Interdiffusion between uranium-bearing reactor fuels and refractory-metal thermionic emitters

Advanced Energy Conversion. Vol. 3, pp. 101-111. Pergamon Press 1963. Printed in Great Britain

INTERDIFFUSION FUELS

AND

BETWEEN

URANIUM-BEARING

REFRACTORY-METAL

THERMIONIC

REACTOR EMITTERS*

A. F. WE1NBERGand L. YANG'~ Summary--Material compatibility as related to the "marriage" of reactor fuels to refractory metal thermionic emitters is under investigation. Four fuel materials, UC, UCs, UC-ZrC, and UOs, are studied in combination with tungsten, molybdenum, tantalum, and niobium over the temperature range 1200 °2000°C. A technique has been developed for simultaneously studying the interdiffusion between a given uranium-bearing material and each of the four refractory metals. Uranium monocarbide in contact with niobium and tantalum at 1875 ° ::k 20°C and 1925 ± 30°C respectively reacted to form a liquid phase identified as almost pure uranium metal. Molybdenum also reacted with U C at 1800 ° ± 20°C while at the same temperature tungsten appeared relatively unaffected. However, after being contacted with UC at 2000 ° -k 20°C for 30 hr tungsten exhibited a mild reaction. The reaction mechanism for all of these metals appears to be UC + M = MC + U. Formation of this liquid phase was suppressed at 1800°C when UC-UC2 mixtures were substituted for the UC. The kinetics of uniform metal carbide layer formation were studied for UC-UC2 mixtures in contact with the four refractory metals at 1800°C. In addition to the uniform carbide layers formed, severe carbide penetration of the grain boundaries was observed in niobium, tantalum, and molybdenum. Investigation of the interaction between UZrC (6.0 mol ~o U ; 45.8 tool ~ Zr; 48.2 mol ~ C) and the same four refractory metals at 1800°C for 50 hr showed that the stabilization of the carbide phase by the addition of zirconium dramatically improves the diffusion characteristics. Electron micro-probe analyses of these diffusion couples have been made and the results are discussed. Preliminary investigations of UOz-refractory metal compatibility have been completed for three temperatures 1200, 1800 and 2000°C. Indications of transport of one or more of the fuel components through grain boundaries have been observed. INTRODUCTION

IN THE quest for means of direct conversion of nuclear heat to electrical energy, thermionic emission from cesium-coated refractory-metal cathodes has received much attention. However, little information has been available concerning what is probably the major problem area in the utilization of such thermionic cathodes, i.e. the "marriage" of the fuel to the emitter. Retention of the mechanical integrity of the clad fuel element and containment of the fuel and fission-product components must be assured for long periods at operating temperatures of 1400°-1750°C. Candidate fuel materials are limited in number because of the high operating temperatures; they include UC, UCs, UOs, and mixtures of UC with other carbides, e.g., UZrC. Investigations of the compatibility of these fuels with possible cladding materials have been limited to temperatures considerably below the temperature range of interest except for some very short-time high-temperature investigations on UC [1] and UO2 [2, 3]. Insufficient information has been available to predict the behavior of the materials over longer periods of time or to distinguish the operative mechanisms leading to incompatibility and failure. To obtain the necessary information, studies were initiated to investigate the

compatibility of four refractory metals--niobium, tantalum, tungsten, and molybdenum--in contact with the candidate fuel materials. * This work was sponsored in part by the National Aeronautics and Space Administration under Contract NAS 5-1253 and in part by the Rocky Mountain-Pacific Nuclear Research Group and the San Diego Gas and Electric Company. t John Jay Hopkins Laboratory for Pure and Applied Science, General Atomic Division of General Dynamics Corporation, San Diego, California. 101

102

A.F. WEIr,mEgoand L. YANG EXPERIMENTAL PROCEDURES

A technique was developed that would permit the simultaneous study of the interactions between all four refractory metals of interest (tungsten, niobium, tantalum, and molybdenum) and a given uranium-bearing material. A tantalum die assembly, as illustrated in Fig. 1, was machined from electron-beammelted stock. Cylindrical specimens of all four refractory metals (0.25 in. in dia. by 0. 125 in. high) were inserted into the die together with carbide or oxide cylinders which were sandwiched between the metals. To ensure that the fiat surfaces of each cylindrical specimen were perpendicular to the cylinder axis, each piece was ground in a specially designed fixture. After grinding, the specimen surfaces were metallographically polished; each specimen was held in a 2 in. dia. hardened-steel fixture to maintain alignment. A suitable clearance was maintained between the specimens and the die walls to prevent reaction between the specimens and the container material. A torque wrench was used to tighten the tantalum bolt against the specimen stock to ensure uniform pressure on all runs; a torque of 10 lb in. was exerted. At the temperature of the anneal, additional bonding pressure was exerted because the expansion of the carbide or oxide was greater than that of the tantalum container.

To BOLT

To CONTAINER

TEMPE:RAT

StGHTHOLE~

FIG.1. Diffusionassembly. The entire assembly was then suspended in a furnace for the diffusion anneal. A platinum resistance furnace was used for the runs at 1200°C, and a tantalum resistance furnace was used for higher-temperature studies. During the 1200°C run, thermocouples were employed for temperature measurement; optical pyrometry was used for temperature measurements at the higher temperature. All runs were conducted in vacuo. Some of the material combinations studied resulted in liquid-phase formation and the liquid permeated the space surrounding the other specimens and caused spurious reactions. Therefore, the technique of studying all four refractory metals simultaneously is not feasible when liquids are formed. After each run the specimens were sectioned and examined by metallographic and X-ray diffraction techniques. Electron-microprobe analyses were also used, when applicable.

InterdiffusionBetweenUranium-bearingReactor Fuels and Refractory-metalThermionicEmitters 103 EXPERIMENTAL RESULTS UC-metal interactions

Interactions between the four refractory metals and UC were studied in the temperature range 1800°-2000°C. A two-phase region of liquid plus UC was formed in the UC-Nb diffusion couple annealed at 1875°± 20°C for 5 rain, as illustrated in Fig. 2. Figure 3 shows a similar region in a UC-Ta diffusion couple treated at 1925° ! 30°C for 90 rain. Electron-microprobe analyses identified the liquid phase as essentially pure uranium. A carbide layer (Mo2C) that formed on the metal half of a UC-Mo diffusion couple annealed at 1800°C 4- 20°C for 10 hr is shown in Fig. 4. Since the UC was of a composition very close to stoichiometric, the formation of an Mo2C layer and the associated removal of carbon from the UC indicates that a liquid-uranium phase would have been formed. This was not observed, probably because of the small amount of liquid that formed during the short annealing time. After a 50 hr anneal at 1800°4- 20°C, a UC-W diffusion couple showed no visual evidence of either tungsten carbide formation or liquid-phase formation, as illustrated in Fig. 5. An additional sample of this material combination was annealed at 2000 ° 4- 20°C for 30 hr. As shown in Figs. 6(a) and 6(b), there was no visual evidence of tungsten carbide formation, but there was a very slight amount of liquid formed; the quantity of liquid was so small that the specimen retained its cylindrical shape. These experimental results illustrate that when UC is in contact with any of the four metals--niobium, tantalum, tungsten, or molybdenum--at a sufficiently high temperature (less than 1800°C for niobium, tantalum, and molybdenum, but approximately 2000°C for tungsten) the following reaction occurs: UC + metal ~ metal carbide + uranium (liquid). It was hypothesized that this reaction might be suppressed by either of two techniques: I.

Incorporation of excess carbon to give the reaction xUC2 + yUC + zM ~ z M C + ( x - - z) UC2 + (y + z) UC;

2.

Stabilization of the uranium-bearing carbide, to prevent the reduction reaction from taking place: stabilized carbide + metal -~ metal carbide ÷ liquid. Both of these approaches were explored, as discussed below. UC2 + UC-metal interactions

The suppression of liquid-phase formation by incorporating excess carbon was studied, using mixtures of UC + UC2 having 75-99 tool ~ UC2. Since at a constant temperature the activity of the carbon is a constant any place in a two-phase field, the large amount of excess carbon did not change the nature or the kinetics of the reactions but ensured that sufficient carbon would be present to prevent liquid-phase formation even at the longest annealing time employed. The nature of the reaction products formed at 1800° q-- 20°C is illustrated in Figs. 7(a) through 7(d) and is summarized in Table 1. Of particular interest is the observation that in addition to uniform carbide-layer formation in tantalum, niobium, and molybdenum, the metal grain boundaries were very severely penetrated by the carbide which, in effect, created a very brittle cermet.

104

A. F. WEINBERGand L. YANG TABLE 1. REACTION PRODUCTS FORMED DURING U C 2 Jr- UC-METAL INTERACTIONS AT 1 8 0 0 ° ~ 2 0 ° C

Metal

Reaction products*

Tantalum Niobium

TaC; Ta,~ NbC; Nb2C

Tungsten

WC ;W2C

Molybdenum

Mo2C

* Identified by X-ray diffraction.

I100 1050

950 90O

O • 0

850

NIOBIUM TANTALUM TUNGSTEN MOLYBDENUM

800 OENUM

750 Z o ¢ u w 2 o

700 650 BOO 550

z

500 4 bO

ft

400

.d

350

J

TUNGSTENN~

J

300 NIOBIUM

250 200

J

J

150

8

I00

A

50 0

, TANTALUM

0

I

2

I 3

I 4

I S

I 6

TI ME"l/z (HR I/~ j

FIG. 8. Penetration of reaction layers at 1800° ! 20°C.

I

?

f

/

~d<

"2 (500

• )

FI(;. 2. rcJC Nb diffusion couple after annealing :.ll 1875 i 20 C for 5 n3in, illustrating liquid-phase formation; liquid identified by electron-microprobe analysis as essentially uranium metal.

(5C0

FIG. 3. U C - T a diffusion couple after annealing at 1925 90 rain, showing formation of liquid phase.

• )

3 0 C for [/~zcin~, p.

I(H]

Mo

Mo~C

(250.)

Fit;. 4. /JC Mo diffusion couple after treatment at 1800 10 hr, showing carbide- layer formation.

20 (" I\~r

W UC

(75 × )

FIG. 5. Tungsten reaction with UC at 1800° ± 20'C for 50 hr (note lack of tungsten carbide formation).

UC A

w

(so,) FIG. 6(a). Tungsten reaction with U C at 2000 !_ 20 C for 30 hr, illustrating bond (note lack of tungsten carbide formation and also liquid-phase formation at uranium carbide grain boundaries).

(1000 ~ )

FIG. 6(b). Tungsten reaction with UC at 2000°± 20~C for 30 hr, illustrating liquid-phase formation at uranium carbide grain boundaries (note spherical particles in liquid phase; these have not been identified).

-. TaC +Ta,>C

(20o • )

Fie,. 7(a). UCz i UC refractory-metal interactions at 1800 ;/: 20°C; tantalum after 10 hr (note uniform carbide layers at top and severe grainboundary penetration).

Nbc ~cNbc i Nb,.C J Nb,,C

(75 ~,: )

FIG. 7(b). UC2 + UC-refractory-metal interactions at 1800"± 20°C; niobium after 50 hr (note severe grain-boundary penetration by carbides).

FIG.

7(C).

R e f r a c t o r y - m e t a l i n t e r a c t i o n s at 1800 20 C : m o l y b d e n u m after 50 hr.

-

(75 ~ )

Mo

Carbide layer formed

FIe;. 7(d). R e f r a c t o r y - m e t a l interaction5 at 1800 20 C ; t ungst e n after 50 hr.

:

(7s • )

Carbide layer formed

~-Nb

t DiffusionZone -<-UZrC

(250 > )

FIo. 9. Niobium reaction with UZrC (6-0 tool % U; 45.8 tool % Zr; 48.2 tool % C) at 1800< ~ 20'C for 50 hr (note lack of niobium carbide formation and diffusion of niobium into the UZrC).

-Ta

G t Diffusion'Zone
(250 x )

FIG. 10. Tantalum reaction with UZrC (6-0 tool ~ U; 45.8 mo! ~ Zr; 48.2 tool ~ C) at 1800" ± 20c'C for 50 hr (note lack of' tantalum carbide formation and diffusion of tantalum into the UZrC).

< Mo

UZrC

(250)

t:i(i. 12. Molybdenum reaction with UZrC 16.0 tool "<, U; 45-8 tool ",, Z r ; 4 8 . 2 tool 'Y,,C) at 1800 ! 2 0 C for 50 hr (note that there is no diffusion of molybdenum into the carbide).

<-Mounting Plastic

<-Carbide pieces ripped from massive sample


(150~)

FIG. 13. Tungsten reaction with U Z r C (6.0 mol O//o U ; 45.8 mol %i Zr; 48.2 tool "/,, C) at 1800' J: 20'C for 50 hr.

UO~

Mo

(so • )

Fu(;. 15(a). Molybdenum UO~ compalibility at 2000 I 20 C for 10 hr (note prominent broadening of gr~tin boundaries near interface, which is indicative of penetration by components emanating from the UO2).

-<

U02

Mo

(50 × )

FIG. 15(b). Molybdenum-UO2 compatibility at 1800" ± 20°C for 10 hr (note prominent broadening of grain boundaries near interface, which is indicative of penetration by components emanating from the UO2).

lnterdiffusion Between Uranium-bearing Reactor Fuels and Refractory-metal Thermionic Emitters 105 Electron-microprobe analyses failed to detect any uranium penetration into the metal carbide layers formed. It was also observed that for all of the metals studied, the carbide grew into the metal from the original interface. The kinetics of the formation of the uniform carbide layers at 1800 ° :~ 20°C were ~NTERFACE

li

=

HOLE, CARBON, OXYGEN

IO0-

~

f

d

BO70-

~ 4030'-

°2or, f 0

~IRCONIUM

~

l

.-I

200 I~ 55 45

#NIOBIUM

I--GOES TO ZERO

U.AN,OMII\ Ar35°-4°°" \

~

40

~k5 30

25 20

15

I0

5

,

i

0

5

,

I0

~

1

II~r-~ES TOZERO ~ I

u

30 50 70 90100

=

i --"T"---.O

125 150 17"5 200

DISTANCE FROM INTERFACE (MICRONS)POSITIVE INTO Nb NIOBIUM/IO MOL-% UC-90 MOL-% ZrC 1800*+ 20*C; 50 HOURS (6"0 M/O U-45"B M/O Zr-4B.2 M/O C) (23-1 W/O U-67-7 W/O Zr -9"2 W/OC)

FIG. ll(a). Electron-microprobe trace showing niobium reaction with UZrC (6"0 mol ~ U; 45"8 mol % Zr; 48.2 mol % C; 23"1 wt % U; 67'7 wt % Zr; 9.2 wt % C) at 1800° ~ 20°C for 50 hr. observed to be very rapid and to follow a parabolic law, as illustrated in Fig. 8. The growth of the carbide layers can be expressed as follows: Ta: Nb: W: Mo:

X : 12-6 p/2, X = 48 p/2, X --= 52.6 t 1/2, X = 1536/2 ,

where X is the thickness of the uniform carbide layer in microns and t is the time in hours. It is noted that the metals considered to be the strongest carbide formers, tantalum and niobium, have the slowest rates of reaction. This indicates that although the reduction of UCz to U C must be occurring, it is not the rate-controlling step. The growth of the carbide into the metal indicates that the rate-controlling step is the transport of carbon through the refractory-metal-carbide layers formed. This is in agreement with the observations of Accary and Trouve [4] for the Z r - U C system at lower temperatures. That the rate of carbon transport should be highest in those metals (molybdenum and tungsten) having the least-stable carbides is in agreement with the work of Samsonov and Latisheva

106

A.F. WEINBERGand L. YANG

[5] in which the diffusion constants for carbon diffusion in metal carbides were related to the free energy of formation of the carbides. The reaction rates observed, and in particular the grain-boundary penetration of carbides, appear to preclude structural applications in which UC~ or UC-UC2 mixtures are in contact with refractory metals for extended times at temperatures of 1800°C or higher. Studies of the interaction between UC2-UC mixtures and the refractory metals were extended to 1200°C for a period of 1000 hr. With all four metals, a metal-carbide reaction layer less than 1.5 mil thick was formed. The carbide layers were similar in appearance to

- - T A NTALUM IO0

0 0 0 0

I

90

8O

~. 70 z 0

60 5O

~-

§

40

z

30

8 2O

o---o.--o-.o

I0

20

30

40

50

60

70

BO

90

I 0 0 I10 120 130 f40

SAMPLE EDGE OPPOSITE CONTACT ZONE

DISTANCE FROM BEGINNING OF SCANNED AREA (MICRONS) (GOING FROM UZrC'--,-- METAL)

FIG. 1l(b). Electron-microprobe trace showing tantalum reaction with UZrC (6.0 mol ~ U; 45.8 tool ~ Zr; 48"2 mol ~ C; 23.1 wt ~ U; 67.7 wt ~ Zr; 9.2 wt ~ C) at 1800° 4- 20°C for 50 hr. those layers formed at 1800°C. It appears that UCz would be compatible with these materials for extended use at 1200°C. UZrC-metal interactions The second technique for the suppression of liquid-phase formation was the stabilization of UC by the addition of zirconium. A carbide of nominal composition I0 tool UC, 90 mol ~o ZrC (actual composition 6.0 tool ~o U, 45.8 tool ~o Zr, 48.2 mol ~o C) was reacted with the four refractory metals at 1800 ° ± 20°C for 50 hr. The results very dramatically indicated that if the carbide mixture was stabilized so that the refractory metals could not cause a reduction reaction to occur, then no liquid would be formed, nor would carbon be made available for refractory-metal-carbide layer formation. Figures 9 and 10 illustrate the N b - U Z r C and the T a - U Z r C couples, and it is evident that no niobium carbide or tantalum carbide layers have been formed. However, zones of metal diffusion into the UZrC may be observed. Electron-microprobe analyses for these

lnterdiffusion Between Uranium-bearing Reactor Fuels and Refractory-metal Thermionic Emitters 107 specimens are presented in Figs. ll(a) and 1l(b). These are completely analogous and will be discussed together. The salient features of the analyses are as follows: 1.

The refractory metal penetrated about 40/~ into the carbide. This corresponds quite well to the "diffusion zones" observed in the photomicrographs. The zirconium content of the carbide increased, starting about 200/~ from the interface. The uranium content of the carbide decreased, beginning about 200/~ from the interface and reaching about zero at a distance of about 40/~ from the interface. A low concentration "tail" of both uranium and zirconium penetrated into the refractory metal a distance of 300-350/~.

2. 3. 4.

It is especially to be noted that the trough in the uranium-concentration curve occurs in the region of the carbide which was penetrated by the refractory metal. An explanation for this behavior is that these metals, which are strong carbide formers, grasp the carbon atoms from the uranium, thereby increasing the activity of the uranium in this region from the value corresponding to uranium atoms in the carbide to the value of metallic uranium. An activity gradient ensues, causing migration of the uranium out of this region. Although the bond between tb.e carbide and the tantalum or niobium appears sound and gives promise of long-time stability for structural applications, the uranium and zirconium penetration deep into the refractory metal brings to light problems that may result if such materials are used to clad UZrC fuel elements, especially for thermionic applications. A typical cladding would be about 20 mils thick and, assuming a conventional parabolic diffusion law, it would be expected that the uranium and zirconium would penetrate this

,/--MOLYBDENUM IO0 9O

80

.

~

Zl RCONIUM

70 z o

60

§

40

l

20

J,~4.~a.t..~ -RO - 8

-6

-4

-2

0

2

4

6

8

I0

12

15 2 0

30

I

I

4,0

50

6----,>&~

I

h'~,~O IE~60 IB?O

DISTANCE FROM M o - U Z r C INTERFACE (MICRONS} POSITIVE INTO MOLYBDENUM

FIG. 14. Electron-microprobe trace showing molybdenum reaction with UZrC (6"0 mol ~ U; 45"8 mol ~ Zr; 48"2 mol ~ C; 23.1 wt ~ U; 67.7 wt ~ Zr; 9.2 wt ~ C) at 1800° 4- 20°C for 50 hr.

108

A.F. WEINBERGand L. YANG

20 mils in about 200 hr (on the basis of a 300/z penetration in 50 hr). If the arrival of these elements at the surface is faster than their rate of removal by vaporization, the cesium coverage on the surfaces of'these metals may be severely influenced. Since it is on the cesium coverage that these metals depend for good electron emission, the work function and thereby the electron emission in the cell would be severely altered. Consideration is currently being given to future experiments which would determine the magnitude of such an effect. Microstructures of the molybdenum and tungsten that were placed in contact with UZrC are presented in Figs. 12 and 13; there is no visual evidence of molybdenum diffusion into the UZrC. The tungsten specimen did not stick to the UZrC, but small pieces of UZrC did adhere to the tungsten surface, which gave evidence of good contact over the entire surface. Electron-microprobe analysis of the tungsten specimen failed to reveal any interdiffusion of the various components; that is, the diffusion distance was less than the resolution of the microprobe, or less than 1/~. The electron-microprobe trace for molybdenum is shown in Fig. 14. Analysis of these data gave results that are quite different from those obtained for niobium or tantalum; i.e., (1) penetration of molybdenum 7/z into the carbide and (2) penetration of a low concentration of uranium and zirconium (~-~3wt %) 15/z into the molybdenum. No evidence was found that would preclude the use of a molybdenum--10 mol % UC, 90 mol % ZrC structural joint or the use of molybdenum as a cladding for a carbide of this composition. Tungsten also appeared compatible with UZrC of this composition. UOz--metal interactions Materials compatibility studies were extended to include UO2, using the same techniques as those used for the carbides. Studies were made at two temperatures, 1800° and 2000°C, for a period of 10 hr at each temperature. The UO2 was sintered in hydrogen and thoroughly degassed prior to being placed in contact with the metals. Chemical analyses proved that the oxygen content was very close to stoichiometric. Niobium, tantalum, and molybdenum specimens adhered to the UO2 pellets after the exposure at each temperature. The UO2 did not adhere to the tungsten at either temperature. An orange-red coating adhered to the tungsten surface and was also found on the surfaces of the other metals if the adherent UO2 pellets were broken away. Electron-diffraction analyses identified only UO2; however, the presence of a suboxide is suspected from the visual appearance. Metallographic examination of the UOz--W interfaces failed to reveal any evidence of a reaction between the UO2 and tungsten at either temperature. Examination of the other metals failed to reveal any evidence of a gross reaction but did indicate a penetration of the grain boundaries near the interface by some component from the UO~. This is illustrated by grain-boundary broadening near the interface, as shown in the molybdenum specimens in Figs. 15(a) and 15(b). After breaking the UO2 from the molybdenum treated at 1800°C, further evidence was found which indicated grain-boundary attack on the contact surface, as shown in Fig. 16. Interactions between tungsten and ThO2, UO2, and La2Oa were studied during investigations of dispenser cathodes for the electronic industry [6--8]. These studies proved that after suitable pretreatments, free thorium, uranium, or lanthanum metal was made available for diffusion through the tungsten grain boundaries to the surface of the metal. It was feared that a similar reaction was taking place between the UO2 and molybdenum, tantalum, and niobium, which would also cause thermionic cathodes composed of UOz

(1000

. )

FIG. 16. Surface of molybdenum after heating in contact with UO2 at 1800 ± 20°C for 10 hr. Photomicrograph was taken after UO2 pellet had been broken free of molybdenum. The surface was not polished or etched. The apparent grain structure conforms to the molybdenum grains and illustrates the penetration of these boundaries by some component from the UO~.

[]acing p.

108]

InterdiffusionBetweenUranium-bearingReactor Fuels and Refractory-metalThermionicEmitters 109 clad with these metals to behave as dispenser cathodes. Subsequent studies described by Yang et al. [9] did indeed prove that in a molybdenum-clad UO2 specimen, diffusion through the molybdenum of some component (or reaction product) emanating from the UOz did alter the vacuum electron emission from the surface of the molybdenum. Whether this phenomenon will influence emission from a cesium-coated metal surface must be determined. The compatibility of these four metals in contact with UO2 was also investigated at 1200°C for a period of 1000 hr. No reactions were observed between the UOz and any of the metals. CONCLUSIONS In the utilization of refractory-metal-clad nuclear fuel elements as thermionic cathodes, the major problem is the "marriage" of the nuclear fuel to the electron emitter. With this application in view, studies were made of the compatibility between four refractory metals-niobium, tantalum, molybdenum, and tungsten--in contact with the candidate fuel materials UC, UC2 ÷ UC, UZrC, and UO2. Investigations of UC in contact with niobium, tantalum, molybdenum, and tungsten at 1800°C indicate that the first three of these metals react with UC as follows: UC ÷ metal = metal carbide + uranium (liquid). Tungsten appears to be relatively unaffected by contact with UC at 1800°C, but after exposure at 2000°C for 30 hr a very mild reaction was observed. Studies of the compatibility of the same four refractory metals with UCz-UC mixtures at 1800°C were also made for periods up to 50 hr. The use of carbon content greater than stoichiometric UC suppressed liquid-phase formation in accordance with the following reaction: xUC2+yUC+zM--zMC+(x--z) U C 2 + ( y 4 - . z) UC. However, the usefulness of couples of these materials is limited by a rapid growth of uniform carbide layers and severe carbide penetration of the grain boundaries (except in tungsten). The growth of the uniform carbide layers is controlled by the rate of carbon diffusion through the refractory-metal-carbide layers formed during the diffusion anneal and proceeds according to a parabolic rate law: Ta: Nb: W: Mo:

X -----12.6 t 1/2, X : 4 8 t 1/2, X = 5 2 . 6 t ~/2, X = 153 t ~/2,

where X is the thickness in microns and t is the time in hours. Observations of the interaction between UZrC (6.0 mol % U, 45.8 mol % Zr, 48.2 mol % C) and the same four refractory metals at 1800°C after 50 hr showed that the zirconium greatly increased the stability of the carbide, so that carbon was not free to diffuse into the refractory metals to form metal carbides. Electron-microprobe analyses indicated that niobium and tantalum did diffuse into the UZrC and succeeded in grasping carbon from the uranium atoms, which increased their activity and caused them to diffuse in both directions out of the region containing the refractory metal. A low concentration "tail" of both uranium and zirconium was found to penetrate both niobium and tantalum to a distance of 300-350 t~. Molybdenum was found to diffuse slightly (7/z) into the UZrC, and both uranium and zirconium penetrated into molybdenum to a distance of 15 t~. lnterdiffusion between tungsten and the UZrC was found to be less than that discernible using electron-microprobe techniques.

110

A . F . WEINBEROand L. YANG

U r a n i u m d i o x i d e a n d the same f o u r metals were p l a c e d in c o n t a c t at 1800 ° a n d 2000°C. Tungsten was f o u n d n o t to react with the UO2 at either t e m p e r a t u r e , b u t the o t h e r three m e t a l s gave evidence o f g r a i n - b o u n d a r y a t t a c k b y some c o m p o n e n t e m a n a t i n g f r o m the UO2. A d d i t i o n a l experiments p r o v e d t h a t a UO2 pellet c l a d with m o l y b d e n u m has a beh a v i o r similar to a dispenser-type cathode. O n the basis o f these studies o f m a t e r i a l c o m p a t i b i l i t y , it is possible to m a k e a prel i m i n a r y assessment o f the suitability o f the various c o m b i n a t i o n s o f m a t e r i a l s studied for a p p l i c a t i o n as n u c l e a r - h e a t e d p l a s m a diodes. This assessment is s u m m a r i z e d in Table 2. TABLE2.

SUMMARY OF SUITABILITY OF CARBIDE-FUEL REFRACTORY-METAL THERMIONIC EMITTERS*

Fuel UC UC2 10 mol ~ UC, 90 mol ~o ZrC 90 mol % UC, 10 mol ~o ZrC UO2

Temperature (°c) r Tungsten 1800-1900 1800 1200 1800 1800 1800 2000 1200

Suitable Unsuitable:~ Suitable Suitable ([I) Suitable Suitable Suitable

Refractory metal A

Molybdenum

Niobium

Tantalum

Unsuitablet Unsuitable~ Suitable Suitable ([I) Unsuitable¶ Unsuitable¶ Suitable

Unsuitablet Unsuitable~ Suitable Unsuitable§ ([1) Unsuitable¶ Unsuitable¶ Suitable

Unsuitablet Unsuitable~ Suitable Unsuitable§ (ll) Unsuitable¶ Unsuitable¶ Suitable

* Based on conventional diffusion studies. More definitive studies of the influence of reactions between the fuel and cladding materials on electron-emission characteristics are currently in progress. t Liquid-phase formation. Excessive carbide-layer formation and grain-boundary penetration. § Uranium and zirconium diffusion through metal at too rapid a rate. 11Work in progress. ¶ Fuel component diffusing through grain boundaries causes behavior similar to that of a dispenser cathode. Acknowledgements--The authors wish to gratefully acknowledge the contributions of Dr. Kurt Fredrikkson of Scripps Institute of Oceanography in connection with the electron microprobe analyses, D. L. Menken for metallographic services, and C. C. Morris for general technical assistance.

REFERENCES [l] J. W. R. CREAGH,NASA Research on Uranium Carbide and Refractory Ceramics, Proe. of Uranium Carbide Meeting, December 1-2, Oak Ridge, Report TID-7603, 55 (1960). [2] J. J. GANGLER, W. A. SANDERSand I. L. DRELL, National Aeronautics and Space Administration, Report NASA TN-D.-262 (February, 1960). [3] J. J. BYERLEY,Atomic Energy of Canada, Ltd., Report AECL-1125 (October, 1960). [4] A. ACCARYand J. TROtrVE, Bull. Soc. Chim. (France), 1, 26 (1961). [5] G. V. SAMSONOVand V. P. LATISHEVA,Fiz. Metal i Metalloved Akad Nauk S.S.S.R. 2, 309-319 (1956) (AEC-tr-3321). [6] I. LANGMUm,Phys. Rev. 22, 357 (1923). [7] I. LANGMUIR,J. Franklin Inst. 217, 543 (1934). [8] S. DUSHMAN,D. DBNNmONand W. D. R~YNOLDS,Phys. Rev. 29, 903 (1927). [9] L. YANG, R. C. HUDSON, F. D. CARPENTER,and A. F. WE~BERG, Advanc. Energy Cony. 2, 737 (1962). DISCUSSION Question; In your investigation of refractory metal emitters, why didn't you include rhenium? A previous

paper by Dr. Rasor indicates that rhenium is an ideal emitter at the temperatures of your investigation (1800-2000°C). It is also known that rhenium does not form a carbide when in contact with UC at these temperatures.

Interdiffusion Between Uranium-bearing Reactor Fuels and Refractory-metal Thermionic Emitters 111 A. F. WEINBERG: The primary reason rhenium was not chosen for study during the initial stages of this investigation is the large penalty one pays in neutron cross-section in both thermal and fast reactors. This is illustrated by the table below in which the rhenium cross-section is compared to that of tungsten at several neutron energies [1 ]: Cross-section (barns) Neutron energy r A_ Tungsten Rhenium Thermal 16" 9 75' 7 10-25 keV 0.4 2.3 180-302 keV 0' 1 0.5

This is not to imply that one could not construct a reactor which included rhenium, but only that it would be advantageous if other materials could be used. In addition to its thermal properties one must also consider the material characteristics. Although rhenium is not known to form a stable carbide, this alone does not certify its suitability. Rhenium has a maximum solubility for carbon of about 1 wt. ~ [2, 3] and, dynamic and continuous mass transport of this carbon can occur by: (1) transfer along a temperature gradient in the metal if the temperature coefficient of carbon solubility is great enough and (2) precipitation and incomplete re-dissolution during thermal cycling. This last process was studied at our laboratory in a similar situation, nickel contacted with graphite. Even though the nickel did not form a carbide, it dissolved graphite when it was heated and precipitated graphite when the system was cooled. The graphite precipitate particles did not completely redissolve on re-heating and therefore continued thermal cycling caused their growth. Eventually the mechanical properties of the nickel were ruined by the large graphite particles. Whether these processes will occur in rhenium contacted with U C is dependent in part upon the concentration of carbon dissolved in rhenium which has the same chemical activity as the carbon in the U C at equilibrium. These data are not available so one certainly cannot dismiss these possibilities. It is evident that experimentation including the effects of thermal cycling is required to fully establish the suitability of the materials combination Re-UC. [1] R. L. DOUGH, Multigroup Nuclear Data for G E - A N P D Computer Program S, APEX-645, Aug. 1961. [2] W. TRZEI31ATOWSKI,Z. Anorg. Chem., 233, 376-384 (1937). [3] R. SCHENCK, F. KURZEN and H. WESSELKOCK,Z. Anorg. Chem., 203, 159-187 (1931).