C and nnflnft-,nhlo ov^r

PuC + PuyCa. The first two equations wure used to obtain the free energy

of formation shown in Figure 2.1. The lines marked (2) and (5) should have

a common value at 2000°K if this is indeed the decomposition temperature of

PuCa. The fact that they do not indicates an inconsistency between the two

studies which could be due to an error in the pressures reported by Mulford

et al. (1962). Ward, (see Olson nni Mulford, I968) foimd that when Pu

evaporates from a graphite cell, the vapor does not leave the orifice with

a cosine distribution. As a result, the measured pressures tend to be too

low. Since Mulford et al. (1962) used a graphite cell and the measvired

pressures are indeed low compared to a tungsten cell which does not show

this effect (Olson and Mulford, I968), the former measurement should be

considered a lower limit to the Pu pressure.

A transpiration study by Harris et al. (1967) which cannot be quoted,

is in much better agreement with (2) than with (3).

Ainsley et al. (1964) measured the Pu pressure over oxide-carbon mixtures

and obtained values which were about 0-1 of those over PviaCo + c. Apparently,

the presence of oxygen has a stabilizing effect on this system.

B. EMF Measurements

Campbell (1968) has studied cells of the type M/PU ^, LiCl-KCl/

PiiRua + Ru where M is Pi,'/.\> '=C + PU2C3 or PU2C3 + C. The resulting free

energy of formation of PuaCo is shown in Figure 2.1. A comporison to the

vapor pressure studies is limited by the fibsence of thermal values for this

compoimd. Nevertheless, the ,';''<->p'-' <!.jv-'n V)y the ZW measurement would appear

- k2 -

to be in error* In addition, since the EMF and vapor pressure studies were

made in a two phase region, a change in the PusCs composition as the .tempera­

ture is changed could also introduce a large uncertainty in the slopes.

While the free energy of this compound is still uncertain, it is

clearly more stable than indicated by the combustion measurements of Huber

and HoUey (1962)*

II. PoC + PugCa

A. Vaporization Behavior

Olson and Mulford (1968) obtained

log P - -(19160 ± 260)/T + (5.30 ± O.IT) (1566*-1835°K)

for the pressure of Pu over PuC + PugCa. The resulting partial molar free

energy of formation is Shown in Figure 2.2* If a value for the free energy

of formation for PugCa is taken from Olson and Mulford (1968) and if this

value is assumed not to Change with the composition of the PueCa iftiase,

then the free energy of formation of PuC varies according to the equation

A G = -110^3 - 0.6' T (cal/mole of PuCo^sr)*

The transpiration study by Harris et al* (I967) gives pressures which

are basically consistent but the slope is much different from that found

in the above study.

The Langmuir measurements of Potter (196'^) cannot be used to generate

thermochemical data because of composition gradients which were demonstrated

to be present in their sample.

B . BttnS

Campbell (196B), using the c e U PueCa + PuC/Pu'^^, idCl-KCl/

PuRue 4- Ru, obtained a partial molar free energy of formation as indicated

in Figure 2.2* Again the slope.Is inconsistent with that obtained from the

vapor pressure measurements*

.>3.

o NO. 3 4 1 - M DICTZBEN SPAPH PAPER MIU.IMETER o CUQENE OlETZacH CD.

MADE tN U. a. A. o o

300 iSOQ Temperature, *K LASL-AEC-QFFICIAL

It is generally accepted that the high carbon phase boundary of PuC

moves to a lower stoichiometry as the temperature is increased. This will

lead to an increasing Pu pressure compared to the behavior if the composition

remained fixed. Therefore the slope of lines shown in Figure 2.2 may be too

steep and not directly related to the entropy of PuCo^ar* ^ « change in Pu

pressure at I600*R between the various 2 phase regions Is diagramed in Figure

2.3. Carbon activities, obtained from a Gibbs-Duhem integration of the curve,

are also shown.

KJ*

io-"r

E B

i

to

— I 1 1 1 1 1 1 1

I PoHriymon and Polltr (1964) • Mulford (1966o>

A . , , 0 - , .. i ^ . — Mul'o"* tl966b)

0, •0029

n, • 0083

- ^ U • ^ ' ^ i i * PuC-^f^PuC + PujCj-H)

A/^i* 233.8 kem/mol*

&/i','93.0licot/mola' 0 , 'U)

. K P u / : , * C

J L 4- JL L X _L Ri oS a* 06 08 10 \2 i4 iS 18 So

C/Pu, olom ratio

Figure 2.5. Pressure of Pu/ \ over various compositions in the Pu-C system.

- U5 -

Ainsley, R., Wood, D. C , and Sowden, R. G. (1961+) in "Carbides in Nuclear Energy" Vol. 1 (L. E. Russell, ed.) p. ^k^, Macmillan and Co.

Burnham, J. B., Skavdahl, R. E. and Chikalla, T. D. (196^) in "Carbides in Nuclear Energy," Vol. 1 (L. £. Russell, ed.) p. 1, Macmillan, New York.

Campbell, G. (1968) IA-39^^> Los Alamos Scientific Laboratory.

Dalton, J. T. {l96h) iQ **Carbides in Nuclear Energy," Vol. 1 (R. E. Russell, ed.) p. 77, Macmillan, N.Y.

Harris, P. S., Phillips, B. A., Rand, M. H. and Tetenbaxm, M. (1967)

AERE-R 5553.

Kruger, 0. L. (1962), J. Nucl. Mater. J, l42.

Kniger, 0. L. (I963), J* Am. Ceram. Soc. 46, 80.

Kruger, 0. L. (196' ) in "international Sympositu on Compounds of Interest in Nuclear Reactor Technology" (j. T. Waber, P. Chiotti, and W. N. Miner, eds.) p. 387, Met. Soc. AIME.

Mulford, R.N.R., Ellinger, F. H., Hendrix, G. S. and Albrecht. E. D« (i960) in "Plutcmium" (E. Grison, W.B.H. Lord and R. D. Fowler, eds.) p. 301, Cleaver-Hume Press, London.

Mulford, R.N.R., Ford, J. 0., and HofAnan, J. G. (1962) in "Thermodynamics of Nuclear Materials," p. 317, IAEA, Vienna.

Olson, W. and Mulford, R.N.R. (1968) in "Thermodynamics of Nuclear Materials," IAEA, Vienna.

Falfreyman, M. and Potter, P. E. (196 ) in "Carbides in Nuclear Energy," Vol. 1 (L. E. Russell, ed.) p. 336, Ifocmillan, N.Y.

Potter, P. E. (1964) j. Nucl. Mat. 12, 3^5.

Quarterly Status Report (1962) LAMS-2813, Los Alamos Scientific Laboratory.

Rosen, S., Nevitt, M. V. and Mitchell, A. W. (1963), J. Nucl* Mater. 1£, 90.

Storms, E. K. (1967), The Refractory Carbides. Academic Press, New York, 1967»

1*6 -