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Numerous irradiation experiments have been carried out and several hundred specimens have been tested to assess the effects of low dose neutron radiation (∼1 dpa or ∼1 appm helium (He)) on the mechanical properties of the near core permanent structures of fast breeder reactors. The experiments conducted include irradiations in fast and thermal reactors lasting from a few days to well over 15 years. In all cases, particular attention has been given to the correlation between service conditions and irradiation parameters such as temperature, time, dose, dose rate, spectrum, and sodium environment, as well as to materials requirements through the use of nuclear-grade welded plates of stainless steel Types 304L and 316L-SPH. The postirradiation properties measured are those usually obtained with tensile, instrumented Charpy (U and V notches), fracture toughness (J-R curves), fatigue, and fatigue-creep testing. The neutron metrology of the experiments has been performed with conventional detectors distributed among the specimens and in some cases, doubled with parallel irradiation of heavy isotope detectors in adjacent reactor sites.
The results obtained show that for lower core structures, where the operating temperature is about 400°C and the ductility and toughness are primary design concerns, the changes induced by radiation doses up to 1.3 dpa in the overall mechanical properties of welded joints are small. For upper-core structures, where the operating temperature is about 550°C and fatigue and creep resistance are major design requirements, the changes induced through the formation of up to about 3 appm helium in conventional fatigue properties or fatigue with short hold times are negligible. With increasing hold time, intergranular rupture in irradiated specimens is enhanced, but the limited number of tests conducted in this study do not allow definite conclusions to be drawn.
low dose neutron irradiation, dpa, helium, fast breeder reactor, above core structure, below core structure, design codes, austenitic stainless steels, Type 304L, Type 316L, Type 316L-SPH, mechanical properties, tensile properties, impact toughness, fracture toughness, fatigue, creep-fatigue, fatigue with hold time, low cycle fatigue
Tavassoli, Ali Asghar
DMECN, IRDI, C.E.A. CEN-Saclay, Gif sur Yvette,