Journal of Nuclear Materials 61 (1976) 229-231 0 North-Holland Publishing Company
IDENTIFICATION OF PRECIPITATES ASSOCIATED WITH INTERGRANULAR
FISSION GAS BUBBLES IN IRRADIATED UO, FUEL
I.J. HASTINGS, D.H. ROSE and J. BAIRD Atomic Energy of CanadaLimited, Chalk River Nuclear Laboratories, Chalk River, Ontario KOJ lJ0, Canada Received 15 April 1976
We found that a platinum pre-shadowed single stage carbon technique produced an excellent precipitate-extraction replica from irradiated U02 fuel exhibiting intergranular fracture. The result is a low activity sample replica with the extracted precipitates in positions corresponding with those they occupied in the bulk sample. An added advantage is that small (GO.2 w diameter) precipitates can be analyzed without considering the contribution from U in the matrix, which can occur in a bulk sample . Examination and qualitative analysis of the precipitates were carried out using an unshielded scanning electron microscope (SEM) with an attached energy dispersive X-ray spectrometer (EDS). Details of the extraction and examination techniques are given elsewhere [lo]. Replicas containing extracted precipitates were obtained from fracture-surfaces of UO, fuel irradiated to 75 and 220 MWh/kg U; in each case lhd0 was 4.0 kW/m. Samples originated from the equiaxed grain growth region where the operating temperature was (1800 + 100) K; the fracture mode was predominantly intergranular. Examination of replicas in the SEM is not standard procedure; initially it was necessary to reduce the operating voltage to 4 kV to obtain a reasonable image [lo]. Precipitates were detectable, but the remainder of the replica was transparent. However, increasing the thickness of the Pt./C shadowing deposits made it possible to both observe and analyze samples at the normal operating voltage of 26.4 kV.
Optical metallography of sections from irradiated UO, and (U, Pu)02 fuel usually reveals inclusions up to 5 pm in diameter, identified as fission products [ l51. The most common inclusions in UO2 have been found to be metallic, containing MO,Tc, Ru, Rh and Pd [3,4]; less common oxide inclusions contain Ba, Sr, Zr and Ce [3,4]. Scanning and replica electron microscopy of intergranular fracture-surfaces show precipitates, assumed to be fission products, associated with individual fission gas bubbles on the grain boundaries [6-81. The technological significance lies in the possible modification of fission gas bubble migration , swelling and gas release due to the bubble-precipitate interaction. In this note we report the identifitiation of precipitates attached to individual intergranular fission gas bubbles in U02 irradiated to burnups of 75 and 220 MWh/kg U.
2. Experimental Two techniques have previously been used in the examination of inclusions in irradiated fuel [2-S]. In one, the sample is reduced in size to minimize gamma activity, allowing analysis in an unshielded microprobe. A variation of this technique is the chemical or mechanical extraction of inclusions; a disadvantage is that the position of the inclusion in the microstructure is not accurately known. The second major approach utilizes a shielded microprobe, which permits examination of larger samples with high gamma activity.
3. Results and discussion Fig. 1 is a secondary electron micrograph of a replica from a fracture-surface in UO, fuel with a burn229
Fig. 1. Replica from an ixl.tergranuler fracture surface in UO2 fuel irradiated to 75 MWh/kg pi. Secondzuy eiectron micrograph at 26.4 kV shows extracted precipitates (Q) associated with fission gas bubbles (b) in the ptar.e of a grain boundary.
up of 75 MWh/kg U, Eenticula~ fission gas bubbles a ~axi~~~ major axis of up to 2 pm lie in tke plane of a grain boundary; extracted precipitates about 0.2 pm in diameter axe associated with the bubbles. Sim-
X-RAY fig. 2. X-ray intensity/energy and 220 MWh/kg U.
spectra from precipitates of tile type shown in fig. i.) extracted from aj& Euei
I.J. Hastings/Identification of precipitates
One unexpected result was the presence of Fe in the precipitates. An early study [l] of irradiated UO2
and ZrO,-UO, containing 70-760 ppm Fe showed white “spots” 2-5 m in diameter associated with the presence of metallic Fe. No positive identification of the “spots” was made. Since our as-sintered fuel contains 5200 ppm Fe, the interesting possibility arises that Fe at the grain boundaries is acting as a nucleus for the depositing solid fission products. If so, this presents a means of controlling solid fission product precipitation; subsequent grain boundary bubble migration may thus be modified by the bubble/precipitate interaction, as suggested previously for uranium metal [ll]. In a Chalk River study , UO was bombarded with 1Olg (40 keV Fes6) ions/m3 and annealed at 1300- 1640 K to produce Fe-rich precipitates ranging from 0.02-0.3 w in diameter. Following a subsequent bombardment with 10lg (40 keV Xelsl) ions/m2, it was suggested that the precipitates may have been dissolved, or redistributed into much smaller precipitates, by the energetic xenon ions. Consequently, uncertainty exists regarding the initial stability of the Fe and the later stability of fission product precipitates under reactor operating conditions.
4. Conclusions (a) An extraction technique allowed the SEM/EDS examination of small (0.2 m diameter) intergranular precipitates in irradiated U02 fuel; there was thus no matrix contribution to affect the intensity/energy spectra. (b) 86 precipitates associated with intergranular fis-
sion gas bubbles in UO, fuel irradiated to 75 and 220 MWh/kg U showed identical constituents: MO,Tc, Ru, Fe and Pd. No U or rare earths were detected. (c) The presence of Fe in the precipitates invites the speculation that Fe at grain boundary sites in the assintered U02 may be acting as a nucleus for the depositing fission products. If so, the possibility exists of controlling solid fission product precipitation, and thus subsequent intergranular bubble migration, through the bubble/ precipitate interaction. The main uncertainty is the stability of the precipitates under reactor operating conditions. References [l] J. Belle, L. Berrin, J.C. Clayton, I. Cohen, J.M. Markowitz and T.R. Padden, Westinghouse (USA) report WAPD-251 (1961).  M. Bleiberg, R.M. Berman and B. Lustman, ibid WAPDT1455 (1962). [ 31 B.T. Bradbury, J.T. Demant, P.M. Martin and D.M. Poole, J. Nucl. Mater. 17 (1965) 227.  B.M. Jeffery, ibid 22 (1967) 33. [S] J.I. Bramman, R.M. Sharpe, D. Thorn and G. Yates, ibid 25 (1968) 201.  W.J. Chatwin and R. Sumerling, UKAEA report TRG1595 (1968).  G.L. Reynolds and G.H. Bannister, J. Mat. Sci. 54 (1970)  I!tHastings, J. Nucl. Mater. 54 (1974) 138.  C.E. Johnson, Physical Aspects of Electron Microscopy and Microbeam Analysis, Ed. Benjamin M. Siegel and Donald R. Beaman (Wiley Biomedical-Health Pub 1975) p. 373. [lo] LJ. Hastings and D.H. Rose, J. Am. Ceram. Sot. 59 (1976) 168. [ll] S.F. Pugh, J. Nucl. Mater. 4 (1961) 177. [ 121 A.M. Ross, quoted by P.O. Perron and P.A. Morel, Atomic Energy of Canada Limited report AECL-3329 (1969).