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of the uranium will have been fissioned to produce a wide variety of stable and radioactive fission products. A significant fraction of these are insoluble in UO2 and can form precipitates within grains and at grain boundaries. These include the noble gases Xe and Kr, some elements that are volatile at fuel operating temperatures (e.g., Cs and I), and the metals Tc, Mo, and Ru. Many others, such as the lanthanides and actinides like Pu and Am, which are produced by neutron capture from U, substitute for uranium in the UO2 lattice. The high temperatures (-800 to 1400°C)

Figure 1. Transmission electron micrograph of spent LWR fuel with a burnup of 45MW d/kg U. The dark particles along the grain boundary are e-ruthenium (Mo-Ru-Tc-Pd-Rh). Courtesy of L.E. Thomas.

experienced by fuel during in-reactor irradiation allow diffusion of the more insoluble species out of the grains to the grain boundaries. A fraction (typically several percent) of the inventories of noble gas and volatile fission products accumulate there and can be released to the periphery of the fuel as a result of the formation of fission gas tunnels at grain boundaries and cracks in the pellets. A typical microstructure in the vicinity of a grain boundary in spent light water reactor (LWR) fuel is shown in the transmission electron micrograph in Figure 1. Both nm-size fission gas bubbles and e-ruthenium phase (MoRu-Tc-Pd-Rh) particles are visible along the grain boundary. Examination of an intergranular fracture surface of a spent CANDU (Canada deuterium uranium) fuel sample using x-ray photoelectron spectroscopy2 illustrates the buildup of insoluble fission products at grain boundaries. Figure 2 shows photoelectron spectra of the fracture surface of four samples. Peaks indicative of the presence of Cs, Rb, Ba, and Te are clearly apparent. Figure 3 shows composition depth profiles of these surfaces, demonstrating segregation of Cs in a thin film of approximately 1 nm depth. These fission products can be released from grain boundaries during in-reactor irradiation, leading to their accumulation at the cooler gap between the fuel and the cladding. Studies of LWR fuel using leaching techniques3'4 suggest that, on average, 1-2% of the Cs inventory in the fuel is present in the gap, with up to 1% present at grain boundaries. Slightly larger percentages are observed for CANDU fuel, which typically experiences higher in-reactor irradiation temperatures.5* Other microstructural features that may affect fission-product release include grain growth in the hot central region of the fuel, and the subdivision of grains that occurs in the cool outer region of the fuel. The latter feature occurs because of accumulated radiation damage arising from very high burnup in this region.7 It is not yet clear if this effect can lead to enhanced fission product leaching. It should be emphasized that, except for this feature, solid-state radiation effects in UO2 fuel are generally thought to be small, and UO2 does not become amorphous even at very high burnups.8 Dissolution