Irradiation effects on beryllium oxide materials

JOURNAL

OF NUCLEAR

MATERIALS

IRRADIATION

14 (1964) 125-134,@

EFFECTS

NORTH-HOLLAND

ON BERYLLIUM

PUBLISHING

OXIDE

CO., AMSTERDAM

MATERIALS

t

R. G. MILLS, John

Jay Hopkins

J. 0. General Atomic

BARNER,

Division,

Laboratory for Pure and A@lied D. E. JOHNSON

General Dynamics

The results of irradiation experiments on 134 beryllium oxide specimens are presented. The variables that have been studied include the effects of the density, grain size, composition, shape, and temperature of irradiation of the specimens. The peak exposure in the irradiation capsule was 1.0x 10” nvt (> 1 MeV) fast neutron flux. The temperatures of the specimens ranged from about 340” C to 720” C. The most striking result is the excellent resistance to cracking exhibited by the high density, fine grain size specimens. The highest strength and strength retention

Science

and M. T. SIMNAD

Cor$oration,

San Diego,

Calif.,

USA

after irradiation was exhibited by the high density, small grain size, 1 per cent MgO-containing specimens irradiated in the high neutron flux-high temperature section of the capsule. Thermal expansion measurements provide a unique means of establishing the annealing behaviour of the defects that are produced by irradiation. In irradiated beryllium oxide, a contraction sets in at 860’ C. Growth of Be0 specimens as high as 1.4 per cent was noted. Strengths of irradiated pieces decreased to values of 14 to 70 per cent of the pre-irradiated. values.

tonite, magnesium oxide, alumina) appeared to improve the irradiation stability of the propThe Experimental Beryllium Oxide Reactor erties measured. In the majority of the will use a dispersion type fuel consisting of specimens that were irradiated the increase in uranium dioxide particles in a beryllium oxide dimensions was less than 1 per cent, the density matrix. The core moderator and reflector will decreased by less than 3 per cent, the axial be beryllium oxide. compressive strength decreased by 60 per cent An irradiation experiment was designed to to 80 per cent, and the thermal conductivity furnish information on the influence of grain decreased by 15 per cent to 40 per cent. size, density, composition, shape, method of There was evident need for further exfabrication, and temperature of irradiation of periments to determine the manner in which the beryllium oxide. This paper is a progress the numerous variables influence the behaviour report on this experiment. of beryllium oxide under irradiation. A new The status of irradiation work on beryllium irradiation capsule (MGCR-2) experiment was oxide has been reviewed in a number of recent designed to irradiate specimens containing papers l-18). The results of previous work at three different fluxing additives; namely, MgO, General Atomic on the irradiation of beryllium MgO + Al,O, , and bentonite. Both high and oxide were presented in a summary report 18) low density, and small and large grain size in 1962. A wide selection of beryllia materials specimens were included in each composition, was irradiated at temperatures of 540’ to with a total of 134 specimens in the capsule. 1040” C, to integrated fast neutron flux of The peak exposure in the capsule was estimated 2 x 10zl nvt (> 1 MeV). The results indicated to be 1.25x 10zl nvt ( > 1 MeV), and the temthat the use of some fluxing additives (ben- perature ranged from 340’ C at the low neutron1. Introduction

t This work was supported

by the U.S. Atomic Energy

Commission

under contract

At (04-3)-187. III.

RADIATION

EFFECTS

126

R. G. MILLS, J. 0.

flux

regions up to 720” C at the high neutronregions in the capsule. The properties

flux that

have

been

and diametral sity, strength, in

progress

measured

BARNER,

include

the

D. E. JOHNSON

that

axial

dimensions, microstructure, denand thermal expansion. Work is to

measure

thermal

diffusivity,

the

AND

Be0

grains

in the

large

grain

size

specimens were rectangular in cross section, with the larger sizes approximately 50 by 150 microns. The grains in small grain size specimens were 24 to 50 microns

stored energy, modulus of elasticity, and the rates of release of helium and tritium upon post-

3.

irradiation

were sealed

annealing.

M. T. SIMNAD

in cross section.

Irradiation One hundred

and thirty-four

in a helium

Be0

atmosphere

specimens inside

a

TABLE 1 Dimensions of Be0 specimens Specimen 1Number oi specimens t-e 16 10 10 32 32 32 3

TOTAL

2. Preparation

Outside diameter (cm)

Inside diameter (cm)

Length (cm)

4.27 4.27 1.92 4.27 3.47 2.44 4.27

zero 1.93 zero 3.48 2.45 zero zero

1.27 1.27 1.27 1.27 1.27 1.27 0.64

134

of Materials

The specimens were formed in the beryllium oxide facility at General Atomic. They included solid discs and hollow rings of various diameters, and these were subjected to a wide range of temperatures, stresses, and neutron-flux levels in the capsule. The fluxing additives included 1.25per cent bentonite, 1 per cent magnesium oxide, and 1 per cent of a mixture of equal parts of magnesium oxide and alumina. The grain sizes varied from 16 to 50 microns, and the densities from 83 per cent to 98 per cent. The dimensions of the several types of specimens that were used are summarized in table 1. The starting powders were calcined GC-grade or UOX-grade beryllium oxide with the required amounts of the fluxing additives. The specimens were prepared by cold pressing at 700 kg/cm2 (10000 psi) followed by sintering at various temperatures from 1430"C to 1640"C. Five specimens were prepared by a hot pressing technique. Metallographic examination revealed

stainless steel can. The irradiation was conducted in the General Electric Test Reactor during the last half of 1962.The heat flux at the outer surface of the capsule was calculated to be 5.5 x 10’ watts/cm2 and the capsule surface temperature was calculated to be 80-100’ C. The axial temperature distribution as calculated is shown in fig. 1. Temperatures of the specimens ranged from 360'C to 720” C along the centreline of the specimens, and from 340” C to 550” C along the edges. The integrated fast flux is also illustrated in fig. 1.

4. Results and Discussion 4.1.

APPEARANCE

OF SPECIMENS

A photograph of a set of irradiated specimens is shown in fig. 2. An analysis of the distribution of cracked specimens is presented in table 2. A total of 55 specimens with fluxing additives had broken into several pieces. The ones that fractured included 17 of the disc type (out of 60), 16 of the inner ring type (out of 32),

TABLE

;r

No. of Specimen 1 2 3 4

Integrated Diistance from ‘b :utron Flux to p of capsule n t(> 1MeV) (inches) x 1080 2 0

5 6 7 8

5

3.5

9

10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44

10

6.5

Summary Fluxing Additive

Nensity(=I

Bentonite I, 8, II >. I> ,> II ,, ,I MgG ,I >. I, ., I> ,, 3, MgO-Al,O, .I ,, >, >. 8, ,# ,> ,, >, ,, ,I I, ,I 8, Bentonite

lb

20

9

$8 3, 8) >. ,. ,, 3. I, I> MgG *> ,> ,I ,I

10

High

II Small

>. I, 9, High

9, Small

II ,, I, Low

>I Small

I, 3, ,, High

I, I, ,I Medium

LOW

,I

a,

8,

8, .,

High

46 47 48 49 50 51 52 53 54 55 56 57

I* I, I> I, >, I> *> I, 8, I, 8. II ,, 3, Hot Pressed

8, ,I ,I Low

I> ,, ,, ,I 30

(a) (b) (c) (d)

4

8, 2, 2, .>

dedium

,, I. ,. Low

MgO-Al,O,

8

Grain Size (b)

,. I, ., >. I, I> II I, ,I Large

,, I> ,> >, ,I 8, ,, >, ,, I> I> ,J I,

.I

of specimens

.P 9. 8, 3, ,> I, II II ,,

45

25

2

of appearance

., .> .> High >> I> ,, >.

I> Large

Large

>, Large

,> ,I I, I, ,J I, ># ,. ,, Large I> Small I, Large ., Small 8, Large I, Small ,, Large 8, ?I 3, .I 8. I, I> I>

Specimen

Shape (C)

-

nner Ring

Disc

-

c)uter

Ring

Intact I, ,> II ,, 8, I> ,I II .. I, ,I ,* >, I, ,, >I 8, ,. I, ,> ,I ,, ,, ,, ,I ,I ,I II II 3. Broken (2) I, (2) ,, (2) >, (2) >, (2) Intact ,I I, II Broken (4) ,I (4) ,I (2) I, (2) Intact ,, ,, I, I> I> I, I> Broken (2) ::

$(c

::

(2)(? (x)(1

Intact ,, 9, II 13rok& (3) Intact

Intact >I I, I>

,I Broken (4)

Broken (2) Intact ,, 2, ,I I> >> Broken (2) ,, ,, I>

(3) (3) (2)

>> >* ,. I, Intact 0 ,I ,# Broken

(3) (3) (3) (3)

(4)

,> (4) 8, (2) ,I (3) Intact ,I >I ,, II

Intact Broken (2) Intact

Broken

,I Broken

(4)

>, Broken (5) ?

,, (4) >, (2) .? (3) 1 crack 1 crack Intact

>. (b)? I, (b)? I, (b) ? I> (4) I, Intact

,, Broken II >> >,

(2) (2) (3) (3)

(4)

,I (4) Intact

$8 Broken (4) ? ,, ,, >,

(4)? (4)? (4)?

High density is > 93 ye of theoretical density, low density is < 93 ye of theoretical density. Large grain size is about > 30 microns, medium is about 20 to 30 microns, small is about < 20 microns. The numbers in bracketsdenote the number of fragments in the broken specimens. These hot pressed specimens also crumbled into powder in the outer half.

128

R.

G. MILLS,

J. 0.

BARNER,

D.

and 22 of the outer ring type (out of 42). This correlates well with the fact that the rings were at relatively lower temperatures during irradiation than the corresponding discs, and were therefore more subject to damage by irradiation. The most striking correlation is between the density, the grain size, and crack sensitivity under irradiation. Specimens with high density and small grain size exhibit good resistance to irradiation over the whole range of integrated neutron fluxes and the corresponding range of temperatures, whereas the spec-

E.

JOHNSON

AND

M. T. SIMNAD

tion under the radial thermal gradients. The cooler outer sections grew more than the central sections to give rise to stresses that initiated the cracks from the centres of these specimens. None of the bentonite discs with smaller diameters (1.92 cm) showed any signs of cracks, even though their structures and densities were similar to the large diameter discs. Of the MgOcontaining discs, the four specimens that fractured were all of low density and were in the high-neutron flux region. Of these, two had large grain size and broke into more than four T 700

OF -

1200

600

1.0

- 600

0.6

II FAST FLUX DOSE INVTb

II 6.55 X 106 (FLUX)

0.6

L = LARGE S = SMALL

GRAIN SIZE GRAIN SIZE

Fig. 1. Centreline temperature and fast flux dose as a function of specimen position in MGCR-2 capsule irradiation.

imens with low density and large grain size showed very poor resistance to irradiation. The following description of the specimens confirms this conclusion: 4.1.1. Disc

Tyfie Specimem

The seven bentonite-containing specimens that fractured had medium grain size (26 to 30 microns) and high density. However, the diameter of these specimens was evidently too great (4.27 cm) to sustain the stresses and strains induced by the inhomogeneous dimensional changes, which were caused by irradia-

pieces, while two had small grain size and broke into two pieces. None of the high density MgO-containing discs showed any sign of cracks. None of the MgO-A&O,-containing discs fractured or cracked. The five hot-pressed, very high density pure Be0 specimens at the bottom of the capsule all cracked into pieces. 4.1.2. Inlzer Ring

Ty$e

S$ecinaerts

Of the MgO-containing specimens, six were fractured. Five of these were low density (three large grained, two small grained), and one was high density large grained. The latter was in

129

IRRADIATION EFFECTS ON BE0 MATERIALS

the highest neutron flux region in the capsule. The fractured small grained specimens were in two pieces, whereas the large grained specimens were broken into four or more pieces. None of the high density, small grain size specimens containing MgO broke or cracked. The eight MgO-Al,O,-containing specimens that fractured all had low densities and large grain sizes. Two other MgO-Al,O,-containing specimens exhibited a crack. These were in the high neutron flux section and had a high density and large grain size. None of the high density,

Fig. 2. Post irradiation photograph of a typical set of specimens in MGCR-2 capsule. Specimen composition is 99 % BeO, 1 y0 MgO+Al,O,. (The scale is in inches).

small grained MgO-A&O,-containing specimens broke or cracked. 41.3.

Owter Ring Type Sfiecimens

These had the lowest relative temperatures at any given level in the capsule and showed the largest number of broken pieces. Of the twelve MgO-containing specimens that fractured, eight had low density (four large grained, four small grained), three had high density and large grain size, and one had high density and small grain size. The latter specimen was the only high density, small grained specimen that cracked. It was in the lower temperature range where the irradiation effect was not mitigated by thermal annealing. Ten of the MgO-Al,O,-containing specimens fractured. Of these, eight were low density, large grained,

and two were high density, large grained specimens. Again, none of the high density, fine grained specimens broke up or showed cracks. 4.2. DIMENSIONAL CHANGES Each pellet was placed in between parallel, spring-loaded plates, and the dimensions were read with a dial-gauge micrometer to the nearest 10 microns. The diametral measurements of the cracked specimens were not accurate. Because of the radial temperature gradient in the specimens, the changes in diametral dimensions represent integrated values over a temperature range. Consequently, measurements of the changes in axial lengths are more reliable measures of dimensional changes and are more readily amenable to interpretation. The results are shown in fig. 3. The results indicate that the combined effects of integrated fast neutron flux and temperature govern the changes in dimensions that take place under irradiation. In addition, the results of the dimensional measurements corroborate the conclusions made above from examination of the appearance of the specimens. The outer rings, which were at the lowest relative temperatures at any given position in the capsule, grew more than the central discs or inner rings. Similarly, the large diameter discs grew more at the periphery than at the centre; for example, one of the large bentonitecontaining discs grew 1.40 per cent near the outer edge and 1.22 per cent at the centre. The length changes in the specimens, as a whole, ranged from zero to 1.40 per cent; the changes in diameter ranged from 0.24 per cent to 1.48 per cent. The average linear growth for the disc type specimens increased fairly uniformly to a maximum value at the central region of the capsule where the highest neutron flux was present. The extent of growth in the specimens can be related to the density and grain size and also to the diameter. This conclusion is supported by the following observations: III.

RADIATION EFFECTS

130

R.

G. MILLS,

J.

0.

BARNER,

D.

4.2.1. Disc Type Specimens The only fractured diameter

specimens

were the large

(4.27cm) discs that were in the high-

1.22per cent at the centres and from 1.16per cent to 1.40per cent near the edges. Evidently, the stresses that were generated by the radial strain gradient resulted in the initiation of in these specimens.

AND

M. T.

SIMNAD

region, and by over 1.3per cent with crumbling in the coldest region of the capsule.

4.2.2.Ring Type Specimens of

On the other hand,

In almost all these specimens, the extent growth was governed primarily by the

neutron flux and the temperature.

smaller diameter discs (1.92 cm) and ring shaped MGCR-2

JOHNSON

specimens, respectively. Other MgO-containing high density discs close to these specimens did not crack, and they grew in length from 0.24per cent and 0.48per cent in the warmer

est flux region. The length increase in these specimens ranged from about 0.87per cent to

fracture

E.

The structure

Be0 IRRAOIATION

LINEAR EXPANSION,%; VERSUS CAPSULE POSITION ( EXPANSION MEASURED AT THREE RADIAL POSITIONS FOR EACH SAMPLE OR SAMPLE GROUP1 I”

L

I.-.

AL,%FOR Rm2.OCM AL,% FOR R* I.OCM AL,%FOR

R-O.OCM

I.0 ap g-

0.6

J, 0.6

f t:

0.4

0.2

0 0

IO

20

30

40

SO

60

M FROM TOP

BENTONITE

HI = HIGH DENSITY LOW * LOW DENSITY

BENTONITE

L = LARGE GRAIN SIZE S = SMALL GRAIN

Fig. 3.

SIZE

Linear expansion of irradiated

(4.27 cm by 1.93 cm) bentonite-containing materials situated close to the large discs did not fracture even though comparable increases in length took place; namely, about 0.96 per cent to 1.22per cent in the rings and 0.5 per cent to 0.58 per cent in the discs. The other set of disc type specimens that fractured were the four low density, MgOcontaining discs that were in the high neutron flux region in the capsule. The linear growth in these specimens was 0.54per cent and 0.6per cent in the two small grained, and 0.98 per cent and 1.40 per cent in the two large grained

samples as a function

of specimen

position.

of the specimen influenced the growth only insofar as it governed the strength. For example, bentonite-containing rings did not crack even though the linear growth was as much as 1.22 per cent in one specimen. Similarly, some of the MgO and MgO-Al,O,-containing rings grew over one per cent without cracking, and these generally were the high density, small grain size specimens. In the specimens that fractured, the linear growth ranged from 0.12 per cent to over one per cent. The fractured specimens with low growth corresponded to the weak low density, large grained materials,

IRRADIATION EFFECTS

whereas fracture

those with larger growth before generally had small grain size.

ON BE0

MATERIALS

specimens

ranged

131

from

about

14 per cent

to

to

per cent of the pre-irradiation strengths. The factors that influence the strength include integrated neutron flux, temperature, density, and grain size. The highest composition,

measure the densities of the specimens. All the specimens decreased in density after irradiation and the general features of the changes in

strength retention in the irradiated specimens was exhibited by the MgO and the MgO-Al,O,containing, high density specimens that were

density paralleled the changes in dimensions. The density decreases ranged from about 0.24

in the section

per cent to 6 per cent. The density shown in fig. 4.

in both

4.3.

70

DENSITIES

The

water-buoyancy

method

was

VERSUS z a 2

used

changes are

ditions

CAPSULE

’

-6.0

-5.0

8

-40

the mm-radiated was obtained

and irradiated

with

the

high

con-

density,

POSITION

INDICATES FRAGILE ? INDICATES

4’ c ii

high neutron flux-high temperature of the capsule. The highest strength

SAMPLE TOO TO MEASURE NO DATA

c’ g +

-3.0

zl Y

-2.0

L I y

-1.0

a 3

0 0

20

IO

,,

30,*

40

I

50

*,60

,TO

C M FROM TOP OF CAPSULE

1BENToN’TE lL;“~;$~~~jjBENToNITE 1$:&,&,;~;~;j,f+

HI = HIGH

DENSITY

BENTONITE

LOW = LOW DENSITY

Fig. 4. 4.4.

L = LARGE

GRAIN

SIZE

S = SMALL

GRAIN

SIZE

Change in per cent of theoretical density of irradiated samples as a function of specimen position.

STRENGTH

A diametral compression test was used as a measure of the tensile strengths of the specimens. The test samples were obtained by a core-drilling technique. Although the load is compressive along the radial direction, failure starts where the circumferential tensile stresses are greatest since beryllium oxide is much weaker in tension than in compression. The results are shown in fig. 5. Irradiation caused a loss in strength in all the specimens. The post-irradiation strengths of the untracked

small grain size MgO-containing specimens, with strengths of about 20 000 psi pre-irradiation and up to about 13 600 psi post-irradiation. 4.5.

THERMAL EXPANSION

Several

irradiated specimens were heated to of about 4’ C and 8’ C per minute during determinations of their thermal expansion coefficient. The thermal expansion curves were compared with those of identical m&radiated specimens. Fig. 6 illustrates the type of curves obtained in these experiments, 1000’ C at rates

III.

RADIATION EFFECTS

R. G. MILLS,J. 0. BARNER,D.

132

in this case with a MgO-containing, high density, small grained specimen. The curves for the irradiated and the unirradiated specimens were parallel up to about 860” C. Beyond this temperature the irradiated specimens showed a marked contraction and the downward bend in the curve continued to 1000” C, which was the limit of the apparatus. Upon cooling the irradiated specimen to room temperature, it was found to have decreased in length by about 0.45 per cent, which was a return to its preirradiation length. Upon subsequent heating

E.

JOHNSONAND M. T. SIMNAD

beryllium oxide at temperatures above 800’ C, which is remarkably close to the temperature at which the beginning of contraction is observed in the thermal expansion measurements. 4.6. OTHERMEASUREMENTS Work is in progress to carry out measurements of the modulus of elasticity, thermal diffusivity, stored energy (using a high temperature differential thermal analysis calorimeter), and the release of helium and tritium upon post-irradiation annealing. Structural

MGCR -2 Be0 IRRADIATION DIAMETRAL COMPRESSION STRENGTH, VERSUS CAPSULE POSITION OUTER CORES, BENTONITE DISCS b INNER

CORE,

CONTROL IRRADIATED

BENTONITE

PSI X lO-3

DISCS

STRENGTH STRENGTH

C M FROM TOP OF CAPSULE

Fig.

6.

Diametral

compression

strength

of irradiated

to 1000” C the specimen did not exhibit any further deviations in its expansion curve, which paralleled the heating and cooling curves for the unirradiated specimens. Specimens that had shown little dimensional changes upon irradiation showed correspondingly small contractions during the thermal expansion measurements at temperatures above 860” C. This technique appears to be a useful tool for studying the annealing behaviour of the defects that are present in irradiated materials. It is of interest to note that Hickman 16)has predicted the release of stored energy from irradiated

samples as a function

of specimen

position.

studies are being made with the aid of electron microscopy. 5* SummaW 1. The results of irradiation experiments on beryllium oxide materials have furnished information on the effects of a number of variables, including the density, grain size, composition, shape, and temperature of irradiation of the specimens. 2. Three different fluxing additives were used in preparing the 134 test specimens: 1.25 per cent bentonite, one per cent MgO, and

IRRADIATION

EFFECTS

ON BE0

one per cent of a mixture of MgO-A&O,. The Be0 starting materials included the UOX- and GC-grades. The peak exposure in the irradiation capsule was estimated to be 1.25x 1021nvt (> 1 MeV) fast neutron flux. The temperatures of the specimens ranged from about 340° C to 720” C. The visual examination of the irradiated specimens indicated that 55 out of the 134 specimens had fractured and broken into several pieces during irradiation. The 1.2

7.

1.0 0. G 0.6 0.4

8.

0.2 0

i? !3 w

-0.2

-

RUN 2 ./

-0.4

,


-6. I 0

200

I 400

GO0

TEMPERATURE,

800

i 1000

1200

*C

Fig. 6. Linear thermal expansion of irradiated and unirradiated samples. Specimens are 99 y0 BeO, 1 y0 MgO, high density small grain size.

most striking correlation is the excellent resistance to cracking exhibited by the high density, small grain size specimens. The, low density, large grain size specimens showed very poor resistance to fracture under irradiation. The high temperatures in the highest neutron flux section of the capsule mitigated the effects of the higher integrated neutron flux on the specimens that were situated in this position. Measurements of changes in axial lengths of the irradiated specimens are more reliable measures of dimensional changes than diametral measurements. The growth in length of the specimens is governed by the combined effects of neutron flux and temperature, and by the density, grain size,

9.

10.

133

MATERIALS

and shape of the specimens. The length increases ranged from zero to 1.40 per cent. In the fractured specimens, the stresses that were generated by the radial strain gradients caused the initiation of the cracks. These strains were the result of the radial differences in irradiation growth caused by the radial temperature gradients in the specimens. The stronger high density small grained specimens withstood these strains (over one per cent increase in length) whereas the weak low density-large grained specimens fractured at relatively low strains. The densities of the irradiated specimens decreased by about 0.25 per cent to 6 per cent. The general features of the changes in density paralleled the changes in dimensions. The strengths of the irradiated specimens decreased to about 14 per cent to 70 per cent of the pre-irradiation values. The factors that influence the strength include integrated neutron flux and temperature, and the density, composition, and grain size of the specimens. The highest strength and strength retention under irradiation was exhibited by the high density, small grain size, MgO-containing specimens irradiated in the high neutron flux-high temperature section of the capsule. The thermal expansion of irradiated specimens shows a marked contraction above 850” C. This is directly related to the amount of linear growth under irradiation and is probably an indication of the annealing behaviour of the defects that are present in the irradiated material. Measurements of other properties are in progress.

6. Acknowledgement This paper has drawn on the work of many of the members of the Metallurgy and Chemistry Departments who are contributing to this project. These include the following: R. D. Abbey, H. F. Bothman, G. Buzzelli, B. A. Czech, III.

RADIATION

EFFECTS

134

R.

G. MILLS,

J. 0.

BARNER,

D.

P. E. Gethard, D. G. Guggisberg, K. Koyama, F. H. Lofftus, H. E. Shoemaker, L. R. Zumwalt. References

E.

JOHNSON

AND

M. T.

SIMNAD

lo) J.

Elston, International Atomic Energy Agency Conf. on Irradiation Damage, Venice 1962, paper No. DM1182 11)J. Elston, C. Labbe, H. Nouguier and M. Simbozel, “6th Colloquium on Metallurgv. Saclav. 1962”. (Presses Universitaires de France, Paris, France) I*) H. Frisby, A. Bisson and R. Caillat, J. Nucl. Mat. 1 (1959) 106 I*) High Temperature Materials and Reactor Component Development Program, Second Annual Report, General Electric Company Report No. GEMP-177 (1963) ip) B. S. Hickman, R. A. Sabine and R. A. Coyle, J. Nucl. Mat. 6 (1962) 190 ia) B. S. Hickman, Australian Atomic Energy Commission Research Establishment Report No. AAEC/E99 (1962) Ia) A. J . Rothman. Lawrence Radiation Laboratory Report No. IJCRL-6743 (1962) r7) R. P. Shields, J. E. Lee and W. E. Browning, Oak Ridge National Laboratory Report, ORNL-3164 (1962) l*) J. M. Tobin, General Atomic Report No. GA-2648 (1962) -u.

‘1 J. Aslanian, R. Caillat, M. Salesse and L. Weil, Comptes

Rendus 253 (1961) 1032 S. B. Austerman, NAA-SR-‘7654 (1963) P. 0. Budniker and R. A. Belyayef, Zhur. P&lad. Khim. 33 (1966) 1921 T. W. Baker and P. J. Baldock, UK Atomic Energy Research Establishment, Report No. AERE-M 1017 (1962) A. Bisson and H. Frisby, J. Nucl. Mat. 4 (1961) 133 F. J. P. Clarke, Progress in Nuclear Energy Series IV, 5 (1963) 221 F. J. P. Clarke and J. Williams, J. Nucl. Mat. 4 (1961) 121 F. J. P. Clarke, U.K. Atomic Energy Research Establishment, Report No. AERE-R 4276 (1963) J. Elston and C. LabbC, J. Nucl. Mat. 4 (1961) 143

1,’ 9

:; ‘1 9 9

<

.

.