An electron microscope examination of matrix fission-gas bubbles in irradiated uranium dioxide

JOURNAL

OF NUCLEAR

AN ELECTRON

MATERIALS

38 (1971) 319328.

MICROSCOPE

0 NORTH-HOLLAND

EXAMINATION

IN IRRADIATED

OF MATRIX

URANIUM

PUBLISHING

FISSION-GAS

CO., AMSTERDAM

BUBBLES

DIOXIDE

R. H. CORNELL

CEGB, Berkeley Nuclear Laborator@ Berkeley, Gloucestershim, UK Received 26 August 1970

Uranium dioxide irradiated over a wide temperature range to a dose of greater than N 3.2 x 101s fissions/ mm3 is always found to contain a high concentration of intragranular fission-gas bubbles which contain a constant amount of gas approximately independent of both irradiation ~m~rature and burn-up. These bubbles, which are assumed to be nucleated heterogeneously upon the sites of the energetic fission fragments, have a lifetime which lies in the range of 4-40 h with thermal neutron fluxes of - 5 x 10” n/mm2 . sec. The collection of fission-gas at grain boundaries is controlled by resolution and atomic diffusion of gas atoms through the uranium dioxide lattice rather than by the migration of bubbles up the temperature gradient. The diffusion coefficient of the fission-gas atoms in irradiated uranium dioxide has been shown to be considerably enhanced at temperatures below N 1300 “C.

Le bioxyde d’uranium irradie dans un large domaine de temperatures a une dose sup&ieure it 3,2 x 1016 flssions/mm3s’est toujours revel8 contenir une concentration &e&e de bulles intor~~ula~es de gaz de fission. Ces bulles eontiennent une quantitti de gaz constante, approximativement independante 8, la fois de la temperature d’irradiation et du taux de combustion. Ces bulles que l’on suppose nucleer de fqon heterogene sur les sites de fragments de fission Bnergetiques ont une dur&e de vie situ&e entre 4 et

1.

Introduction

It is of considerable importance to be able to predict accurately the amount of fission-gas released from uranium dioxide fuel pins during their irradiation in a power reactor. Before this can be successfully achieved it is necessary to obtain information about the location and behaviour of fission-gas atoms in uranium dioxide during irradiation. The amount of

40 h avec des flux de neutrons thermiques de N 5 x 1011 n/mn+~sec. Le rassemblementdes gaz de fission sur les contours de grains est contr616par les processusde redissolution et de diffusion atomique des atomes de gaz it travers le reseau du bioxyde d’uranium plutot que par la migration des bulles sous l’effet du gradientthermique. On montre que le coeffioientde diffusion des atomes de gaz de fission augmente considerablement aux temperatures inferieures a 1300 “C environ. In Urandioxid, das in einem grossen Temperaturbereich bestrahlt wurde (Bestrahl~~dosis > 3,2x 10’6 Spaltungen/mm3), ist stets eine hohe Konzentration an Spaltgasblasen in den Kornern festgestellt worden. Die Gasblasen enthielten jeweils eine kon&ante Gasmenge, die von der Bestrahlungstemperatur und vom Abbrand nahezu unabhiingig war. Es wird angenommen, dass die Blasen heterogen an energiereiehen Spaltfragmenten gebildet werden. Sie haben eine Lebensdauer zwischen 4 und 40 Stunden bei einem therm&hen Neutronenfluss von ungefahr 5 x 1011 nlmm2.s. Die Anreicherung des Spaltgases an den Korngrenzen ist em Prozess, der eher durch die Auflosung sowie Diffusion der Gas&tome durch das Ur~dioxid-Gitter als durch die Wandernng der Blasen in Richtung des Temperaturgradienten bestimmt wird. Die Ergebnissezeigen, dass der Diffusionskoeffizient von Spaltgasatomen in Urandioxid unter Bestrahlung bei Temperaturen unter 1300 “C bet~riichtlioherhoht wird.

fission-gas released from uranium dioxide during irradiation is related to the collection of fissiongas atoms at grain boundaries and several different mechanisms for this process have been proposed. For many years the formation of grain-boundary fission-gas bubbles was assumed to be controlled by the atomic diffusion of gas through a bubble-free matrix 1). A later proposal was that the accumulation of gas at the grain319

320

R.

boundaries

was controlled

M.

by the migration

CORNELL

of

sections facilitated the selection of small samples

fission-gas bubbles within the grains up the temperature gradient in the fuel 2). Recently it

from

has

of thin foils the selected samples were ground to N 0.15 mm in thickness and were broken into pieces N 3 mm across. These

been

migrate

to

proposed s-5) that

gas

grain-boundaries

to

atoms form

can inter-

granular bubbles but a re-solution process operating simultaneously returns some of the

3.

The samples examined were from uranium dioxide pellets 14.5 mm in diameter which had been irradiated to doses between 3.2 x 10164.6 x 1017 fissions/mm3 at temperatures between 700-1600 “C. Doses are listed in table 1. Transverse cross-sections of each pellet N 1 mm thick were obtained using a remotely operated diamond slitting-wheel. The macroscopic cracks present in all the pellet cross-

to contain a high concentration of intragranular gas bubbles. Assuming that these bubbles were equilibrium fission-gas bubbles and using the

TABLE

Doses (fissions/mms) oentre temperature

Pellet surface temperature Bubble

concentmtion

Bubble

diameter

Gas content Total

(“C)

(mm3)

(mm3)

(mm3)

1

Pellet A

Pellet B

3.5 x 10’6

2.3 x 1017

1640

920

700

650

640

1.2-3.8

x 10’4

1.7-2.8

(nm)

in bubbles

gas content

(“C)

Results

During the irradiation the dose-rate and temperatures of the four pellets examined in this study changed from time to time. However, the conditions during the last 24 h of irradiation are assumed to be valid in establishing the temperature profiles for the four pellets examined in this study for reasons which will appear later. Samples from four pellets were examined in the electron microscope and all areas which had been irradiated above N 800 “C were found

Experimental

Pellet

0.95-1.30

x 1016

0.7 x 10’6

for the

700 “C annealing treatment omitted. Each area of sample examined in the Philips EM 200 electron-microscope was identified with respect to the original pellet cross-section and an irradiation temperature allocated to it by reference to a calculated temperature profile (fig. 1). Measurements taken from the photographic plates enabled the mean concentrations and diameters of the matrix fission-gas bubbles to be determined with varying irradiation dose and temperature. Several foils were annealed at elevated temperatures to study the stability of the matrix fission-gas bubbles.

daries and shows that the flux of atoms reaching grain-boundaries depends strongly upon this concentration of bubbles. The purpose of this report is to illustrate the variations of concentrations and diameters of matrix fission-gas bubbles in a selection of uranium dioxide samples which were irradiated for a variety of times with different thermal neutron fluxes. The results are discussed with particular reference to the various models proposed for the behaviour of fission-gas atoms in uranium dioxide during irradiation.

)

positions in the pellet radius examination. To simplify

samples were electropolished using the standard technique for uranium dioxide 6) but with the

gas atoms to the matrix so that a slower rate of growth of intergranular gas bubbles with irradiation time results. Speight’s theoretical treatment considers the effect of a high concentration of matrix fission-gas bubbles on the rate of collection of gas atoms at grain boun-

2.

all

microscopic production

2.9-3.3

x 10’4

1.6-2.1 0.80-1.45 x 10’6 5.0 x 10’6

I

I

Pellet C

Pellet D

4.6 x 101’

2.6 x 10’7

910

1275

2.6-3.4

x 1Ol4

1.7-2.1 1.0-1.3 x 10’6 10.0 x 10’6

560 1.0-2.4 x 1014 2.1-3.1 1.15-1.50x

10’S

5.6 x 101’3

MATRIX

TIME-AVERAGED TEMPERATURE

6500

= EO

FISSION-GAS

PELLET PROFILES.

20 30 40 50 60 PELLET RADIUS hW&).

70

321

BUBBLES

LAST

DAY OF

IRRADIATION PROFILES.

PELLET lEMPER&~URE

P

Fig. la. Time-averaged temperature pro-filasca&~uIated for the four samples of irradiated uranium dioxide examined in this work.

I

*

0

I.0

.

20 3Q 40 5-O 6-O PELLET RADIUS {MMS).

70

e0

Fig. lb. Last-day-of-irradiation temperatureprof%~ c&cd&ad for the three sampk~sof irradiated IE~~~I.IIXI dioxide examined in this work.

‘SAMPLE lRRADIATlON TEMPERATURE ON t&ST DAY IN REACTOR.*C, Kg. 2. The variations of the matrix fissiongas bubble ~on~%ntratio~sand diameters with la&-day ~~~~~0~ temperature and dose as determined in this work. The pellet A results are plotted against time-averaged irradiation temperatures,

322

R.

Van-der-Waals’

gas constant

for xenon,

itI.

the

numbers of gas atoms located within them were determined. In all cases the variations of

CORNELL

small. In most samples bubbles lie in long straight

were seen to

lines which were probably

concentration and diameter of the matrix fissiongas bubbles with last#-day irradiation tempera-

the sites of fission fragment, tracks. A sample from pellet A which had been irradiated at - 1450 “C was annealed for 2 h

ture and dose (fig. 2) appeared

in moist hydrogen

to be rather

of the matrix

at this temperature.

fission-gas

bubbles

Some

observed

in

this sample before the anneal (fig. 3) lay in clearly defined straight lines. After t’he annealing treatment (fig. ‘i), some of the bubbles still lay in straight lines, but a more important observation was that many had grown a little in size and no bubble shrinkages were detected. A sample from pellet C which had been irradiated at 1065 “C was annealed for several hours in moist’ hydrogen at 1400 “C. After irradiation this sample contained - 1Ol6 atoms/ mm3 in matrix fission-gas bubbles at’ a concellbut after the tration of - 3 x 1014/mm”.

Fig.

3.

Typical

electron taken

micrographs

from pellet A.

of

samples

Fig. 4.

Typical

electron micrographs t,aken from pellet, R.

of

samples

MATRIX

Fig. 5.

FISSION-GAS.

Typical electron micrographs of samples taken from pellet C.

annealing treatment this figure rose to - 1017 atoms/mm3 in a concentration of - 5 x 1012 bubbles/mm3 of mean diameter - 18 nm (fig. 8). This latter gas content corresponded to the total gas content of the sample which had been produced in it during irradiation.

BUBBLES

Fig. 6.

323

Typical electron micrographs of samples teken from pellet D.

The concentrations of gas bubbles/cm3 of sample were all obtained assuming that the foil thicknesses were 1500 A. Measurements made later on several foils suggest that the actual thicknesses were - 1400 i- 200 L%in the areas containing the bubbles examined. The

324

Fig.

R.

7.

Electron

pellet A irradiated

micrograph

of

a

sample

M.

CORNELL

from

ret 1450 “C and then annealed for

two hours at 1450 “C. Fig.

concentration figures therefore contain an error of - f 15%. The variations observed in the number of bubbles/unit area of foil at any temperature were found to vary by up to f 10%. The measurement error on all the gas bubble diameters was - f 10% whilst their statistical variation

on the mean size within an area at

constant temperature was - & 4.5o/o. The results obtained in this study, listed briefly in table 1, are shown in fig. 2 where the matrix fission-gas bubble diameters and concentrations are plotted against the last day of irradiation temperatures, except for pellet A where the values are time-averaged only. Typical electron micrographs taken from the samples are shown in figs. 3-6. 4.

Discussion

Thin foils of uranium dioxide irradiated between 800 and 1600 “C to doses of 0.324.6 x 1017 fissions/mm3 have been examined in

pellet

8.

Electron C irradiated

micrograph at

1065

of

&

“C and

sample then

from

annealed

for two hours at 1400 “C.

the electron microscope and all found to contain what appear to be matrix fission-gas bubbles. It has been suggested that these bubbles are not present during the irradiation but form during the cooling-down period after the irradiation has ceased. However, Whapham and Sheldon 7) have shown that specimens of uranium dioxide irradiated at 900 “C to N 3 x 1016fissions/mm3 and cooled at, - 6 “C/see after irradiation showed the presence of matrix fission-gas bubbles - 2.5 nm in diameter at a concentration of - 8 x 1013/mm3. These results, which are close to those of the current work, provide strong confirmation for the presence of these bubbles in uranium dioxide during irradiation. The absence of any reduction in size of the bubbles on annealing at elevated temperatures provides additional evidence that they are stable gas bubbles and not voids.

MATRIX

During

this process the bubbles

FISSION-UAS

were actually

325

BUBBLES

day of the irradiation

is the one which must

observed to increase in size slightly. An important feature observed in all the

be used in establishing the variations in matrix gas bubble sizes and concentrations with ir-

samples was the absence of any observable variation in the diameters of the fission-gas

radiation temperatures in fig. 2. The mean concentrations of matrix

bubbles across a grain in the direction of the temperature gradient. There was also no evidence for the directed motion of fission-gas

gas bubbles constant

were found

over

the

fission-

to be approximately

temperature

range

1600 “C. This was a most surprising

800-

result, bubbles towards the grain boundaries. Speight 5) being in marked contrast to those obtained on annealing similar irradiated material out-of-pile has shown that the random motion of fissionin which a strong temperature dependence was gas bubbles by volume diffusion 8) is an unfound for the fission-gas bubble concentrationia). important process in the redistribution of fissionTaking the number of fission-gas atoms gas within a grain ; the uniformity in size of contained in a small gas bubble as that described the gas bubbles throughout the grains in the by Van der Waals’ equation and using the present observations tends to confirm this results in fig. 2, the concentration of fission-gas hypothesis. It appears, therefore, that between 800 and 1600 “C! the formation of grainatoms in matrix bubbles was found to be approximately constant between 800and 1600"C boundary bubbles is not controlled by the (table 1).This same concentration was also diffusion of fission-gas bubbles. It is concluded found to be independent of dose over the range that the collection of gas at grain boundaries is controlled by atomic diffusion. This process 0.35-4.6 x 1017fissions/mma. Taking the results from pellet C as an example, only N 10 y0 of predicts a rate of collection of gas atoms at grain-boundaries which is far higher than that all the fission-gases were precipitated in matrix observed but it can be reduced to an acceptable gas-bubbles in the as-irradiated state. Annealing rate if it is assumed that an irradiation-induced a sample of this material at 1400 "C caused re-solution mechanism is operative on the grainthe remaining gas to precipitate to give a low boundary bubbles. This process has already concentration of large matrix bubbles (fig. 8). been demonstrated to be effective in destroying It is postulated that the curious variation matrix fission-gas bubbles during the irradiation in fission-gas content of matrix bubbles with of uranium dioxide at low temperatures 9). In temperature and burn-up is brought about by this manner fission-gas atoms are delayed in the re-solution process. Gas atoms are constantly their migration to grain-boundaries by being conbeing precipitated into bubbles during the stantly trapped in and redissolved from matrix irradiation due to their high local concentration. fission-gas bubbles. The exact nature of the reAt the same time gas atoms are being returned solution process is unknown but several workers to the lattice from the existing bubbles by the have proposed possible mechanisms which re-solution process. At equilibrium the rate of might be operative in uranium dioxide in-12). re-solution exactly balances the rate of precipitation and a fixed concentration of gas in The re-solution parameter for matrix fissiongas bubbles in uranium dioxide has been matrix bubbles results. Each bubble has a finite determined as 10-dto lo-5/set @) and therefore lifetime therefore and this has been shown to the lifetime of each bubble is only about 4-40 h be approximately 4-40 h from separate experiwith thermal neutron fluxes of N 5 x 1011 ments 9). From a detailed analysis of the electron n/mm2. sec. micrographs it is proposed that the bubbles are destroyed in a single re-solution event when a This result has relevance to the temperature fission fragment directly hits or passes very profile used for each pellet examined, for if the lifetime of each matrix gas bubble is only close to the bubble. The bubble, once nucleated, N 4-40 h then the temperature on the last therefore grows by absorbing gas atoms from

326

R.

M.

CORNELL

solution until the moment, it is destroyed. No evidence has been found to support a gradual

However, if there are M matrix gas bubble/cm3 of radius R each containing m * atoms per

re-solution

bubble,

of gas bubbles

by a series of re-

then :

solution events. A detailed analysis of the variations in concentrations and diameters of

m = Drn * = %cyR3N/3(kTR + 2yB),

matrix fission-gas bubbles in irradiated uranium dioxide is given elsewhere I*).

where kT has its usual meaning, y is the surface energy and B is a Van der Waals’ gas constant,.

The presence

of lines of matrix

fission-gas

bubbles in irradiated uranium dioxide has been established over a wide temperature range. This is not a new observation since the presence of matrix fission-gas bubbles in straight lines has already been reported 7). These authors suggest that fission-gas bubbles are aligned by their movement to a fission spike. However, the results obtained from these experiments suggest that matrix gas-bubbles do not readily migrate during irradiation. The very precise lining-up of bubbles such as is regularly observed would also be difficult to achieve if the directed mot’ion of bubbles in a temperature gradient were the controlling mechanism. An alternative explanation for the regular observation of matrix fission-gas bubbles lying in straight lines is t,hat these bubbles are nucleated upon the fission-fragment t’rack sites. This proposal runs contrary t,o currently accepted ideas in that, it suggests t’hat the nucleation of mat’rix gas bubbles in uranium dioxide is heterogeneous. Under this regime the number of fission-gas bubbles present at equilibrium is approximately independent

of dose rate and temperature,

as

is observed. Further work is being carried out to study more fully the nucleation a,nd stability of these bubbles in uranium dioxide during irradiation. The amount of fission-gas present in all the matrix bubbles during irradiation (m) when resolution is operative can be defined by the following relationship,

where b is the probability per second of a gas atom in a bubble undergoing re-solution, g is the probability per second of a gas atom in the lattice being trapped in an existing bubble, @ is the rate of production of fission-gas atoms/ ems.set of material and t is the irradiation time.

Hence gyt/(b + g) = 8nyR3N/3(kTR + 2yB).

(1)

But it can also be shown 15) that g = 4nDRN,

(2)

where D is the gas atom diffusion coefficient In the matrix. Substituting for g from eq. (2) into eq. (1) results in: D = 2ybR2/[3/3t(kTR + 2yB) - 8nyRSN].

(3)

From the results of the current experiments, therefore, it is possible to calculate at each temperature a gas-atom diffusion coefficient and to compare it with the values expected from out-of-pile measurements 1%16). The results of this analysis are shown in fig. 9, where the curved line represents the estimated enhanced diffusion coefficient for the rare gases in UOZ due to the irradiation 17). The results obtained show that there is a considerable enhancement of the diffusion coefficient below about 1300 “C. The effect is brought about owing to the large number of irradiation-induced vacancies present in the material

at

low

tempera’tures.

A

detailed

analysis of the enhancement of the diffusion coefficient is given elsewhere, but it can be seen from fig. 9 that the variation in the coefficient follows the predicted curve reasonably closely if the resolution parameter is taken as lo-s/see. The slopes of the curve however appear to be somewhat lower than predicted, suggesting that the activation energy for the process is lower than that of Speight’s analysis. If the resolution parameter is taken as 5 x IO-5/set [approximately in the middle of the range as determined by Turnbull and Cornell 9)] then a greater enhancement of the diffusion coefficient

MATRIX

I....

57

.

60

e3

.

.

.

66

FISSION-GAS

..\.............‘s.......

49

7.2

75

lo.ooo T

Fig.

9.

327

BUBBLES

78’

84

84

87

5

(OK)

The variation in the gas-atom diffusion coefficient with temperature

during irradiation as determined

from the results of t*his st.udy.

is predicted, as shown in fig. 9. Until a more accurate determination of the re-solution pnrameter is made it will not be possible from these or similar studies to be able to calculate the enhanced diffusion coefficient more accurately than has been possible

5.

here.

Conclusions

Uranium dioxide irradiated to a burn-up greater than N 3.2 x lOle ~ssio~s/rnrn~ between 800 and 1600 “C is always found to contain of matrix fission-gas a high concentration bubbles. These bubbles contain an approximately constant amount of gas per unit volume which is independent of both irradiation temperature and burn-up. This curious effect can be shown to be caused by irradiation induced re-solution which returns gas from within gas bubbles to solution in the lattice. Under these conditions the mean lifetime of a matrix bubble has been estimated as approxi-

mately 4-40 h when the neutron Aux is N 5 x IOn n/mm2.sec. Evidence obtained from the present work suggests that the nucleation of matrix fissiongas bubbles is heterogeneous in nature, taking place upon the sites of the passage of fission fragments. The diffusion coefficient of fission-gas atoms in irradiated uranium dioxide has been shown to be considerably enhanced at temperatures below N 1300 “C. The exact extent of this enhancement cannot be determined until a more accurate value of the re-solution parameter becomes available.

Acknowledgement This paper is published by permission Central Electricity Generating Board.

References 1) A. H. Booth, AECL-496

of the

Report of Atomic Energy of Canada,

(1957)

328 2)

R. R.

S. Barnes,

UKAEA

Report,

M.

AERE-R

4952

CORNELL

9) J. A. Turnbull and R. M. Cornell, J. Nucl. Mater. 37 (1970)

(1967) 3)

R. M. Cornell, M. V. Speight and B. C. Masters, J. Nucl.

4)

A.

J.

Mater.

Manley,

1681 (W)

UKAEA

M. V. Speight,

6)

A.

Mater.

10 (1963)

International 8)

Nucl.

Whapham

Kyoto,

Report,

TRG

Report

Japan

M. E. Gulden,

355

D. Whapham AERE-R

11) J. A. Turnbull,

and B. 4970 CEGB

E.

Sheldon,

UKAEA

(1965) Report,

RD/B/N

1112

(1968)

D.

A. D. Wbapham

A.

Report,

170

(1968)

5)

7)

30 (1969)

lo)

Sci. Eng.

and B.

E.

37 (1969)

Sheldon,

180

J. Nucl.

157 and B. E. Sheldon,

Congress for Electron (1966)

Proc.

Mater.

R. S. Nelson,

J. Nucl.

Mater.

25 (1968)

227

(1970) 15

9 23 (1967)

)

6th

Microscopy,

p. 375

J. Nucl.

12

R. M. Cornell, Phil. Meg. 19 (1969) 159, 539 9 1727 14) J. A. Turnbull, CEGB Report, RD/B/N

30

1

F. S. Hem, R 4347

9

J. Phys. Chem. Solids 6 (1958)

D. Davies and G. Long, UKAEA

335

Report, AERE-

(1963)

M. V. Speight,

private communication

(1970)