High resolution TEM observation and density estimation of Xe bubbles in high burnup UO 2 fuels K. Nogita * , K. Une Nippon Nuclear Fuel Development Co., Ltd., 2163 Narita-cho, Oarai-machi, Higashi Ibaraki-gun, Ibaraki-ken 311-1313, Japan Abstract High resolution transmission electron microscopy (TEM) observations, nano-diraction and nano-area energy dis- persive X-ray spectroscopy (EDX) analyses were carried out in the outer region of high burnup UO 2 pellets. Samples were prepared from the outer low temperature region of UO 2 pellets, which had been irradiated to the pellet average burnups of 30 and 49 GWd/t in BWRs and 83 GWd/t in a test reactor under a pressurized water reactor (PWR) con- dition (1 GWd/t  2.5 ´ 10 19 ®ssions/cm 3 ). Lattice images and nano-area EDX results indicate the presence of 4±10 nm size Xe bubbles with ®ve-metal particles of Mo±Tc±Ru±Rh±Pd. Nano-diraction patterns from bubbles show two dif- ferent new patterns other than that of matrix UO 2 . From the Xe/U proportion obtained by the EDX peak and nano- diraction patterns, it was suggested that Xe in the small bubbles existed as a solid or in a near solid state at very high pressure. Ó 1998 Elsevier Science B.V. All rights reserved. PACS: 61.80.Hg; 61.61.Bg Keywords: UO 2 ; Neutron irradiation; High resolution TEM 1. Introduction Sintered ceramic fuel pellets of UO 2 are com- monly used in light water reactors (LWRs) for nu- clear energy production. In the outer region of UO 2 pellets irradiated in LWRs, small intragranu- lar bubbles of several nanometers in size are formed due to accumulation of ®ssion-induced va- cancies and the ®ssion product (FP) gas atoms of Kr and Xe. The detailed characteristics of bubbles in UO 2 fuels irradiated to 1±83 GWd/t have been studied by transmission electron microscopy (TEM). The small bubbles, of a few nanometers in diameter observed in low burnup fuels (<18 GWd/t) [1,2], appear to grow to 7±8 nm in high burnup fuel with 45±83 GWd/t [3±6]. The volume fraction of bubbles also increases with increasing burnup, and the bubble size distribution changes from a monomodal shape for the middle burnup fuel of about 20 GWd/t to a bimodal one for high- er burnup fuel [4,5]. It has been thought that very high pressure FP gas atoms of Xe are present in these small bubbles [7]. In addition, solid ®ssion product particles with the hcp structure (a solid- solution alloy of Mo±Tc±Ru±Rh±Pd) are found Nuclear Instruments and Methods in Physics Research B 141 (1998) 481±486 * Corresponding author. Tel.: +81 29 266 2131; fax: +81 29 266 2589; e-mail: nogita@nfd.co.jp 0168-583X/98/$19.00 Ó 1998 Elsevier Science B.V. All rights reserved. PII S0168-583X(98)00040-8