On the techniques for observing fission gas bubbles in uranium

JOURNALOF NUCLEARMATERIALS17 (1966) 237-244.

0 NORTH-HOLLANDPUBL.XlHlNUCO., AM$TERnti

ON THE TECHNIQUES FOR OBSERVING FISSIONGAS BUBBLES IN URANIUM J. E. BAINBRIDGE UKAEA,

Metallurgy Division,

and B. HIJDSON

Atomic Energy Research Emkzb~iahmnt, Harwell, Didcot, Bed&., UK Received 26 June 1965

Replicas taken from cathodically vacuum etched and fractured surfaces have been compared with the resultsfrom thin film electronmicroscopy to determine the limitation of each technique. Cathodic etching enlargesbubbles - 160 A diameter by a factor of - 2; only cleavage surfaces from the fracture produce results comparable with the bubble size distributions obtained by using other techniques. With bubbles > 600 A the thin film technique is no longer reliable. The results give an experimental value for the surface energy of a-uranium of 1000 erg/cm? Des rbpliques de surfaces fractur&s et attaqubes cathodiquement sous vide ont 6th compa&s aux images obtenues par microscopic dlectroniquesup fihn mince par transmission ati de determiner les limites de chaque technique. L’attaque cathodique agrandit lea bulles d’un diam&re de - 1SOA d’un facteur - 2 environ. Seules les surfaces de clivage provenant de la rupture d%chantillons produisent des r6sultats cornparablesaux dis-

1.

tributions de taille de bulles obtenues en utilisant d’autres techniques. Avec des bulles de taille sup&icure & 500 A, la technique des films minces n’est plus valable. Les r6sultats donnent 1 000 erg/cm* comme valeur exp&imentale de 1’6nergie de surface pour l’uranium 0~. Abdriicke, welche von kathodisch vakuumgeiitzten Bruchfliichengemacht wurden, wurden verglichenmit den Ergebnissen, die durch Diinnfilmaufhahmen im Elektronenmikroskop erhalten worden waren. D& Vergleich diente der Featlegung der Grenzen der Anwendbarkeit jeder Methode. Kathodisches &Zen vergriisaertdie Spaltgasblaeen um den Faktor 2. Der Ausgangsdurchmesserdieser Blasen ist etwa 150 A. Vergleichbare Ergebnisse iiber die Gasblasenverteihmgwurden nur an durch Bruch gerissenen Oberfltihen erhalten. Blasen mit grosser 600 A k&men durch die Diinnfilmtechniknicht mehr untersucht werden. Die Ergebnisse fiihren ausserdem zu einem experimentellen Wert der OberfZichenenergie von a-Uran. Sie betriigt 1000 f 600 erg/cm?

Introduction

Various techniques have been used to observe the bubbles of fission gas produced by neutron irradiation of uranium 1~2~6). The electron microscope forms the basis for most methods since gas bubbles are usually smaller than one micron in diameter at the irradiation doses of interest. In some of the earliest work using replica techniques with the electron microscope, the bubble size and density was thought to be typically about 101sbubbles/cm3 of size 0.1 ,u 3). A refinement of the replica technique by one of the authors suggested that typical bubble densities were two orders of magnitude higher than this, with sizes only 80-100 A in diameter; but the possibility that some “bubbles” were artifacts, introduced by cathodic vacuum 237

etching, remained. However, with the advent of the electron transmission technique the opportunity of comparing replicas and thin foils arose, and this comparison, as shown below and in conjunction with other work 16) reveals that the numbers quoted by Greenwood et al. are wrong, a more typical distribution being about 1015 bubbles/cm3 of about 200 A in diameter. Again the replica technique haa been used extensively to calculate volume increases due to the observed bubbles, and to estimate the effect that fine precipitates have on nucleation 4). The latter used an experimental technique with a resolution of N 300 8. This could lead to serious underestimates of bubble number and size since in the results presented in this report bubbles below 100 A in diameter were common.

238

J. E. BAINBRIDUE AND B. HUDSON

With this previous work in mind the authors have examined the same specimens by three different ~chniques and have compared the results to obtain a better understanding of the limitations of each. 2.

Experimental

Usually the parame~rs of interest are bubble size and distribution relative to other features of the structure. The techniques chosen for this study were: I. Fractography followed by replication. 2. Cathodic Vacuum Etching (CVE) followed by replication. 3. The preparation of thin foils suitable for electron microscopy. The material used was taken from two “Adjusted Uranium” Fuel Element bars irradiated as follows: Irradiation Temperature Spec. A 340” C fuel surface temp. Spec. B 390” C fuel surface temp.

Burn-up (% all atoms) 0.11 % 0.05 %

Specimens for each type of preparation were cut from adjacent positions in eaoh bar to reduce real variations in structure; although the area studied was not selected precisely the outer l-2 mm of the bar were avoided for the same reason. 2.1.

FRAOTOURAPHY

Specimens in the form of discs, 2.5 mm thick, were machined from the radiated bar, cooled in liquid nitrogen, placed in a cooled vice (- -20” C) and broken with a hammer blow. The fractured surface was replicated (within 30 set) with cellulose acetate film 0.06” thick softened in acetone ; onto this first layer several more thicknesses of cellulose acetate film similarly softened were added. The whole was allowed to harden for 5-10 min, a 100 g weight placed on top and the thick replica allowed to harden for several hours. The replica was stripped and the contact surface shadowed with evaporated gold-40 y0

palladium alloy and coated with carbon. The plastic was then dissolved in aoetone and the carbon film examined in the electron microscope. The thinnest possible carbon film was used to give a final replica with a resolution less than 100 A. It was found necessary to wash the carbon replica in at least two changes of acetone to remove all traces of plastic. 2.2.

CATHODICVACUUMETCIIINU(CVE)

The specimen was mounted in an adjustable metal mount, ground metallographioally on bakehte wheels covered with silicon carbide papers, and polished down to 1 (u grade on diamond impregnated sueded nylon cloths. The specimen was finally de-mounted and ultrasonically washed in methyl alcohol. An alternative method which was only briefly tried was to ele~tropolish the surface to be etched, in the first of the two solutions used to prepare thin foils. This had the advantage of requiring no mounting and de-mounting, was quicker and specimens could be polished from only a roughly ground surface. ~though the technique was promising some trouble was experienced in the initial induction period of etching which may be due to surface films from the polishing solution. The prepared specimen was mounted in good thermal contact with the cathode of the etoher and bombarded with 4 keV argon ions at a pressure of N 16 Torr. After an initial induction period (which probably includes some degassing of the specimen) the current passing decreased to N 1 mA~orn~when etching began. Early practice at AERE 5) had been to etch for 5-10 min but during this investigation this short etching produced bubble densities two orders of magnitude lower than that observed in thin fdm specimens. Hence much longer etching times 30-40 min were used and after this treatment the agreement between the two techniques was much improved. As observed by Bierlein, Mastel and Legatt 6) etching times longer than this, up to - 90 min, did not reveal further bubbles. The etched surface was then replicated with

OBSERVATION OF FISSION GAS BUBBLES IN URABNIUI cellulose acetate film N 80 pm thick, softened in acetone, this negative replica being allowed to harden for five minutes before stripping. Experience showed that several plastic replicas could be taken from the same etched surface, moreover the first two replicas were usually discarded since they carried much more contamination from the specimen surface than did subsequent ones. The “Bexfilm” replica was then shadowed with evaporated gold palladium and coated with carbon as before. 2.3.

THIN FOIL PREPARATION

Slices 2.5 mm thick were machined from the irradiated bar. This “penny” was mounted on a block of lead previously ground flat and parallel to the grinding wheel, using two-sided adhesive tape. The penny was ground on successively finer silicon carbide papers, using water as lubricant until the surface was parallel to the grinding disc, finishing on a 600 grit paper. The disc was then turned over and the other side ground parallel to the grinding wheel until the final thickness was 0.25 mm, finishing again with a 600 grit paper. Pieces 0.5 cm x 1.5 cm were cut from the disc and electropolished in orthophosphoric acid using 0.2 A/cm2 (6 V open circuit) with a stainless steel cathode and cleaning the surface as before 7) with a solution of 75 vol o/osulphuric acid, 18 vol o/o glycerol and 7 vol o/o water at 0.15 A/cm2 (6 V open circuit) with nickel cathode. After each stage the specimen was washed in alcohol and distilled water. 3.

Experimental results

Photographs of the thinned foils or the replicas were taken on either a Siemens Elmiskop I, Philips EM100 or a JEM 6A electron microscope at suitable magnifications. These photographs were then enlarged and divided into 2 cm squares. The diameters of the individual bubbles were measured and divided into small size ranges (a 0.25 mm interval was convenient). The number and volume were then calculated from these “histograms”.

239

The density of bubbles was calculated in the case of thin Clms by either measuring the thickness in areas where a twin or slip trace appeared, or by using an estimate based on previous experience of uranium foil thicknesses. From eighteen measurements of foil thickness on a-uranium foils fifteen (85 %) lay in the range 300 & 150 ,& and the figure 300 f% was taken as estimated value in cases where no real measurements could be made. Throughout the calculations the thickness of the foil was always taken as (t+d) within each size range, where t is the measured or estimated thickness and d the diameter of the average bubble within that size range. This correction allows for bubbles whose centres lie outside the foil thickness 8). In the case of the replicas the density (and hence volume) of the bubbles was calculated using the average bubble diameter within each range as the replica “thickness” and these densities summed to give the final density 9). Note that the observedsize was used throughout. This was because of the apparent increase in size caused by cathodic etching to these small bubbles. The correction which estimates the real diameter dt from the observed diameter dog), of a set of spheres intersecting a plane surface,< dt = (3/2)* do makes the discrepancy between the diameters from thin films and cathodic vacuum etched surfaces seem worse, although the authors consider that this correction is not applicable here since the etching effectively makes the bubbles non spherical. The bubble density as measured is probably accurate to a factor 2 in most cases. The volume increase deduced from this number will be much less accurate but general trends should -be detected. The results for thin films. where the thickness was measured and the bubble diameter was less than this thickness, should have considerably increased accuracy approaching f. 20 y. for density and N 60 y. for volume increase.

240

J. E. BAINBRIDC%E

The results are presented in table 1. It is to be noted that there are only a few results from fraotured specimens. It was soon found that only cleaved surfaces gave results comparable to the other techniques and the incidence of clearly recognisable cleavage faces was low. Some specimens were fractured below - 20” C in an attempt to increase the amount of cleavage fracture but this was largely unsuccessful. A more sophistica~d fracturing technique with the specimen at much lower temperature may be more successful. Under specimen A the figures in brackets correspond to the bubble densities obtained if this is calculated using the diameter measured from thin foils, (also in brackets in the column under “observed size”). In this paper “Etch” always means cathodic vacuum etch. 4.

Discussion

4.1.

SPEU~E~

Rgs.

la,

lb,

Fig. la.

Specimen A. As irradiated. and replicated.

CV etched

Fig. lb.

Specimen A. As irradiated. Fractured and replicated, cleavage face.

A AS RADIATED lc show

typical

examples

and replicated,

of

fractured

areas of this specimen, The thin film teohnique has given the smallest observed bubble diameter (we feel that these were also easiest to measure) and the highest bubble density. If one takes the observed size of the thin film bubbles and uses this to caIculate the density of bubbles from the etched and repIicated samples then the densities of the two methods are more comparable. This has been done in table 1, the figures in brackets under “bubble density” representing the density calculated in this way, with the diameter used shown in column under “observed size”. Hence it seems that CVE and replioation is revealing all the bubbles, but the diameters observed on the etched surfaces are (in this case) enlarged by a factor of N 2. This is supported by the volume increase figures showing a discrepancy of N 8 times (23). It is believed that the etching is flattening the profile of the bubble when the latter intersects the surface. replicated

B. HUDSOB

of results

results from etched and

AND

and

thin

foil

Fig. IO. Specimen A.

As irradiated.

Thin film.

The diameters of the bubbles on the fractured surfaoes were more difficult to measure than those from the thinned foils mainly because an optimum shadow angle cannot be ohosen for a

OBSERVATION

OF

FISSION

GAS

TABLE

Specimen A,

0.11 ‘% burn-up.

BUBBLES

330” C fuel surface temperature

9.4X1014 (fig. la) (1.8 x 1015) 8.7 x 1014 (1.75 x 10’5) As irradiated

1.04 x 1014 (2.0 x 10’5)

Etch and replicate

URANIUM

1

Bubble density (cm-r)

Preparation

IN

1.3 x 1015 (3.0 x 10’5) 2.0 x 1015 (6.0 x 1015) 6.4 x 1014 (1.92 x 1015)

Observed size (A)

Volume increase (%) 2.7

(ii) 300

2.2

(150) 310

3.0

(360) 370

3.1

(150) 460

6.6

(150) 460

2.7

(150) 3.1 0.86 1.5

As irradiated Fractured and replicated

2.8 x 1015 (fig. lb) 1.5 x 1016 1.4 x 1015

200 200 200

As irradiated Thin foil

1.9 x 10’5 1.58 x 10’5 1.3 x 1015 5.8 x 1014 2.3 x 1015

160 160 160 150 160

1 hour at 800” C Etch and replicate (only two results reported, many more obtained)

3.2 x 1014 (fig. 2a) 4.2 x 1014 (fig. 2b)

I hour at 800” C Fracture and replicate

2.0 x 10’4 3.6 x 1014

360 350

1.8 1.8

1.76 x 1014 2.8 x 10’4 2.2 x 1014 2.9 x 1014 2.6 x 1014

370 400 400 460 450

0.82 0.2 0.17 0.36 0.49

I hour

Thin foil

Specimen B,

0.06 % burn-up.

0.24 0.43 0.29 0.14 0.21

270

(t measured) (t measured) (t measured) (t measured) (1 measured) 0.29 20.0

200-104

386” C fuel surface temperature

As irradiated Etch and replicate

Not possible to count with confidence

As irradiated Fracture and replicate

Not possible to count with confidence 1.6 x 3.0 x 3.0 x 1.1 x 1.4 x

1015 1015 1015 1015 1016

76 76 100 80 80

0.11 0.09 0.08 0.14 0.11

(t (t (t (t (t

estimated) estimated) estimated) estimated) estimated)

241

242

J. E. BAINBRIDGE

replica taken from a fractured

AND B. HUDSON

surface due to

its uneveness. The volume increases calculated from the thin film results on specimen A as irradiated, where the thickness was measured, together with the volume increase from the solid fission products agree well with the volume increase to be expected from fuel elements with similar irradiation history lo), supporting the view that all the gas is contained in the observed bubbles. In addition the intergranular fractures from this specimen show no grain boundary voids of the type seen by Chatwin and Hyem 11).

Fig. 2a. replicated.

4.2.

Specimen A. lh at 800” C. CV etched and (cf. fig. 2b for heterogeneity

SPECIMEN A. ANNEALED 1~ AT 800” C

Annealing for 1 hour at 800’ C produces a considerable coarsening in the bubbles. The thin film results give a consistent picture of bubbles having enlarged from 150 to 400 A in diameter with a density decrease from N 2 x 1015 to 2.5 x 1014/ems; the two results from cleavage surfaces of fractures agree quite well with this. However, the etched and replicated samples do not agree at all well. The reason for this disagreement is immediately seen in figs. 2a and 2b. The bubble distribution is heterogeneous and only etched and replicated samples reveal this. Probably the heterogeneous distribution is produced by some grain boundary or dislocation line sweeping mechanism 12). However, the size of the smaller bubbles is in good agreement with that observed on fractured surfaces and thin films suggesting that larger bubbles N 300 A are not enlarged as much as those N 150 A in the unannealed samples. The larger areas N 4 mm2 which are examined from cathodic vacuum etched surfaces compared with those of cleaved fractured surfaces (10-s mms) and thin film (10-z-10-3 mmz) has revealed this inhomogeneity. The gas content and volume increase figures from the thin films support this hypothesis since both are much lower than the expected values, indicating that all the gas is not detected by the thin film technique under these conditions. It is considered that cleavage fracture does

of bubble

distribution).

Fig. 2b. replicated.

Specimen B. 1 h at 800’ C. CV etched and (cf. fig.

2~3 for heterogeneity

of bubble

distribution).

not give a representative sample under these conditions and the large bubbles seen in fig. 2b would be much too large to be detected on a thin foil being indistinguishable from large holes sometimes found. The conclusion then, is that when the bubbles are large and distributed heterogeneously the cathodic vacuum etch technique is by far the most reliable. 4.3.

SPECIMEN B. As IRRADIATED

In this case the only technique which gave reliable results was the thin film technique. The surface features of both etched and fractured samples were on a scale similar to the bubbles and hence there was great difliculty in dia-

OBSERVATION

OF

FISSION

GAS

TABLE

BUBBLES

IN URALLNIU&l

243

2

Gas content (%) Volume

Specimen

incream

(%)

Observed

oalcul&ted (y=lOOO)

A

As irrbdiattted

0.24

0.58

0.51

Thickness

0.43

1.03

0.51

rmasured

0.29

0.76

0.61

0.21

0.46

0.51

Specimen

B

As irradiated

(+ 0.28 y0 solid=0.52)

0.20

0.22

0.09

0.27

0.22

0.11

(+0.12

yc solid=0.23)

0.08

0.28

0.22

Thickness

0.14

0.23

0.22

estimated

0.11

0.18

0.22

tinguishing bubbles from this background. The thin film technique is, however, still quite consistent showing N 2 x 1015 bubbles~cm3 at about 75 A size. The volume increase and gas content caloulated from these is again in agreement with that to be expected (table 2).

tilted in and out of contrast but the contrast shows up in the proximity of a thickness contour not a bend contour as fox ~slocations. This is of course a consequence of bubbles being seen by “thickness contrast” is). Errors from this source always lead to measured values of gas content too low, and henoe values for y 5. Observed gas content (surface energy) too high. However, at the Since the burn-up is 0.11 %, the gas content moment the magnitude of this error is unknown, is 0.5 ems/cm3 ref. 13); using the relation that it is not likely to be too serious when the average the pressure in the bubbles is balanced by the bubble size is N 150 A as in the case of the three surface energy restraint the amount of gas results used to calculate y, here. Note that the values of gas content obtained contained in the observed bubbles in the thin foil results may be calculated. This calculation from specimen B table 2 are about equal to involves assuming that the surface energy of the expected value, if y= 1000 erg/cmz. This uranium is 103erg/ems, ref. 14)and by comparing indicates that in these results the number of the apparent volume of gas with the known invisible bubbles is not high. content an experimental check on this value of A further source of error also which leads to values of y too high is that pointed out by the surface energy is possible (table 2). The accuracy of this calculation depends Levy 17). It is shown that most bubbles have upon measuring the thickness of the thin foil, fringes in their images and the outermost size of the bubbles and correcting for deviations fringe may not be easily distinguished from the from the perfect gas laws. The correction for general background in the matrix, hence the deviations from the perfect gas law can be made measured diameter of such a bubble is always using the values of PV from Nichels et al.15) less than the true diameter. Once again the with graphical extrapolation. Vsing the theo- precise extent of a correction to be applied for retical correction of Barnes 12) results in an this is not known. apparent increase on the gas content figures. The results quoted in table 2 do not include The most difficult to measure is the diameter a correction for either of these errors, so that of the bubbles; tilting of areas cont~ning very the average value for y is on the high side. small bubbles N 50 A shows that these can be The average for the four results on specimen A

244

J.

E.

BAINBRIDGE

from thin films is consistent with y=~ 1000 erg/ems. The one result giving a measured gas volume of 1.03 against an expected 0.51 may be due to counting of pits where inclusions have been etched away. These areas being mistaken as large bubbles. 6.

Conclusions

Although not exhaustive, indicates the following :

the work strongly

1. With bubbles less than 150 A in diameter the most reliable technique is by thin film electron microscopy. This is the only one of the three techniques suitable for nucleation studies. 2.

3.

4.

5.

6.

7.

Cathodic etching enlarges the diameter of bubbles by a factor of N 2 when they are in the range 100-200 A. The thin film technique, although more difficult, is the most powerful of the 3 techniques used here, when bubbles are less than the foil thickness. Fractography is the simplest technique but only cleaved surfaces give results comparable with the other methods. However, fractography is particularly useful for grain boundary studies. When the bubble geneous cleavage representative.

distribution is heterofracture may not be

When the bubbles are heterogeneously distributed on a large scale the thin film transmission technique is no longer applicable, since bubbles much greater than the foil thickness are not detected. The results indicate that the surface energy of uranium is N 1000 f 500 erg/cmc.

AND

B.

HUDSON

Acknowledgements The authors acknowledge considerable help with experimental work from the remote handling group Metallurgy Division and from M. A. Lunn. Thanks must also go to Mr. R. G. Bellamy for helpful comments and to Mr. A. Stuttard for providing the specimens and irradiation details.

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