[en] This paper describes the results of investigations of 08Cr16Ni11Mo3 (AISI 316 steel analogue) austenitic stainless steel irradiated in BN-350 breeder reactor at irradiation conditions close to that for Light Water Reactor (LWR) Internals. The pores were found in 08Cr16Ni11Mo3 steel irradiated at temperature 280 deg. C up to rather low damage 1.3 dpa and with dose rate 3.9 x 10^-9 dpa/s. There were obtained dose rate dependencies of yield strength, ultimate strength and ductility for 08Cr16Ni11Mo3 steel irradiated up to 7-13 dpa at 302-311 deg. C. These dependencies show a decrease in both yield strength and ultimate strength when dose rate decreases. There was observed an apparent decrease in total elongation when dose rate decreases, which was presumably connected with the pores formation in the steel at low dose rates. (authors)

[en] The paper presents and discusses new experimental results on studying the state of structural material of hexagonal fuel assembly ducts exposed first to long-term nuclear reactor irradiation while being in contact with liquid sodium and then to post-irradiation 'wet' storage at BN-350 fast nuclear reactor. The changes in microstructure and physico-mechanical properties of austenitic steel 08Crl6Ni11Mo3 (analog of AISI 316) have been studied; the steel samples of 50x10x2 mm were cut off from the duct of fuel assembly H-214(11) at various levels away from the center of the core: -1200, - 900, - 500, 0,+500 mm. There were performed calculations that enabled to gain information about the irradiation regimes at each level; according to that, minimal temperature during irradiation did not exceed 280 Deg.C (at the level ^-1200mm⁾ and maximal damaging dose comprised 15.6 dpa at the level '0'. Changes in microstructure and phase composition of the steel were studied by the methods of optical, transmission (TEM) and scanning (SEM) electron microscopy as well as by means of X-ray structure analysis. Besides, there were studied changes in mechanical properties when performing both mechanical tensile tests and microhardness measurements. For each level along the duct there were determined values of hydrostatic density and content of trans-mutative helium, which are dependent on damage dose and irradiation temperature. Metallographic investigations have revealed that each investigated sample is characterized by considerable nonuniformity in the sizes of grains. At the same time TEM method revealed pores at all levels except -1200 mm in the irradiated steel. Precision mechanical tests of highly-radioactive microsamples revealed the dependence of mechanical strength and plasticity characteristics of the steel on damage dose and irradiation temperature, which supports the model of stage-by-stage process of radiation hardening and embrittlement. Theoretical analysis takes into consideration the following aspects in the change of mechanical properties of irradiated materials: radiation-stimulated deformation (swelling, creep) and radiation hardening. Within these approaches there was performed an analysis of obtained experimental data and calculated relative changes in strength properties of stainless steel due to high-dose irradiation. This work has been performed within the ISTC Project K-437 'Application of the investigation results of construction materials from breeding reactor BN-350 for prolongation of light-water reactor life-time'