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Postirradiation tension and burst tests were performed at 593 and 704 C on materials from various heats of Incoloy-800 to establish the effect of irradiation on short-term mechanical properties for fast fluences to 6.25 × 1021 n/cm 2. Two types of tension specimens were tested: precut sheet specimens that were prepared prior to irradiation, and fuel clad specimens prepared from irradiated Mark II EVESR fuel rods. At the highest fiuences, uniform elongations of 1.5 to 2.0 percent have been measured for precut specimens. The uniform elongation was found to be dependent upon the strain rate at which the specimen was tested. Comparable fuel cladding properties at this fluence are not yet available. At a fast fluence of 5.7 × 1020 n/cm 2, precut specimens exhibited uniform elongations of 11.5 to 16.8 percent whereas fuel clad exhibited 2.6 to 6.0 percent for similar fluence and test conditions. It was concluded that the greater degradation of ductility in fuel cladding specimens was associated with both nonuniform (and often higher) temperature and plastic strain experienced during the irradiation period. Resistance of fuel clad to low-cycle fatigue is a key parameter in the fuel performance of steam-cooled reactors, and to a lesser but still significant degree, in determining the fuel performance in liquid metal cooled reactors. Cladding with high ductility has better resistance to low-cycle fatigue and also has improved ability to accommodate fuel swelling at high exposures. Because ductility is shown to be a function of strain rate, the rate of loading upon a fuel clad may be programmed within the range associated with high residual ductility and thus increase the probability that a load can be sustained by the fuel clad without fracture. Analysis of the mode of operation of EVESR fuel shows that strain cycling achieved during transient modes (scrams, emergency cooling, and so on) took place at high strain rates. In view of the dependence of ductility upon strain rates, it is not surprising that the fuel performed as well as it did. A possible explanation is that the large number of cycles was accumulated at relatively high strain rates, where the residual uniform elongation of irradiated Incoloy-800 is shown to be as high as 10 percent.

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