The nucleation of bubbles and re-solution effects in uranium dioxide irradiated at elevated temperatures

JOURNAL

THE

OF NUCLEAR

MATERIALS

NUCLEATION

41 (1971)

OF BUBBLES

87-90. @ NORTH-HOLLAND

AND RE-SOLUTION

IRRADIATED

AT ELEVATED

R. M. CORNELL

PUBLISHINQ

EFFECTS

CO., AMSTERDAM

IN URANIUM

DIOXIDE

TEMPERATURES

and J. A. TURNBULL

Central Electricity Generating Board, Berkeley Nuclear Laboratories, Berkeley, GEos., UK Received

Uranium

dioxide

samples

have

been

irradiated

29 April

ces bulles

at

prendre

naissance

des trajeotoires

de faGon

de fragments

de fission. La redissolution de oes bulles induites par

of these samples by transmission electron

microscopy

se r&i;lent

h~t$rog~ne k l’aplomb

1200 ‘C to a variety of doses up to 3.2 x 1Oa5fissions/m3. Examination

1971

I’irradiation

has shown that there is a critical dose

s’est r&&lee Btre effective pour degager

below which intragranular fission-gas bubbles are not

les atomes de gaz concentres dans les pores de frittage

observed

du materiau.

dose

these

bubbles are shown to nucleate heterogeneously

to

form.

Above

this

critical

upon

fission-fragment tracks. Irradiation-induced resolution has been shown

to be effective

gas atoms

within

from

in the removal

the sintering

of

Urandioxid

pores of t,his

material.

bei

1200

Bchantillons

de

bi-oxyde

d’uranium

ont

et6

der

kritische

irradies a 1200 “C sous une gamme de doses s’etendant

granulare

jusqu’it 3,2 x 1025 fissions/ms. L’examen

wird.

de ces Bchan-

“C

existiert,

ergibt,

unterhalb

Spaltgasblasenbildung

Oberhalb

Die

Untersuehung dieser Proben

Durchstrahlungstechnik Dosis

versehiedenen

tit

1025 Spaltungen~ms best&&.

elektronenmikroskopische nach

Des

wurde

Dosen bis zu 3,2x

dieser kritischen

dass eine

welcher

nicht Dosis

intra-

beobachtet werden

die

tillons par microscopic

Blectronique par transmission

Blasen heterogen an Spaltfragmentspuren

a montre

une dose critique en dessous

wird gezeigt., dass die Abgabe von Gasatomen aus den

qu’il existait

de laquelle des bulles de gaz de fission intra~an~llaires

Sinterporen

ne se forment plus. An-dessus

induzierte

1.

de cette dose critique,

Introduction

Recent electron-microscope irradiated uranium dioxide intragranular

durch die bestrahlungserfolgt.

which no intragranular fission gas bubbles are observed to form, and that above this dose intragranular gas bubbles are heterogeneously

examinations of have shown that

fission gas bubbles

dieses Materials ~iederaufl~sung

gebildet. Es

are present

nucleated.

in material irradiated above N 800 “C to a variety of doses 1). The irradiation doses of these samples in all cases exceeded 3.2 x 1025 fissions/ma and therefore it was not possible to determine at what irradiation dose intragranular fission-gas bubbles first began to form. Experiments already carried out on these samples have provided evidence that the intragranular fission gas bubbles are heterogeneously nucleated upon fission-fragment track sites 1~2). Experimental confirmation of this conclusion is highly desirable at low irradiation doses when intragranular fission gas bubbles first begin to form. The experiments described below demonstrate that there is an i~adiation dose below 87

2.

Irradiation

details and

results

Uranium dioxide discs of enrichment 3.1y. 2sW were irradiated at 1200 “C in a thermal neutron flux of 5 x 1Ol7 n/m2 set to doses of 8.7 x 1022 and 1.7 x 10z5 fission/ms. The discs were all 3 mm in diameter, N 0.15 mm thick and after i~adiation were electro~hemically polished using the standard techniques), and examined in a Philips EM 200 electron microscope. Discs from the lower-dose irradiation were not found to contain intragranular fissiongas bubbles, but in these and many subsequent foils from other irradiations, bubbles were observed close to sintering pores. Examination

R. M. CORNELL AND J. A. TURNBULL

88 of the higher-dose

samples revealed that intra-

granular fission-gas bubbles were just beginning to form

(fig.

1). This micrograph

was taken

with the reverse to normal contrast (black-on-white) bubbles

since it enabled

to be seen more clearly.

conditions these small

The concen-

tration of these bubbles within the samples was seen to vary considerably from place to place. In many

places the bubbles

were seen

to lie in straight lines. In order to examine uranium dioxide which had been irradiated to a still higher dose, some samples were cut from a section of a fuel pin discharged from a power reactor. Samples were 5.25% 235Uenriched uranium dioxide irradiated at N 1200 “C in a thermal neutron flux of N 1.3 x 1017 n/ma set to a dose of N 3.2 x 1025 fissions/m3. This material was found to contain a high concentration (- 3 x lOaa/ma) of intragranular fission gas bubbles. The bubbles were N 25 A in diameter and evenly distributed throughout, the matrix of the material, suggesting that a stable condition had been reached (fig. 2). Ag ain, ma’np of these bubbles were seen to lie in clearly defined straight lines.

Fig.

2.

a power

A sample

of uranium

react,or at) -

fissions/m3. fission-gas

A

tmiform

bubbles

dioxide

irradiated

in

1200 “C to a dose of 3.2x lo25 dispersion

is present

of

intragranular

in t,his material,

and

many can be seen to lie in clearly defined straight

lines.

In addition the results demonstrated that the observations on irradiated discs were representative of the behaviour of bulk material. Several other irradiations were carried out on uranium dioxide disc samples held at - 1000 “C. None showed any sign of intragranular fission-gas bubbles after irradiation but the presence of bubbles around sintering pores was detected in all t’he samples to some

Fig. 1. A sample of uranium 1200 “C to granular

a dose

of

dioxide

irradiated

1.7 x 1025 fissions/m5.

fission gas bubbles to form

are only locally.

just

at

Intra-

beginning

degree. These samples, of varying 235Ucontents, were irradiated in a thermal neutron flux of N 4 x 1015 n/ma set to t’otal doses varying from 2.5 x lo20 up to 1.2 x 10”” fissions/m3. An observation made in all the samples examined from the experiments above was the presence of gas bubbles surrounding some of the original closed porosity. The example shown in fig. 3 was taken from a sample irradiated to a dose of 3.5 x 1022 fission/ma at N 1000 “C. No fission-gas bubbles were detected in the matrix of this sample but the effect shown was found in the vicinities of many of the original pores. Some of these bubbles were seen to lie in straight lines, suggesting that they were

NUCLEATION

Fig.

3.

OF

A sample

BUBBLES

of uranium

AND

dioxide

RE-SOLUTION

irradiated

at

EFFECTS

Fig.

4.

A

1000 “C to a dose of 3.5 x 1022 fissions/m3. Gas atoms

irradiated

knocked

out of this pore

by t’he re-solution

have formed into intragranular the original

process

IN

pore

URANIUM

in

a

sample

and annealed

at

intragranulrtr

bubbles

pore.

having

formed

showed

no

tendency

to

shrink,

but

some

appeared to grow in size slightly. One particular pore in this sample was extensively studied after annealing at temperatures up to 1400 “C (fig. 4). The photomicrograph was taken with black-on-white contrast so that the small bubbles surrounding the pore could be clearly distinguished. The diameters and numbers of these small bubbles surrounding the large pore were measured and their total gas content calculated as N 4 x 104 atoms, using the Van der Waals’ gas equation (the uranium dioxide surface energy was taken to be 1017 erg/ms). Assuming that the original pore was in equilibrium and using the value of the irradiation-resolution parameter of lo-5-lo-*/see

uranium

1400 “C. A large

from

surround

gas atoms

the re-solution

obtained

of

dioxide

at 1000 “C to a dose of 3.5 x 1O22fissions/m3

gas bubbles surrounding

lying upon the sites of fission-fragment tracks. The bubbles were generally N 25-30 A in diameter and were seen to lie inside a spherical volume surrounding the original pore. The radius of this sphere was always less than 1000 A larger than that of the pore it surrounded. Upon annealing for periods of up to 30 min at temperatures rising to 1500 “C such bubbles

s9

DIOXIDE

from

number

the knocked

of small

original

pore,

out of it by

process.

earlier work 4), the number

of

gas atoms knocked out of this pore during the irradiation at 1000 “C was calculated to lie between 104 and 105 atoms. 3.

Discussion

The experiments have demonstrated that there is a critical irradiation dose before intragranular fission gas bubbles can form in uranium dioxide at 1200 “C. No bubbles are observed after an irradiation dose of 8.7 x 1022fissions/m3 but bubbles are beginning to be seen after a dose of 1.7 x 1025 fissions/m3 and are in a uniform dispersion after a dose of 3.2 x 1025 fissions/m3. The considerable variation in local gas-bubble concentration within the sample irradiated to a dose of 1.7 x 1025 fissions/m3 is believed to be caused by local increases in gas content brought about by the complete resolution of gas from sintering pores in the initial material. The diffusion distance for gas atoms knocked out of these pores during the whole irradiation time at 1200 “C was calculated

R.

90 to be -

M.

CORNELL

AND

400 A and so it is not surprising that

the local gas atom concentration is inhomogeneous. The matrix of the sample did show a marked absence of pores after irradiation support this view. From these experiments that

the

critical

intragranular

dose

for

to

it can be deduced the

fission gas bubbles

formation

J.

studied in more detail appear to confirm that irradiation-re-solution is indeed responsible for the removal of residual gases from sintering porosity in uranium dioxide during irradiation.

of

lies between

3.2 x 10”s fissions/m3 (correof sponding to a gas atom concentration 2.9 x 1022- 1.1 x 1025 atoms/ma). Owing to the effect of local enhancement in gas ~oncentratiolls found in the 1.7 x lOa5 ~ssions/m3 dose samples it seems likely that the critical dose lies towards the high end of the range indicated. Fission-gas bubbles show a marked tendency to lie in straight lines in all the samples examined. This observation was regularly made, even in the sample where intragranular fission gas bubbles were just beginning to form. Previous work 132) has shown that intragranular fissiongas bubbles are nucleated heterogeneously upon fission-fragment track sites in uranium dioxide, and these results provide confirmatory evidence t,o support this view even at gas concentrations at which the bubbles are just beginning to form. Gas bubbles were observed surrourlding some of the original sintering pores in many of the samples examined. Annealing at temperatures up to 1500 “C did not have any effect upon these gas bubbles and it is inferred that these gas bubbles,

presumably

TURNBULL

these bubbles were also heterogeneously nucleated. Results from the particular pore

4.

8.7 x 1022 and

were stable

A.

having

formed from insoluble gases within the pores. The gas atoms had been knocked into the surrounding lattice by the irradiation re-solution process which has already been shown to be operative in uranium dioxide 4). The gas bubbles which formed from these atoms were often seen to lie in short straight lines, just like the intragranular fission gas bubbles already discussed. From this behaviour it is inferred that

Conclusions These experiments

have demonstrated

that,

a uniform distribution of intragranular fissiongas bubbles does not form in irradiated uranium dioxide at 1200 “C until a dose of between 1.7-3.2 x IO25fissions~m3 is reached. The critical dose for the formation of bubbles has been shown to lie fairly close to the top end of the range indicated. When intragranular gas bubbles are observed to form they regularly lie in straight lines and it is believed that they are heteroge~leo~~sly nucleated upon the fission-fragment track sites. Gas at,oms within both bubbles and pores are returned to the lattice during irradiation by the resolution process and a numerical evaluation confirms that the amount of gas knocked out lies within the range expected from the known value of the paramet8er. This process is believed to be effective in the removal of gas atoms from within sintering porosity in uranium

dioxide

during irradiation.

Acknowledgement This paper is pubhshed Central ~l~ctricit,y

by permission

Generating

of the

Board.

References 1) R. M. Cornell, J. Nucl. Mater. 38 (1971) 319 *)

J. A. Turnbull,

3)

A.

4)

Mater. 10 (1963) 157 J. A. Turnbull and R. M. Cornell, J. Nucl. Mater.

D.

Whapham

36 (1970)

161

J. Nucl.

Mater.

and

E.

B.

38 (1971)

Sheldon,

J.

203 Nucl.