Hydrides as neutron moderator and reflector materials

NUCLEAR ENGINEERING AND DESIGN 14 (1970) 390-412. NORTH-HOLLANDPUBLISHINGCOMPANY

H Y D R I D E S AS N E U T R O N M O D E R A T O R A N D R E F L E C T O R M A T E R I A L S * J. B. VETRANO Manager, Nondestructive Testing Section, Battelle-North west, Richland, Washington, 99352

Received 18 August 1970

1. Introduction Metal hydrides are efficient moderators, since the atomic density of hydrogen in many of these materials (NH = number of atoms of hydrogen/cm 3 X 10 -22) is far greater than that in liquid hydrogen itself. Thus, hydrides are particularly well suited to thermal reactor systems in which core weight and volume need to be minimized. The SNAP (Systems for Nuclear Auxiliary Power) reactors, for example, use cylindrical-shaped zirconium hydride rods containing a dispersion of uranium as a combination fuel-moderator element. These reactors, developed by Atomics International, are liquid-metal cooled, and the moderator (fuel) centerline temperatures may go as high as 800°C. For the gas-cooled Aircraft Nuclear Propulsion (ANP) reactor developed by General Electric, the moderator elements consist of large, hexagonal-crosssection rods of clad yttrium hydride with a central axial hole for a fuel element and coolant channel. Moderator elements such as these are easily capable of operation at 1200°C at some sacrifice o f N H. A large sodium-cooled power reactor being developed jointly by Interatom** and the German Federal Government at the Karlsruhe Nuclear Research Center, Karlsruhe, West Germany, employs various shapes of clad zirconium hydride as moderator

* Chapter 39 of the projected third edition of the U.S.A.E.C. Reactor MaterialsHandbook. ** A joint company of Atomics International (United States), Demag (W. Germany), and Babcock & Wilcox (W. Germany).

elements. The maximum moderator temperature in this reactor, which is called the Kompakten Natriumgekuhlten Kernreactor (KNK) is about 600°C. The future potential of hydride-moderated reactor systems clearly lies in the area of compact power sources rather than in large, central-station power plants. Where weight and size are of logistic importance, along with power requirements greater than about 10 kW (e), it will be necessary to use the sophisticated technology of the hydride-moderated system. Such applications might be space auxiliary power, space propulsion (coupled with an ion engine), remote.base power plant (polar regions), emergency or disaster power plant (trailer or barge mounted), or underwater power source for search, rescue, or research missions. When metal hydrides are used as moderators, the metal serves primarily to hold the hydrogen atoms tightly packed at the operating temperature. As long as it has a relatively low capture cross section for neutrons, little else is required. Hydrides are readily formed, since hydrogen reacts in some way with nearly every element. In some cases, the reaction results only in solid-solution formation, while in other cases, compounds with a variety of possible bond types are formed. It is convenient to categorize the nature of the reactions according to the periodic table, since elements within a single group react similarly with hydrogen. This behavior is illustrated in table 1. Naturally enough, although there are similarities, there is a gradual change in bonding type and chemical behavior in successive elements of a given row of the periodic table. Thus, the halogen binary compounds of hydrogen

£B. Vetrano, Hydrides

391

Table 1 Hydrogen reactions with the elements. Periodic chart of the elements Period

VIIA

Reactions

Elements

CI

Ordinary character

Typical family members

At

Primarily ionic with the hydrogen electropositive

Corrosive gases or liquids

HC1, HF

Po

Covalent

Decomposable gases or liquids (except solid A1H 3)

CH4, B2H6, NH3

Br

I

As Ge Ga

Sb Sn In

Bi Pb T1

Cd Ag Pd Rh Ru Tc Mo

Hg Au Pt Ir Os Re W

Endothermic hydrogen occluder; no compound formation (except exothermic palladium)

Metallic; more or less ductile

Fe(H), Ni(H)

VIIB VIB

Zn Cu Ni Co Fe Mn Cr

VB IVB IIIB

V Ti Sc

Nb Zr Y

Ta Hf La a

Metallic; u sually brittle

ZrH 2, Till2

Acb

Exothermic hydrogen occluder; metallic or interstitial compound formation

Ca K

Sr Rb

Ba Cs

Ra Fr

Primarily ionic with the hydrogen electronegative

Saltqike (except noncrystalline Bell2)

Nail, Call2

IliA

F

Bond type with hydrogen

o

s

N

P

C B

Si AI

lib IB Hj

VIIIB

IIA IA

Be Li

Mg Na

a. And all lanthanides. b. And all actinides.

are considered primarily ionic, with the hydrogen donating an electron to the bond and becoming electropositive. The elements of Groups IliA, IVA, VA, and VIA form primarily covalently bonded compounds with hydrogen, which shares its one electron. Many of these binary compounds are gaseous at ordinary temperatures. The elements of the "middle members" of the periodic table (Groups IB, liB, VIB, VIIB, and VIIIB) dissolve hydrogen endothermically but do not form compounds under ordinary conditions. These solutions of hydrogen in the metal retain most of the physical and chemical characteristics of the solvent metal. Unlike the elements in the middle groups, the transition metals of Groups IIIB, IVB, and VB dissolve hydrogen exothermically. As hydrogen is added beyond the solubility limit, the resulting compounds have properties quite different from those of the parent metal, and often have a broad range of stoichiometry.

The groups IA and IIA alkali and alkaline-earth metals form ionic compounds in which the hydrogen is electronegative. These compounds are often violently reactive with protonic compounds owing to the electronegativity of the hydrogen. In this article the hydrides of only Groups IA, IIA, IIIB, 1VB, and VB will be discussed. Only these hydrides have a high enough hydrogen density and sufficiently attractive physical and chemical properties for use as moderator or shielding materials in nuclear reactors. Ref. [ 1] has a complete discussion of the chemistry and behavior of other hydrides. The specific application of hydrides as reactor-shielding materials is not covered in this article.

2. Preparation 2.1. Hydrogen-metal reaction The usual method of preparing the hydrides

392

ZB. Vetrano,Hydrides

discussed in this article is by direct reaction of the metal with hydrogen.* In general, the metal is heated in vacuum to some elevated temperature (about 800°C in the case of zirconium, 600°C for titanium, and 250°C for uranium, etc.). Purified hydrogen is admitted slowly into the vacuum chamber until absorption by the metal is essentially complete at this temperature. Finally, to complete the hydriding cycle, the partially hydrided metal is slowly cooled in an excess of hydrogen. Although a high pressure of hydrogen may be used, 1 atm pressure of hydrogen is generally sufficient for the whole hydriding cycle.** The direct-reaction method can be used with almost any state of subdivision of the metal, since the solubility of hydrogen in the metals is generally high and the diffusion of hydrogen is rapid at elevated temperatures. 2.2. Massive hydriding Unless unusual precautions are employed, hydriding will almost always cause the starting metal to crack and spall when it is of reasonable dimensions. There is no sharp boundary for this effect, but the problem becomes more pronounced when the smallest dimension is greater than 41 in. The basic problem here is that, generally, the density of the hydride is substantially lower than that of the metal (14% in the case of ZrH 2 and 17% less in the case of Till2) and hydride ductility is low even at the elevated temperatures of preparation. During the hydriding cycle, a hydrogen-concentration gradient is established, with a hydride case forming on the surface and with hydrogen in solid solution in the interior of the specimen. At the interface between the hydride case and interior, the growth accompanying hydride formation generates stresses that can easily exceed the fracture strength of the hydride and cause extensive cracking. To prepare large, single-piece forms of metal hydrides for moderator elements, this cracking must be avoided. One method is to hydride very slowly (for example about a 5-day cycle for a 1 in.-diam rod of zirconium) to minimize hydrogen gradients [2]. *Bell2, an exceptional case, is prepared only by the reaction of a beryllium compound with another hydride, e.g., 2Be(CH3)2 + LiAIH4 ~2BeH2 + LiAI(CHa)4. **MgH2 requires the use of high pressure (about 1500 to 3000 psi) for its synthesis.

This technique is only partially successful, and the yield often depends on other uncontrolled factors, such as purity of gas and metallurgical structure of the starting metal. A more successful technique is to control the grain size and orientation of the parent metal. A fine-grained, randomly oriented structure is preferable to a large, columnar-grained structure. For example, in zirconium the presence of zirconium carbide (0.3 to 0.5 w/o) is particularly effective in limiting grain growth during hydriding and in forming a more stress-resistant structure [3]. 2.3. Powder metallurgy It is possible to prepare hydride forms by direct hydriding of the metal without concern for cracking, crushing the product to powder, and reconstituting to a monolithic form by powder-metallurgy methods. Moderator components of metal hydrides prepared by this method are generally inferior in physical and mechanical properties to massively hydrided products. The inferior density of powder-metallurgy products reduces their moderating ability, strength, and heat-transfer properties. The fundamental difficulty with the application of powder. metallurgy methods to the fabrication of hydride moderators is that effective sintering requires temperatures high enough to decompose the hydrides. This restricts the method to the preparation of green compacts of about 70 to 80% of theoretical density by cold compaction, or perhaps of somewhat higher density by vibratory compaction of size-graded powders. Although not too suitable for use in power reactors, moderator elements made by powdermetallurgy methods are used extensively in critical assemblies, since the powder-metallurgy process is relatively inexpensive compared with the massivehydriding technique. Lithium hydride is an exception to the above generalization. It is relatively stable near its melting point (688°C) and can be sintered after cold pressing. Massive blocks of LiH have been prepared by this method [4]. 2.4. Casting LiH is sufficiently stable at its melting point to allow the casting of various-shaped bodies. Although the method is unique to LiH, the extensive use of LiH in reactor shielding makes casting an important

ZB. Vetrano, Hydrides

393

10GO

/--~('\

800

o

~.

' "'":"

6o0

~-T,

2

7___~

~~m--I

K/("t

'

i

rH2i,

400

200

I

0

I

II

I

0

0.5 1.0 1.5 H/Ti, ATOMICRATIO a. Ti-HSYSTEM6 IIXl0 I ! ! o-Hf 800

\

- N\

2.0

I

I

"

II

0.5

1.0 1,5 HlZr, ATOMICRATIO b. Zr-H SYSTEM7 I

I

/

2.0

I

I atm

{.)

2,IC ,.=,

i 400

8 HfH2~

200 0

i

('HfH2"x/ I I l I / 0.5 1.0 1.5 HIHf, ATOMICRATIO c. Hf-HSYSTEMS

0

2.0

0.5 H/Nb,ATOMICRATIO d. Nb'HSYSTEM9

1.0

Fig. 1. Phase diagrams of metal-hydrogen systems, a. Titanium-hydrogen system, b. Zirconium-hydrogen system, c. Hafniumhydrogen system, d. Niobium-hydrogen system.

forming method. LiH may be cast by normal methods if oxidants such as air are excluded and a partial pressure of hydrogen is used to suppress dissociation. The rather substantial contraction on solidification (about 18%) encourages the formation of shrink pipes and voids. Structurally sound bodies can be obtained, however, by using honeycomb reinforcing supports

and by carefully controlling the solidification rate and direction.

3. Machining and working The alkali and alkaline-earth hydrides are generally

J.B. Vetrano, Hydrides

394

1400

l

1700

X lO00

tO0 e~

600

1201),~ LIQUID

y+LIQ

~....,....-,.-~H3+LIQ

,÷u.,

1000 400

~--

6oo

-~YH3-x

ZOO

o'÷UH3 200 i ,

I

,

°'+UH3

I

3.|)

1.0 2.0 HIY, ATOMICRATIO

0.08

0.16

O.~

0.]2

0.40

H/U0 ATOMIC RATIO

e. Y-H sYSTEM10'11

f. U-H SYSTEM1'12'13

Fig. l.e. Y t t r i u m - h y d r o g e n system.

Fig. 1.f. Uranium-hydrogen system.

1000

|

w

|

w

I atm LIQUID 800

J

608

--

¢ - Pu + Pull2. x

6' - P~/+ Pull 2

2,c

..

.J

400 -

I I I 1 I t m .|I | ! t tI

~ -Pu+PuH2. x y -Pu + PuH21

200 -

15 - Pu + PuH2_x o - Pu + Pull2. x ,

i

A

1.0

Pull3 x i -

PuH~, x

1

2.0 HIPu, ATOMIC RATIO

g. Pu-H SYSTEM1

Fig. 1,g. Plutonium-hydrogen system.

3.0

2.88

2.~

J.B. Vetrano, Hydrides

~0

!

'

I

700

o -Th ÷

600

ThH2. x

'

I ~

" !

I

ThH2T x

iJ

ThH2.3 ,

Th4HIs- x ~ \ ;

-

rh4Hls:~ ' I 0.4

0.8

1.2

1.6

,

I 2.0

,It 2.4

,

I s, 2.8

3.2

I

3.6

HITh, ATOMIC RATIO h. Th-H SYSTEM 1

Fig. 1.h. Thorium-hydrogen system. not machinable. They are brittle, saltlike materials which deteriorate in air. A pellet of Call2, for example, will deteriorate to a pile of powder if left in the open overnight. Powder compacts or castings of these materials can be drilled, but even this operation must be done with a cover gas and fracture often occurs during drilling. The hydrides of metals in groups IIIB, IVB, and VB, on the other hand, are machinable if proper care is taken. Except for yttrium hydride, deterioration of these hydrides in air at ambient temperature is not a problem. In general, they are metallic in nature. When the hydrogen-to-metal atom ratio does not exceed about 1.0, these materials can be turned on a lathe (using full coolant flow to prevent thermal decomposition), cold forged to a limited degree, ground, shaped, drilled, or trapped. When the hydride phase predominates (at hydrogen-to-metal atom ratios greater than about 1.0), the hydrides become brittle and can no longer be forged. Lathe turning and shaping are possible, but surface grinding is a far more satisfactory method for machining these hydrides.

4. Physical properties 4.1. Phase diagrams Many hydride systems have been studied but few of the phase diagrams are complete. The diagrams for zirconium, uranium, lithium, and yttrium have been most extensively studied because of their importance

395

to nuclear-reactor technology. Data on these systems, as well as phase diagrams for selected other systems which have been studied in detail are given in fig. 1. In most figures the isobar of 1 arm partial pressure of hydrogen is shown, to indicate the relative thermal stability of the various phases. In regions of one condensed phase plus one gas phase, the equilibrium partial pressure of hydrogen varies with composition and temperature, as required by the Phase Rule. In regions of two condensed phases plus one gas phase, it is independent of composition and depends only on temperature. Note, for example, the horizontal isobar portion in figs. la and lb. Hydrogen pressures in this region are commonly called "plateau pressures". As can be seen from fig. 1, the phase diagrams of these systems can take many forms. Most have several important common features which help in understanding the properties of the hydrides. Generally, on the metal-rich side of each diagram there is a region of solid solubility of hydrogen in the metal (or in its various allotropic forms). On the hydrogenrich side of the diagram there is a hydride phase. The hydride phase almost always has a rather broad range of stoichiometry, with consequent variations in physical and mechanical properties within this singlephase region. Between the solid-solubility region and the hydride phase, there will be a two-phase region containing varying proportions of the hydride and metal phase. Operation of a hydride moderator in this twophase region should be avoided, primarily because of thermal migration of hydrogen to the cold region. This results in a loss of moderating power in the hotter zones as they become depleted in hydrogen. 4.2. Plateau pressures As previously mentioned, the term "plateau pressure" designates the partial pressure of hydrogen over the two-condensed-phase region containing a mixture of hydride and metal phase. Relative values of the plateau pressure can be used to compare the stabilities of different hydrides, since plateau pressure represents the minimum dissociation pressure of the hydride phase at a particular temperature. At constant temperature, increasing the hydrogen content of the hydride phase beyond the plateau region increases the dissociation pressure, which reaches a maximum at the stoichiometric hydride composition.

396

J.B. Vetrano, Hydrides

1200,/

'

'

'

/ /

, ....

q

'

0

1.0

'

t I~-Sr

o

'

/

a'SrH2-x

2.0 0

1.0

2.0

H/Sr, ATOMIC RATIO

H/Ca, ATOMIC RATIO

j. Sr-H SYSI-t.M15

i. ~ - H SYS11[Mbl

Fig. 1. i, Calcium-hydrogensystem,j, Strontium-hydrogensystem.

1200

'

I

'

~

'

I

w

,

w

r

1 iO00 L

I

~

,

8OO ~J

LiHI_x~_

< 600 a~aH2,x~_ 400

200

B

i

I 1.0

J

HIBa, ATOMIC RATIO k. Ba-H sYSTEM 16

i

J

I

L

2.0

I 0.5

I

i

HILi, ATOMIC RATIO I. Li-H sYSTEM l

PhaseDiagramsof Metal-HydrogenSystems. Fig. 1. k. Barium-hydrogensystem. 1. Lithium-hydrogensystem,

~ ~'.0

397

J.B. Vetrano, Hydrides

Table 2 Plateau pressures for various substoichiometric hydrides. Formula for fully hydrided state

BaH 2 CaH2 CeH2 Csl-I DyH ErH2 GdH 2 HfI-I2 HoH 2

KH LaH2 LiH LuH2 MgH2 NaH NdH 2 PrH 2 Pull 2 RbH Sell2 SmH 2 SrH2 TbH 2

ThH2 Till2 TmH2 UH 3 YH2 ZrH2

-A

LogloP(torr) = ~

l~ r~)

Constants

Temperature range

-A loglOP=-~--+B,

B

(°c)

9,150 9,610 8,890 10,760 5,900 12,120 11,900 10,250 7,353 12,110 6,175 10,850 8,224 10,730 4,060 5,958 6,100 11,030 10,870 8,165 5,680 10,477 11,700 10,400 11,320 7,650 8,630 11,750 4,410 9,709 11,630

10.4 10.23 9.48 10.63 11.79 11.16 11.0 9.72 8.728 11.04 11.69 10.64 9.926 10.20 10.145 11.47 11.66 10.48 10.53 10.01 11.80 10.409 11.4 11.10 10.38 9.50 12.43 10.82 9.14 8.80 12.98

470- 550 600- 780 780- 900 600- 800 250- 400 650- 800 600- 800 600- 800 600- 900 550- 800 300- 415 700- 800 600- 800 300- 500 500- 600 250- 415 600- 800 600- 800 400- 800 250- 350 600-1050 600- 800 500-1000 600- 800 500- 800 400- 650 500- 700 450- 650 900-1350 550- 900

(1)

where P = partial pressure o f hydrogen, torr; T = temperature, ° K ; A and B are constants, also appearing in eqs. (2) and (3). This is the form o f the van 't Hoff equation, and can be used with available data to calculate some o f the thermodynamic properties o f

-z2d/p -ASp T (kcal/mole) (kcal/mole) (1 atm,

Ref.

°c)

A

The plateau pressures are commonly expressed in the form

+B

41.87 43.98 40.68 49.24 13.50 55.46 54.46 46.90 33.65 55.42 14.13 49.65 18.82 49.10 18.58 13.63 13.96 50.47 49.74 37.36 13.00 47.94 53.54 47.59 51.80 35.01 39.49 53.77 30.27 44.43 53.22

34.32 33.62 30.20 35.46 20.40 37.89 37.16 31.30 26.76 37.34 20.16 35.51 16.12 33.50 33.24 19.65 20.09 34.78 35.01 32.63 20.41 34.45 39.00 37.62 34.32 30.29 43.49 36.33 43.11 27.09 33.04

944 1062 1115 389 1191 1193 1226 985 1211 428 1125 894 1193 286 422 1178 1148

870 364 1119 1100 992 1236 883 631 1207 432 1367 878

[17] [181 [18] [19]

[201 [211 [22] [23] [241 [211 [201 [191 [25] [21] [261 [27] [20] [191 [19] [281 [20] [29] [22] [301 [211 [31] [8] [211 [12] [111 [321

the hydride (although not the standard thermodynamic properties o f the stoichiometric hydride). For example, let AHp and ASp represent, respectively, the enthalpy and entropy o f formation o f a mole o f metal-saturated hydride (i.e., the composition of hydride at the two-phase boundary) derived from a mole o f metal saturated with hydrogen reacting with ~n~ moles of hydrogen at 1 atm. These quantities approach the standard enthalpy and entropy of formation as the solubilities o f the metal in hydride and o f hydride in metal approach zero. These conditions

398

J.B. Vetrano, Hydrides

are far from being met in some hydrides, such as zirconium hydride (compare stoichiometry range in fig. lb with that in fig. 1~). It can be shown [1] that 2 303 zSJ--/p = ~ R n A

hydride phase. Also: 2.303 n 8~Sp = - - ~ - - t~n (B - 1og10760 ) cal/°K-mole of

(2)

cal/mole of hydride,

hydride.

where R = universal gas constant, 1.987 cal/mole-°K; n = hydrogen-to-metal atom ratio of stoichiometric

(3)

The constants in the plateau-pressure equations for several of the metal hydrides and the derived thermo-

T,K 100,000

18.0 I

/

14.0

16.0 1

1Z.0 1

10.0 1

8.0

lO,I]lO

1000

-

1 ~m

t-

lO0

O-

10

"Cs " 1.0

0.1

I

I

300

400

500

600

700

800 ~

TEMPERA11JRE,C

Fig. 2. Plateau pressures of the monohydrides and dihydrides of various metals.

1000

J.B. l/etrano,Hydrides l0

I

I

I

I

I

tion is usually required as a hydrogen atom density, the quantity N H , the number of hydrogen atoms per cubic centimeter of hydride X 10-22, is used. It can be calculated from the hydrogen-to-metal atom ratio and the density by the expression

I

NH

,_1 £: 0

,

.1

0

I

I

I

200

400

60O

,

cr

,

~0

I000

I~0

Y 18,00

~:MPERAIURE, C

Fig. 3. One atmosphere isobars in various metal-hydrogen systems. dynamic properties are given in table 2. Where available, the temperature range over which the experimental data were determined is also given, along with an extrapolated temperature in which the partial pressure of hydrogen would be 1 atm abs. The plateaupressure data are also shown in fig. 2, where the broad range of thermal stability in the hydrides is quite evident.

4.3. Isobars Isobars denote the change in composition with temperature at constant pressure. Since the composi-

T.K 13

12

OXY~N

' <14W,m

.,.m~

<=~-

ll

'

'

/

9

I0 /

8

~

/

/

1.0

H/Zr

1. /

/

.11-6 E(]UIUIIRIUM •

=

~" 0.1

1.80

0,01

399

800 ~MPERA~IIE. C

Fig. 4. lsochores in the zirconium-hydrogen system.

q00

=

(H/M) (p) (60.23) M.W.

'

(4)

where HIM = hydrogen-to-metal atom ratio; # = density of hydride, g/cm 3 ; M.W. = molecular weight of hydride. The variation o f N H with temperature at a constant pressure of 1 atm is shown for several hydrides in fig. 3. For these curves, the density is assumed not to change with either temperature or composition. While this assumption may introduce up to +- 10% error, it permits ready comparison of various moderator materials with a minimum of data. A significant feature in fig. 3 is the precipitous loss of hydrogen for each of the hydrides at some characteristic temperature. This temperature, at which the hydride phase is in equilibrium with the metal phase (plateau region), must be avoided in operational use of hydride materials. Data such as those presented in fig. 3 are useful in selecting the optimum moderator for a particular application. Optimization for a compact reactor generally starts with a maximization o f N H and then progresses to the consideration of such other criteria as capture cross section, weight, cost, availability, and physical properties. Zirconium hydride was chosen as the moderator for the SNAP reactors because of its highN H in the 600 to 800°C range. Scandium hydride has a higher Nrt in this range but was rejected because its capture cross section is objectionably high. For use at temperatures above 900°C in compact, air-cooled reactors for aircraft nuclear propulsion, yttrium hydride was the logical choice on the basis of its N H in this region. The useful temperature range of all of the hydrides can be extended beyond the limits shown in fig. 3 by using higher pressures. Isobars at higher pressures will be shifted upward and to the right of those in fig. 3. However, for constant composition, the logarithm of the pressure varies as the reciprocal absolute temperature; thus, a substantial pressure increase allows only slightly higher temperatures.

400

J.B. l/etrano, Hydrides 104

T.K 9.0

7.0

8.0

1.0

6.0 O'PHASE LINE 'LATEAU ESSURES)

0.1

a~

0.01

0.~1 ~0

101~

1100

1~0

1300

TEMPERATURE,C

Fig. 5. lsochores in the yttrium-hydrogen system. 4.4. Isochores Isochores denote the change in dissociation pressure with temperature at constant composition. The three hydrides, ZrH2, YH2, and Till2, of principal interest for reactor moderators, exhibit a broad range of stoichiometry, and their dissociation pressures change with temperature and composition in the nonstoichiometric range. Extensive studies on these

materials yielded results summarized by the isochores in fig. 4 [33], 5 [ l l ] , a n d 6 [6]. Such isochores were useful in the SNAP program. For the SNAP 10A and SNAP 8 reactors, the combined fuel-moderator is prepared by direct hydriding of a Zr-10 w/o U alloy. During the hydriding cycle, the uranium is precipitated out, and all of the hydrogen is associated with zirconium. The dissocia-

J.B. Vetrano, Hydrides

401

5.1

I

T,K 15

~

13

12

II

I0

9

8

5.0

1.0

t

I

I

• REF.6 o REF. I 31

I

Q

4.9

4.8

aofl¢¢)

O.

4.7

0.I

-

F-8 +

4.6

1.5

4.4

0.01

1,4

I 1.5

I 1.6

I 1.1

l 1.8

' 1.9

2.0

HIZr. ATOMICRATIO Fig. 7. Lattice parameters in the zirconium-hydrogen system.

O.OOl

TEMPERAKIRE,C Fig. 6. l s o c h o r e s in the t i t a n i u m - h y d r o g e n system.

tion pressures of the SNAP fuels can be determined from the isochore map in fig. 4 by considering the uranium as inert to both the zirconium and hydrogen and by treating the ternary system as a binary of the zirconium-hydrogen system. The dissociation-pressure characteristics of hydrides of bimetallic systems are generally predictable from the dissociation pressures of the simple hydrides of the constituent metals. The metalhydrogen bond is generally stronger than the metalmetal bond in the alloy, with the results that hydriding the alloy produces a mixture of simple hydrides. For these reasons, efforts to increase the hydrogen content (or to decrease dissociation pressure) of such hydrides as ZrH 2 or YH 2 by alloying have been unsuccessful.

4.5. Crystal structure and density The room-temperature crystal structure and density (calculated from lattice parameters) for various hydrides are given in table 3; These data ale for the fully hydrided phase. In general, lattice parameters are different for the nonstoichiometric composition. The hydrogen density at room temperature, expressed as N H (see eq. 4) is also included in this table. Because zirconium hydride and yttrium hydride are of great interest as solid moderators for compact, high-temperature reactors, their lattice parameters and densities have been determined over a broad range of composition. Fig. 7 gives some lattice-parameter data for zirconium hydride, and table 4 lists some bulkdensity data for various hydrides. 4.6. Thermodynamic properties The two-phase plateau pressures of the hydrides can be used to determine some of the standard thermodynamic properties of these materials if certain additional information is available. The plateau-pressure reaction is:

J.B. Vetrano, Hydrides

402

Table 3 X-ray and density data for hydrides. Structure

Lattice parameter, A ao

AcH2 BaH 2 Bell 2 Call2 CeH2 CeH 3 CsH DyH 2 DyH 3 ErH 2 ErH 3 EuH2 GdH 2 GdH 3 HfH 2 HoH 2 HoH 3 KH LaH 2 LaH 3 LiH Lull 2 Lull 3 MgH2 NaH NdH2 NdH 3 PrH 2 PrH 3 Pull2 Pull 3 RbH ScH 2 SmH2 SmH 3 SrH 2 TbH 2 TbH 3 Till 2 ThH1.93 TmH 2 TmH 3 UH 3 YbH 2 YbH 3 YH2 YH 3 ZrH2

Fluoride Orthorhombic Orthorhombic Fcc Fcc Fcc Fcc Hcp Fcc Hcp Orthorhombic Fcc Hcp Fct Fcc Hcp Fcc Fcc Fcc Fcc Fcc Hcp Tetragonal Fcc Fcc Fcc Fcc Fcc Fluoride Hcp Fcc Fcc Fcc Hcp Orthorhombic Fcc Hcp Fcc Fct Fcc Hcp Cubic Orthorhombic Fcc Fcc Hcp Fct

5.670 6.788 5.936 5.575 5.539 6.389 5.201 3.671 5.123 3.621 6.21 5.303 3.73 4.919 5.165 3.642 5.708 5.663 5.604 4.0834 5.033 3.558 4.5168 4.879 5.464 5.435 5.518 5.483 5.359 3.78 6.049 4.783 5.374 3.782 6.364 5.246 3.700 4.454 5.735 5.090 3.599 6.645 5.871 5.191 5.201 3.674 4.9813

Ref.

bo

Co

7.829 6.838

4.167 3.600

6.615

3.77

6.526 7.16 6.71 4.363 6.560

6.443 3.0205

6.76

7.345

6.779 3.875 6.658 4.971 6.489

3.561

6.763

6.599 4.4485

[34] [35] [ 1] [351 [ 19, 36] [19, 36] [39] [381 [381 [381 [38] [39] [23] [23] [8] [38] [38] [40] [41] [41 ] [42] [38] [381 [43] [40] [38] [38] [38] [38] [44] [44, 45 ] [40] [46] [38] [38] [35] [38] [38] [47] [481 [38] [38] [49] [36] [36] [ 11 ] [ 11 ] [50]

Density (g/cm a)

~H

8.35 4.180 0.57 1.913 5.448 5.594 3.410 7.764 7.118 8.358 7.628 6.102 7.077 6.555 11.363 8.047 7.402 1.432 5.154 5.357 0.775 9.220 8.367 1.419 1.372 5.956 6.093 5.650 5.799 10.40 9.61 2.595 2.860 6.523 6.096 3.288 7.416 6.824 3.752 9.511 8.633 7.964 10.914 8.222 8.311 4.293 3.958 5.610

4.39 3.61 6.22 5.47 4.62 7.06 1.59 5.68 7.77 5.95 8.10 4.77 5.37 7.41 7.58 5.81 7.96 2.15 4.41 6.82 5.87 6.27 8.49 6.49 3.44 4.90 7.47 4.76 7.28 5.09 7.03 1.81 7.31 5.15 7.15 4.42 5.54 7.61 9.05 4.89 6.06 8.35 8.18 5.66 8.58 5.68 7.78 7.25

1022 H atoms/cm 3 of hydride

J.B. Vetrano, Hydrides

Table 4 Bulk density of various hydrides at room temperature. Material

Density (g/cm3)

Ref.

YHo:s YHI.o YHt.s YH2.o ZrH2.o + 10 U TiHo.s Till 1.o Till1. s Till2. o

4.406 4.365 4.332 4.295 6.06 4.24 4.02 3.83 3.73

[51] [51] [51] [51 ] [33] [61 [6 ] [6] [61

½nil 2 (g) + M # MHn

(5)

where M represents metal saturated with hydrogen in solution, and MHnrepresents hydride saturated with metal in solution. A conventional thermodynamic treatment, including an appropriate choice of standard states, shows that the standard free energy of formation of the stoichiometric hydride from the elements is A F ° = - R T In K = - R T In aMH n +RTlna M +RTlnP~/2

,

(6)

where K = the equilibrium constant;P = the plateau pressure, atm abs; a = the activity of the indicated substance. Thus, AF ° and In K can be determined from the plateau pressure and the activities of the hydride and the metal, and AH ° can be found from a plot of In K versus l I T . ~ S ° is then found from the relationship ~°=AHO_T~

° '

(7)

Because the activity data are generally unavailable, this method of determining the standard thermodynamic properties is not widely used. Calorimetry of specific reactions also can be used to determine the standard thermodynamic properties of hydrides. For the saline hydrides, a hydrolysis reaction can be used, while for the transition.metal hydrides, heats of combustion or hydriding are usually

403

used. The standard heats of formation determined by these calorimetric experiments are then combined with entropies of formation (derived from heat-capacity data) to give free energies of formation. In a few cases, electromotive-force (emf) methods were used to determine the standard thermodynamic functions. The relationship AF ° = - n F E ° gives the free energy from a measurement of the cell emf, while the temperature coefficient of the emf yields the standard heat of formation (AH°). With heat-capacity data, the values for AF ° and AH ° can be reduced to room temperature. The methods discussed above were applied to hydrides to determine the standard thermodynamic properties summarized in table 5. The heat capacities of some hydrides at 298°K are given in table 6. The enthalpy change for LiH from 30 to 688°C is 951 cal/g [67], which corresponds to an average heat capacity in this range of 11.49 cal/moleC. 4.7. Thermal expansion The thermal-expansion coefficients of various hydrides are given in table 7. Where a temperature range is given, the expansion coefficient is the average over this range. The strong temperature dependence of the thermal expansion of hydrides is noticeable in the values for Till1.54. 4.8. Diffusion coefficients The diffusion coefficient for hydrogen in a hydride may be expressed by the conventional formula D = D O exp ( - z ~ - / D / R T ) , where D O is the limiting diffusion coefficient at infinite temperature, and &/-/D is the activation energy for diffusion. Data for the diffusion coefficients of hydrogen in some hydride systems are summarized in table 8. Measurement of rates of absorption or desorption at various temperatures provided most'of the data. Where only the activation energies are reported, a nuclearmagnetic-resonance technique was used. The volume.diffusion constant for zirconium in zirconium hydride ranges from 3.1. X 10- 12 cm2/sec

J.B. Vetrano, Hydrides

404

Table 5 Standard thermodynamic properties of hydrides. Hydride

Call 2 Call 2 KD KH KH LiD LiH LiH

-AF°298

-AH°298

Ref.

(kcal/mole)

-AS°298 (cal/mole-°C)

Method

(kcal/mole) 17.70 35.79 -

30.60 45.10 13.819 13.819 15.16 21.784 21.666 21.79 21.60 21.34 17.78 13.339 13.487 13.60 29.6 31.02 30.60 30.35 31.14 17.27 40.22 38.90

43.3 31.26 16.40 18.9 32.3 30.3 43.3 10.76 34.75 32.13

Plateau pressure Calorimetric Calorimetric Calorimetric Calorimetric Calorimetric Calorimetric Emf Calorimetric Calorim etric Plateau pressure Calorimetric Calorimetric Calorimetric Calorimetric Calorimetric Plateau pressure Calorimetric Calorimetric Plateau pressure Calorimetric Calorimetric

[18] [ll [401 [40] [521 [401 [ll [531 154] [52J

-

16.78 16.16 8,17 -

LiH MgH 2 NaD Nail Nail Till 2 UD3

-

UH3 UT3 YH1.75 ZrD2 ZrH2

20.6 17.70 29,86 29.32

[55] [40] [40] I521 [59] [56]

[571 [561 I56] [101

[581 [581

a c t i v a t i o n e n e r g y is 2 8 k c a l / m o l e [84]

c o n d u c t i v i t i e s o f z i r c o n i u m a n d y t t r i u m h y d r i d e s as a f u n c t i o n o f t e m p e r a t u r e have y i e l d e d w i d e l y s c a t t e r e d data. Part o f t h e s c a t t e r is d u e to c o m p o s i t o n a l varia-

4.9. Thermal conductivity The many attempts to measure the thermal con-

tions and part to hydrogen redistribution under the t h e r m a l g r a d i e n t i m p o s e d b y t h e test. P r o b a b l y t h e

at 7 7 0 ° C to 3.3 X 10 - 1 1 c m 2 / s e c at 9 8 8 ° C . T h e

Table 7 Thermal expansion coefficients of hydrides.

Table 6 ~leat capacities of hydrides. Hydride

Cp at 298°K (cal/mole-°C)

Ref.

Material

Temperature (°C)

ot (10-6/°C)

Ref.

LiH ZrH2 ZrD 2 ZrHl.ss ZrD1 .s8 ZrH1.2s YH3 YD 3 YH 2 YD 2 MgH 2 UH3

7.2 7.396 9.631 7.5 9.8 7.39 10.363 13.727 8.243 10.773 8.44 11.78

[60] [61 ] [61] [62] [62] [63] [64] [64] [64] [64] [ 26 ] [66]

LiH (97%) ZrHl.54 ZrHI.83 Till1.54

200 200 20 30 500 20 20 20

36.1 + 0.8 14.2 9.15 5 13 17.9 12.2 16.3

[68] [691 [69] [70] [70] [51] [5] [71]

YH1.89 ZrHl. a + 7 U UH a

to 850 to 550

to 1000 to 800 to 500

J.B. Vetrano,Hydrides

405

Table 8 Diffusion coefficients for hydrogen in various hydrides. System

Phase

Zr-H

cz

t~

DO

-AH D

Ref.

7.00 X 10 - 4

7,060

[72]

21.7 X 10-4 5.32 X 10-3 7.37 X 10-3 1.3 X10 -2

8,380 8,320 8,540 12,500 12,690

[73] [74,75] [761 177] [78]

1.95 X 10-2 3.3 X 10-4 1.5 X 10-3

11,070 3,600 11,400 6,100

[79] [801 [80] [81 ]

1.8 X 10-2 1.95 X 10-3

12,380 9,400 6,640

[82] [83] [82]

2.11X 10-4

2,300

[311

H/Zr,ATOMICRATIO 0.5 I 30,000

8 G U-H

Ti-H

Th-H

flu #-U 3,-U UH 3

-

a ot /3

-

ThH2

1.0

'

~

1.5

2.0

* ~ T [ D t.TO $ R[OIOH_.I

BEFORE TESTINO

~"

,,[

~ z0,~

~_ lo,ooo

o

30

4O

50

6O

HYDROGENalo ,

Fig. 8. Tensile properties of zirconium hydride at 600°C. best value for engineering calculations is 2 watt/cm-C for either YH 2 or ZrH 2 at 350 to 700°C, which is of the same order as the thermal conductivity of the 300-series stainless steels. The nonmetallic saline hydrides have thermal conductivities lower than those of the metallic hydrides. Lithium hydride, for example, has a thermal conductivity of about 0.05 watt/cm-°C at 350°C. At 717°C, liquid LiH has a conductivity of 0.04 watt/ cm-°C [67]. In heat conduction, LiH is more in a class with materials such as TiO 2 or other refractory oxides.

5. Mechanical properties 5.1. Tensile strength The tensile properties of zirconium hydride and fueled zirconium hydride depend strongly on temperature and H/Zr atom ratio. This makes it particularly difficult to give a "handbook value" for the tensile strength. Fig. 8 [ 1] shows some tensile data on zirconium hydride at 600°C [6, 85]. Most of these data were taken under conditions that did not permit establishment of the equilibrium phases at 600°C. Nonetheless, the data shows that the 6- and e-phase zirconium hydrides are quite brittle even at elevated temperatures and are not too strong in tension.

There is practically no ductility in yttrium hydride, For YH1. 7 at 870°C [86], a 0.2% offset yield point 80 to 170 psi below the ultimate breaking stress has been reported. In short-time tensile tests at temperatures up to 1100°C [1 ], YHI. 9 shows essentially no elongation. In view of the brittleness of yttrium hydride, it is not surprising that the reported tensile data exhibit a high degree of scatter. Average tensile strengths from a large number of experiments on YH1.9 are given in table 9. 5.2. Transverse rupture strength The room-temperature transverse rupture strength of ZrH1.85 is reported to be 9000 psi. The addition of 2.3 w/o niobium to the hydride results in a rupture strength of 20,000 psi, a substantial improvement [87]. Additions of 7 or 10 w/o uranium to ZrH1.9 have much the same strengthening effect, and result in rupture strengths of 18,000 psi and 16,000 psi, respectively, at room temperature and about 24,000 psi at 600°C [87]. In transverse bend tests at room temperature, Till0.93 has a strength of 63,000 psi and undergoes brittle fracture [70]. As the temperature is increased, the/3 phase is formed (see phase diagram in fig. 1a), and the material becomes quite ductile. At 400°C no breaking was observed in transverse bend tests of Till0.93.

J.B. Vetrano,Hydrides

406 Table 9 Ultimate tensile strength of YH ; ,9.

Temperature

20

540

870

1100

7000

7000

4500

4000

(°c) Ultimate tensile strength (psi)

The abrupt changes in hardness of zirconium hydride at H/Zr ratios of 1.65 to 2.0 are a result of the 6 ~ e phase transformation. The microhardness of gadolinium hydride increases abruptly between the composition range of GdH2.0 and GdH3.0 as a result of the transformation from a fcc to a hcp structure.

6. Special properties The room-temperature modulus of rupture of coldpressed LiH is about 1800 psi, while that of cast LiH is about 4000 psi [88]. For the cast material, the modulus of rupture is quite dependent on grain size. 5.3. Compressivestrength Unalloyed zirconium hydride of reasonably high purity has a low compressive strength. Values of 13,000-psi yield and 19,000-psi ultimate have been reported [87] for ZrH1.8_ 1.9" Alloying with 2.2 w/o niobium raises the yield strenth to 48,000 psi and the ultimate to 91,000 psi. The addition of 10 w/o uranium to zirconium hydride increases the compressive yield strength to about 30,000 psi and the ultimate to about 70,000 psi [87]. In a search for high pressure polymorphs of LiH the fractional change in volume with pressure was measured. These data yielded a compressibility value of 3.7 X 10 -6 cm2/kg [89].

5.4. Hardness The room-temperature hardness values for a few hydrides are given in fig. 9 [6, 7, 33, 86].

300

I

[

I

I

2 "''~'f,..

. . . . . 1_.5 Gd-HYDRIDE

HYDRIDE)~

--

=E

0 0

I

I

I

I

0.5

l.O

1.5

2.0

H/M, ATOMICRATIO

Fig. 9. Hardness values for various hydrides.

6.1. Thermal migration Under the influence of a thermal gradient, the hydrogen in substoichiometric hydrides migrates out of the high-temperature zones into the low-temperature zones. In zirconium hydride, the steady-state hydrogen-concentration profile approaches an isobaric distribution, i.e., the isobar (concentration versus temperature curve) which satisfies the condition of conservation of hydrogen. In the case of a/3 + 6 two.phase mixture (see fig. 1b), one end of the isobar is in the/3-phase region and the other is in the 6-phase region, so that hydrogen migration is very pronounced. This process of thermal migration is best defined quantitatively by nonequilibrium (irreversible) thermodynamics. For the case of a hydrogen in a metal, the defining equation is

DnH( dlnn H Q* dT], JH ="--~ RT dx +-T -~ ]

(8)

/

ZOO

IO0

Special problems which arise from the high mobility of hydrogen in hydrides and which must be solved to permit the use of hydrides as a reactor material are described in this section.

where JH = hydrogen flux passing a plane at time t, moles/cm 2-sec; D = hydrogen diffusivity, cm2/sec; n H = hydrogen concentration in plane at time t, moles/cm 3 ; x = distance along direction of hydrogen movement, cm; Q* = heat of transport, kcal/mole of hydrogen; T = temperature,°K; R = universal gas constant. Integrating eq. (~) under steady-state conditions (i.e., J n = 0) yields

J.B. Vetrano, Hydrides

a~

nH = C e x p ~ ,

(9)

where C is an integration constant. The Soret coefficient, Q*, characterizes the thermal-migration situation. A value of Q* can be found from the slope of a plot of the logarithm of the hydrogen concentration versus reciprocal absolute temperature under steady-state conditions. In practice, then, thermal-migration experiments in hydride systems take the form of subjecting a long rod of uniformly hydrided material to a temperature gradient (usually linear, as this simplifies the boundary conditions) in a closed system where there is no possibility of loss of hydrogen. The void volume of the system is also kept at a minimum to maintain essentially all of the hydrogen in the condensed phase. The resulting hydrogen-concentration profiles on

101

_

~-" lO'Z

,

1.~

fs E

1.~

,

oY,

1.~

,

B , ,ASTEU.OY ' "

_

uM

10-3

10-4

,I 4OO

5O0

60O

70O

80O

TEMPERATURE, C

Fig. 10. Permeability of various metals to hydrogen.

407

specimens treated in this manner are then determined, and an "effective" Q* is calculated from eq. 9. By plotting the effective Q* found in this manner against reciprocal time at temperature and extrapolating to zero (i.e., infinite time), the Q* for steady-state conditions is found. For ZrH 1.6_ 1.8 the value of Q* is 1.27 kcal/mole, and for Zr-6 w/o U containing 100 ppm hydrogen, it is 0.8 to 1.8 kcal/mole [90]. 6.2. Hydrogen retention Since metal hydrides dissociate to metals and hydrogen at elevated temperatures, they require some type of cladding protection for use under these conditions. Three types of hydrogen barriers have been used with varying degress of success: (1) metal cladding, (2) glass-coated metal cladding, or (3) selfprotecting oxide or nitride layers on the surface of the hydride itself. While metal claddings are good hydrogen barriers at low temperatures, at temperatures required for the operation of most reactors, the metal claddings are quite permeable to hydrogen if the surfaces are free of oxide films. Fig. 10 shows permeability data [91 ] for a number of clean metals for a 1 atm hydrogenpressure differential across a I-ram-thick section. The permeability under other conditions is readily calculated, since the flux is proportional to the square root of the pressure and inversely proportional to cladding thickness. The thin (1 to 5 mil) glass-enamel coatings developed for oxidation protection in hot-gas exhaust pipes are quite impermeable to hydrogen. Because reproducible data were unattainable with these materials, it is not possible to give an exact value, but certain types of glass coatings have less than one-tenth the permeability of molybdenum [92]. The nonreproducibility of data can be traced primarily to variations in bonding at the metal-glass interface and to aging of the glass at high temperatures [92]. The aging process can result in volatilization of the less stable species (primarily alkali oxides), or in the occurrence of nucleation and precipitation reactions in the complex supercooled liquid. Some evidence points to a rate-controlling mechanism involving surface adsorption (rather than diffusion) for the passage of hydrogen through glass, since the permeability is not proportional to the square root of hydrogen

408

J.B. Vetrano, Hydrides

Table 10 Properties of some complex ternary hydrides. Compounda

TiNiTiHx TiC "Till o.46 ZrC'ZrH1.46 HfC'HfHo.86 ZrN'ZrHx ThC "ThH2 ThC'2ThH2

3.04 - 3.05 3.083 3.347 3.291 3.288 3.816 6.50

bo

Co

A

B

3.80

5.01 -5.04 5.042 5.469 5.385 5.468 6.302 10.91(~=119 °)

-

10.91 9.82 9.24 -

pressure as expected for a diffusion mechanism [92]. Oxide and nitride layers on the surface of hydrides of zirconium or titanium are very ineffective hightemperature hydrogen barriers in a vacuum. The escaping hydrogen puts the ceramic layer in tension, which causes it to fracture, and there are no mechanisms for healing the coating. In environments such as air, where the barrier can be continuously reformed, hydrogen losses are less [70]. In the KNK reactor in West Germany, hydrogen losses from unclad zirconium hydride exposed to a flowing stream of sodium at 550°C are about equivalent to losses from stainless-steel-clad hydride over a period of several thousand hours [93]. Small amounts of oxygen in the sodium are probably reacting with the zirconium hydride to form a protective oxide film.

7. Radiation effects The growth of zirconium hydride-10 w/o uranium during irradiation has been studied extensively in the SNAP reactor program. Analysis of data from many experiments [94] indicates that at 1400°F (760°C) and for values of burnup (b) between 0.05 and 0.8 metal a/o, the diametral growth is %ADID = 0.566 + b

-A Log P(torr) = T ~ + B

Lattice parameter (A)

(10)

with a standard deviation of 0.159. Since the scatter is quite large, further work is in progress to isolate the significant variable. The growth at temperatures

9,560 10,670 9,870 -

Ref.

199] [100] [ 1001 [ 100]

llOll [ 1021 [102]

other than 1400°F (760°C) can be determined by using the following temperature dependence: %AD/D o: 0.3 + 235 exp [-12,500/T(F)] .

(11)

Some irradiation experiments have been conducted by Interatom in connection with the KNK-reactor development program. The results of these experi. ments [93] indicate that there are negligible changes in the density of ZrH1. 7 after irradiation to fluences of 7 X 1020 to 1.1 X 1021 n/cm 21 ( E > 1 MeV) at temperatures up to 570°C. Fueled yttrium hydride is dimensionally stable. In one experiment, a yttrium hydride-10 w/o uranium alloy irradiated to 6% burnup of the 235U at 950°C in 640 hr showed practically no change in diameter. The irradiation stability of a hydrided Y-5 Cr alloy (N H = 5.3 to 5.5) clad with an Fe-20Cr-4.5 A1 alloy has been investigated [ 9 5 - 9 7 ] in conjunction with the ANP program. Volume changes were less than 1% at 925°C after a fluence of 2 X 1020 n/cm 2 (E > 1 MeV), accumulated over 10,000 hr. Titanium hydride is also relatively stable under irradiation. No significant changes in dimensions or weights were noted after irradiation at 450°C [98] to fluences of 6 X 1020 n/cm 2 thermal and 4.5 X 1019 (E > 1 MeV). The irradiation behavior of LiH is quite complex. In some experiments, the irradiation produces very little change in the material aside from darkening due to F-center* information. Other irradiations cause * A vacant site normally occupied by a negative ion will trap an electron to form a stable configuration called an "F center'

J.B. Vetrano,Hydrides

409

Table 11 Hydrides of intermetallic compounds. Hydride

Lattice parameter (~)

Structure ao

Hf2CoH s HflMnH s HfV2H4 PrsGa2 H6 ThAIH2 .s Th 2AIH4 ThCoH4.2 TiCuHo.s Ti2CuH4 Ti2GaH 3 Ti2NiH3 TiaAuH 3 Zr2AIH 3 (LiH4)Rh ZrCr2Ha.s ZrNiH 3 ZrV2Hs

? ? C15 (fcc) ? Bf Bf B 11 (tetragonal) E9s Bcc E9a A15 Hexagonal Tetragonal C15 (fcc) Bf C15 (fcc)

12.58 7.610 7.600 3.139 11.46 6.383 11.896 5.288 5.590 11.240 7.654 3.53 7.900

severe swelling and spalling. Welch [88] analyzed all available data and concluded that the cases o f extensive damage are usually associated with the presence of LiOH as an impurity. High-purity LiH is stable to fluences o f 1019 n / c m 2 thermal or 1016 n / c m 2 (E > 1 MeV) above about 1000°C. At higher temperatures even higher fluences can be tolerated.

8. Ternary

systems

There are a few known cases in which hydrogen reacts with a binary compound to form a ternary hydride that has a structure and stoichiometry distinctly different from any o f the combinations o f binary compounds in the system. These compounds can be of the form MxM'yl-Iz,where M and M' are b o t h metals, or Mx.4yHz, where A is a nonmetal (usually carbon or nitrogen). For the most part, studies have determined only the structure of these compounds because they do not seem to offer any advantages over the simple binary hydrides for moderators in compact systems. The thermal stability o f some o f these compounds is good, but the hydrogen density is generally low. Table 10 lists structural data on some

Ref.

bo

Co

-

-

11031

-

-

[1031

_

_

[103

-

-

[1031

l

-

-

iI03l [1031 [1031

_

_

[1031

-

-

11031

-

-

[1031

-

-

-

-

-

9.706 8.932

_

_

-

6.527

-

-

10.48

4.30

-

-

[103]

11031 [103] [104] [103] [105] [103]

of the MxAy ternary hydrides, and table 11 gives the available information on the M x ~ y hydrides.

R e f e r e n c e s

[ 1] W.N.Mueller, J.P.Blackledge, and G.G.Libowitz, Metal Hydrides (Academic Press, New York, 1968). [2] J.B.Vetrano, Method of making delta zirconium hydride monolithic moderator pieces, U. S. Patent 3,018,169, to USAEC (January 23, 1962). [3] R.Van Houten and W.G.Baxter, Development and evaluation of high N H metallic hydrides of zirconium (NH 6.0-6.6) and titanium (N H 5.5-5.8), Trans. Am. Nucl. Soc. 5 (November 1962) 488. [4] A.C.Neeley, J.M.Googin, and J.J.Asbuty, The role of powder metallurgy in the fabrication of very large parts, USAEC Report Y-DA-1053 (Union Carbide Corporation, February 18, 1966). [5] J.D.Watrous, Thermal expansion of hydrided zirconium alloys, USAEC Report NAA-SR-TDR-6046 (Atomics International, Janurary 1961). [6] R.L.Beck, Research and development of metal hydrides, USAEC Report, LAR-10 (Denver Research Institute, November 1960). [7] R.L.Beck, Zirconium-hydrogen phase system, Trans. ASM 55 (1962) 542. [8] G.G.Libowitz, The nature and properties of transition metal hydrides, J. Nucl. Mater. 2 (1960) 1-22.

410

J.B. Vetrano. Hydrides

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