Thermodynamics of the URuC system

Journal of the Less-Cameos

THERMODYNAMICS

Metals, Z67( 1991) 373-379

OF THE U-Ru-C

373

SYSTEM

H. KLEYKAMP Kernforschungszentrum Karlsruhe, 7500 Karlsruhe 1 (F.R.G.)

Institut fiir Material- und Festkiirperforschung I, Postfach 3640,

(Received July 13, 1990)

Summary Thermodynamic measurements by the electromotive force method were made on the binary intermetallic phases URu, and U,Ru, and on the ternary carbides URu,C,, and U,RuC, of the U-Ru and the U-Ru-C systems between 950 and 1200 K using galvanic cells with CaF, single crystal electrolytes: U, U,Ru,, URu,; Ru, URu,, UF, ICaF#-JF,, URu,, Ru; U, UF,ICaF,IUF,, Ru, C; U, UF,jCaF,IUF,, URu,C,,,, U,RuC,, C. The WI CaJ%l UF,, URGI.7, Gibbs energies of formation of URu,, U3Rug, URu&, and U,RuC, were evaluated from the measured electromotive force which give ‘A G”(URu,) = - 199100+35.9~Jmol-’ iAG”(U,Ru,)=

-398600+436TJ

‘AG”(URu,C,,,)

= - 192600-t

fAG”(U2R~C2)=

mol-r 2.51’J mol-’

- 380 200 + 52.5 1”J mol- ’

The implications of these thermodynamic data for the behaviour product ruthenium in irradiated carbide fuels are discussed.

of the fission

1. Introduction Ruthenium as a high yield fission product is a component of two distinct families of precipitates in irradiated nuclear carbide fuels, (U,Pu),(Tc,Ru,Rh)C, and (U,Pu)(Ru,Rh,Pd)&, [ 11. The occurrence and the composition of these phases depend on the fission yield and on the carbon-to-metal ratio in (U,Pu)C, +X.However, knowledge of the constitution and the thermodynamics of these fuel-fission product compounds gives information on the chemical potential of carbon in carbide fuel pins during the irradiation process. The phase (U,Pu)(Ru,Rh,Pd), is furthermore observed in defective sodium-cooled oxide fuel pins; it is an indicator of the local chemical potential of oxygen in (U,Pu)O, _ x after the defect formation. The constitution diagram of the binary U-Ru system was investigated over the entire composition range by Park [2]. It is characterized by five inte~etallic 8 Elsevier Sequoia/Printed

in The Netherlands

374

phases: URu melts congruently in an approximately equimolar composition, U,Ru, U3Ruq, U,Ru, and URu, are formed peritectically. The latter is the only solid compound at 1300 “C and crystallizes in the cubic AuCu,-type structure with the space group Pm3m and the lattice parameter a = 398.8 pm [3]. The constitution diagram of the ternary U-Ru-C system was established at 1300 “C [4]. It is characterized by the tetragonal compound U,RuC, [5] and by the cubic phase URu,C, (CaTiO, type) by filling the octahedral voids of the AuCu,-type structure with carbon up to x = 0.7 at 1300 “C; the lattice parameter is a = 405.1 pm [4]. Thermodynamic measurements on URu, and URu,C,,, were performed for the first time by the electromotive force (e.m.f.) method with second-type galvanic cells using single crystal CaF, electrolytes [4]. Unfortunately, side reactions between the platinum leads and the electrodes were not recognized. Therefore, an amended value of the Gibbs energy of formation of URu, was given in the footnote of ref. 6. The reasons for misleading e.m.f. measurements by use of electrodes which are incompatible with the electrical leads are discussed in ref. 7. In an earlier paper, the Gibbs energy of formation of URu, was determined by mass spectrometry and a 233Utarget collection apparatus between 1680 and 2100 IS according to the reaction (URu,) = (U) + 3 (Ru) which gives, by use of the enthalpy and entropy of vapourization

(I) of uranium, [8]

‘A G “(URu,) = - 79500+35.2TJmoll’

(2)

The Gibbs energy of formation was determined further between 1090 and 1180 K by e.m.f. measurements with the galvanic cell Ni,NiF2]CaF2]UF3,URu,,Ru

(3)

according to the cell reaction 3Ni + 2UF3 + 6Ru = 3NiF, + 2URu,

(4)

which gives, by use of the Gibbs energies of formation of NiF, and UF,, [9] ‘AG”(URu,) = - 178 500 + 16.3 T J mol-’

(5)

The heat capacity of URu, was measured between 5 and 890 K by adiabatic and drop calorimetry which gives, at 298 K, C ,298= 101.4 J (K mol)-‘, S&s = 144.5 J (K mol)-’ and Hi,, - Hi = 21200 J mol-’ [lo]. The third law enthalpy of formation of URu, was calculated as fAH&, = -(150.8*0.3) kJ mol-’ [9, lo]. However, the given error seems to be too low due to the inaccuracy of the Gibbs energies of formation of UF, and NiF, used in eqn. (4).

2. Experimental

details

The two-phase URu,-Ru and U,Ru,-URu, samples were prepared from uranium filings (Nukem, Hanau, F.R.G.; impurities: 0.01% oxygen, 0.05% carbon) and ruthenium powder (Schuchardt, Munich, F.R.G.; purity, greater than 99.95%)

37.5

by arc-melting and subsequent annealing at 1500 “C for 18 h and 1100 “C for 40 h respectively. URu,C, in excess with ruthenium and carbon was prepared by annealing a mixture of pre-alloyed URu, with 2 mol of ruthenium and 3 mol of carbon at 1500 “C for 58 h. The lattice parameter was found to be a = 405.2 pm and the composition was x = 0.7 1. The three-phase U,RuC,-URu&,,,,-C sample was prepared by annealing a mixture of pre-alloyed URu, with 1.35 mol of uranium and 2.8 mol of carbon at 1300 “C for 80 h. The preparation of the ternary carbides was more successful by use of pre-alloyed URu, instead of UC, ruthenium and carbon as the agents. UF, (supplied by Professor C. Keller, KfK; single-phase, oxygen impurity 0.03%) was mixed with the crushed metallic samples. This material was pressed and annealed at 1000 “C. The pellets were used as the electrodes for the e.m.f. measurements. Single crystal CaF, discs were taken as the electrolyte. Tungsten was used for the electrical connections. Experimental details of the galvanic cell arrangements are described in ref. 7. The following galvanic cells were used for the determination of the Gibbs energies of formation of the intermetallic phases URu, and U,Ru, of the U-Ru system and-in extension of this method to ternary phases-of those of the carbides URu,C,,, and U,RuC, of the U-Ru-C system U, UF, 1CaF, 1UF,, URu,, Ru U, UF, ICab IUF,, U&u,,

(I)

URu,

(II)

Ru, URu,, UF, ICaF, 1UF,, URu&,,, U, UF,

ICaF,IW, URU.&~.~, U&G,

Ru, C

(III)

C

(IV)

The overall cell reactions of cells (I) to (IV) are U + 3Ru = URu,

(6)

4U + SURu, = 3U,Ru,

(7)

URu, + 0.7C = URuJ,,,,

(8)

5I-J + 5.3C + URu,C,,., = 3U,RuC,

(9)

The corresponding

Gibbs energies of the cell reactions are given by

rAGf = - 3FE, = ‘AG”(URu,)

(IO)

‘AG;, = - 12FE,, = 3’AG”(U,Ru,)-

5fAG”(URu,)

(11)

‘AG;,, = - 3FE,,, = ‘AG”(URu,C,,,)

- ‘AG”(URu,)

(12)

rAG;v= - 15FE,,= 3’AG”(U,RuC,)

- ‘AG”(URu&,,,)

(13)

3. Results The electromotive forces of cells (I)-(IV) are illustrated as a function of temperature in Figs. l(a) to l(d). Calculations by the least-squares method for the cells

376 1lOOK

lOOOK

iiii,;/

650

700

750

BOO

850

900

T I” OC’

ia)

IlOOK

lOOOK

W 1U,UF3 ( CaF,

(b)

) UF,,U3RuS.URu31

T IOOOK I

120,

I

c

in OC

1lOOK I

I

W ’

1

I

0

2 c

100

80

w

-

.*

l

UF3 JR+,

i

’ 601

I

700

I

750

(Ci

t

I

800 T

1000

.,, Ru,C

850

900

in “C h

K

1100 K

1200

K

560 I

I

.500

w

I

-

W 1 U.UF,

1 CaF,

1 UF,,

URu,C,,,

U,RuC2

C 1 W

+

L60 700

(d)

750

800

850

900

950

T In “C

Fig. 1. Electromotive force E of cells (I)-(IV) as a function of temperature: (a) cell (I), (b) cell (II), (c) cell (III), (d) cell (IV). Different runs are represented by the respective symbols.

result in (e.m.f. E in millivolts and temperature

T in Kelvin)

I?,( k 14)=(688+50)-(0.124kO.047)T

950-l 130 K

(14)

E,,( k 7) = (173 + 43) + (0.042 f 0.040)T

960-l 130 K

(15)

E,,,(f3)=

1040-1140

K

(16)

1020-1200

K

(17)

-(22.6f34.3)+(0.1153fO.O314)T

E,v(f7)=(655f30)-(0.107k0.027)T

The thermodynamic data of the auxiliary electrolyte UF, are not required for the calculation of the Gibbs energies of formation of URu,, U,Ru,, URu$& and

377

U,RuC2 by use of the arrangements (lO)-( 17): ‘AG”(URu,)=

of cells (I)-(IV).

- 199100+35.9T+4000

950-1130K

Jmoll’

‘A G”(U,Ru,) = - 398 600 + 43.6 T+ 4800 J mall ’ ‘A G”(URu,C,,,,) = - 192 600 + 2.5 Tk 4100 J mall ’ ‘AG”(U,RuC,)

It follows from eqns.

= - 380 200 + 52.5 Tk 3600 J mall ’

(18)

960-1130

K

(19)

1040-1140

K

(20)

1020-1200K

(21)

The Gibbs energies of formation of URu,, U,Ru,, URu,C,,,, and U,RuCI per mole uranium are illustrated as a function of temperature in Fig. 2.

4. Discussion The Gibbs energy of formation of URu, of this work which was previously cited in ref. 6 fits well with that of Wijbenga and Cordfunke [9], though different reference electrodes in the galvanic cells, i.e. U, UF, and Ni, NiF,, were used. However, the Gibbs energy of formation of URu, measured by mass spectrometry at higher temperatures by Edwards et al. [8] seems to be too positive. The enthalpy of formation of URu, at 298 K can be calculated by a third-law evaluation ‘AH”,,, = fA G ; - T( @;(URu,) - Q;(U) - 3@;(Ru))

UC.

-100

evaluated

by

(22)

IAEA

-120

-160

I t -160

f U,RUC,

W~,benga.Cordfunke

UR",

J

900

1100

1000

1200

T ,n K

Fig. 2. Gibbs energies function of temperature.

of formation

of URu,.

U,Ru,,

URu,C,,

, and UZRuC,

per mole uranium

as a

378

The Gibbs energy functions Qq of URu, [lo] and of uranium and ruthenium [ 1 l] at 900, 1000 and 1100 K together with the Gibbs energy of formation of URu, measured in this work were used to calculate the enthalpy of formation at 298 K ‘AH&(URu,)

= - (154 k 4) kJ mall ’

(23)

The Gibbs energy of formation of URu& cited in ref. 4 was incorrectly determined because the platinum leads which were originally used for the e.m.f. measurements of the galvanic cell initiated UPt, and UF, formation in the U-UF, reference electrode. The correct results for URu,C,,, in eqn. (20) and for URu, in eqn. ( 18) can be used to calculate the Gibbs energy of formation gain ‘A Go by dissolution of 0.7 mol carbon in URu, URu, + 0.7C = URu3C0,,

(24)

‘AGO= +6500-33.4TJmollI

(25)

It can be inferred from eqn. (25) that carbon solubility in URu, becomes zero at about room temperature. The evaluated Gibbs energies of formation of URu,, U3Rug, URu& and U,RuC, result in the following relative partial molar Gibbs energies and thermodynamic activities of uranium at 1100 K A&(URu,, A&(U,Ru,,

Ru)= - 159500 J mol-‘, a,= 2.7 x lo-* URu,)=

- 63450 J mall’,

a, = 9.7 x 10e4

A~‘,(URu,C,,,,Ru,C)=-189800Jmoll’,a,=9.7x10-1n A&(U,RuC2,

URu#&,

C)= -155540Jmol11,a,=4.1X10-8

(26) (27) (28) (29)

The constitution and the thermodynamic data of the U-Ru-C system imply that the reaction of UC, +x with the fission product ruthenium is possible according to the reaction 2UC + Ru = U,RuC,, where A& = - 108 000 J mol- ’ at 1100 K. Further reaction of ruthenium implicates the equation U,RuC, + 5Ru = 2URu,C,,, + 0.6 C, where A& = - 11450 J mall I. Both types of phases were observed in irradiated nuclear carbide fuels. However, irradiated stoichiometric UC and the fission product palladium, which does not form a ternary carbide with uranium, favour the formation of intermetallic phases, e.g. U(Tc, Ru, Rh, Pd), or U (Tc, Ru, Rh, Pd), +x. Acknowledgments

The assistance of Mr. W. Laumer for the e.m.f. measurements Spate for the X-ray microanalysis is gratefully acknowledged.

and Mr. H.

References 1 H. Kleykamp, in J. Leary and H. Kittle (eds.), Advances in LMFBR Fuels, Tucson, ERDA 4455, 1977,~. 166. 2 J. Park, J. Res. Natl. Bureau Stand., 72A (1968) 1.

379

3 4 5 6 7 8 9

10

T. J. Heal and G. I. Williams, Actu Crystullogr.. 8 ( 1955) 494. H. Holleck and H. Kleykamp, J. Nucl. Muter., 35 (1970) 158. H. Holleck, J. Nucl. Mater., 28 ( 1968) 339. H. Holleck, H. Kleykamp and J. I. France, Z. Metallkd., 66 ( 1975) 298. H. Kleykamp, Ber. Bunsenges. Phys. Chem., 87( 1983) 777. J. G. Edwards, J. S. Starzynski, D. E. Peterson, J. Chem. Phys., 73 ( 1980) 908. G. Wijbenga, E. H. P. Cordfunke, J. Chem. Thermodyn., I4 (1982) 409. E. H. P. Cordfunke, R. P. Muis, G. Wijbenga, R. Burriel. M. To, H. Zainel and E. E Westrum, J. Chem. Thermodyn.,

17( 1985) 1035.

11 I. Barin, 0. Knacke and 0. Kubaschewski, (Supplement), Springer, Berlin, 1977.

Thermochemical

Properties of Inorganic Substances