Effect of Neutron Irradiation on the Mechanical Properties, Swelling and Creep of Austenitic Stainless Steels

Abstract

Austenitic stainless steels are used for core internal structures in sodium-cooled fast reactors (SFRs) and light-water reactors (LWRs) because of their high strength and retained toughness after irradiation (up to 80 dpa in LWRs), unlike ferritic steels that are embrittled at low doses (<1 dpa). For fast reactors, operating temperatures vary from 400 to 550 °C for the internal structures and up to 650 °C for the fuel cladding. The internal structures of the LWRs operate at temperatures between approximately 270 and 320 °C although some parts can be hotter (more than 400 °C) because of localised nuclear heating. The ongoing operability relies on being able to understand and predict how the mechanical properties and dimensional stability change over extended periods of operation. Test reactor irradiations and power reactor operating experience over more than 50 years has resulted in the accumulation of a large amount of data from which one can assess the effects of irradiation on the properties of austenitic stainless steels. The effect of irradiation on the intrinsic mechanical properties (strength, ductility, toughness, etc.) and dimensional stability derived from in- and out-reactor (post-irradiation) measurements and tests will be described and discussed. The main observations will be assessed using radiation damage and gas production models. Rate theory models will be used to show how the microstructural changes during irradiation affect mechanical properties and dimensional stability.

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“…Exposure to irradiation is known to produce a number of detrimental consequences on materials. In structural materials these range from hardening and embrittlement with loss of elongation to changes of dimension and shape due to swelling and creep [76][77][78][79][80][81]. In addition, if the neutron spectrum leads to transmutation with production of helium (α particles) and hydrogen(protons), depending also on material composition, the mentioned effects may be significantly exacerbated and the temperature ranges of susceptibility increased on the high side, this problem being especially serious for fusion and Ni-containing materials [77][78][79][80][81].…”

“…In structural materials these range from hardening and embrittlement with loss of elongation to changes of dimension and shape due to swelling and creep [76][77][78][79][80][81]. In addition, if the neutron spectrum leads to transmutation with production of helium (α particles) and hydrogen(protons), depending also on material composition, the mentioned effects may be significantly exacerbated and the temperature ranges of susceptibility increased on the high side, this problem being especially serious for fusion and Ni-containing materials [77][78][79][80][81]. Radiation-induced hardening with subsequent loss of elongation and embrittlement typically occurs when irradiating at low temperature, where "low" depends on the material [77,79,82], for instance in steels the threshold is roughly below 400 • C, but in tungsten alloys it is below 800 • C [82].…”

“…Hardrning and subsequent embrittlement appear to some extent from the very beginning of the irradiation and increase with dose, but generally saturate at high enough dose [77,79]. In contrast, dimensional changes occur only at high enough dose, on the order of tens of dpa [77][78][79][80][81]: they do not necessarily saturate with increasing irradiation (only the rate does) and typically they only appear above a certain irradiation temperature (about 400 • C in steels) [77,83]. Clearly, these high temperature/high dose effects, which are hardly observed in current generation reactors, are expected to be significant in next generation ones.…”

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“…The 304 and 316 stainless steels were used in the PWR and BWR cores. The corrosion resistance of austenitic stainless steels is better than that of both ferritic and martensitic stainless steels [ 115 ].…”

“…They have also been considered as structural components in innovative nuclear fusion test reactors due to the very good results they have obtained in applications in extreme environments [ 117 , 118 , 119 ]. The 300 series steels (Fe-Cr-Ni alloy) have good corrosion and mechanical properties for applications at high temperatures, making them viable to be applied in aeroengine parts, turbochargers, oil and gas pipelines [ 115 , 116 , 117 , 118 , 119 , 120 , 121 , 122 , 123 , 124 , 125 , 126 , 127 , 128 , 129 , 130 , 131 ], and as structural components in nuclear reactors [ 116 , 117 , 132 , 133 ]. The 304 SS is one of the austenitic steels that is most used in nuclear reactors.…”