A study into the structure and physical properties of the Cr18Ni9-grade steel following long-term irradiation as part of the BN-600 reactor internals
The microstructures and physical properties of the austenitic Cr18Ni9-grade steel after 22 and 33 years of operation as part of the reactor internals were tested to assess the state of the BN-600 reactor unchangeable components (internals) and to use these further to predict the reactor ultimate life. The paper presents block diagrams of the porosity distribution, depending on the pore size, in samples from portions that were subjected to neutron irradiation with displacement rates of 1.0⋅10–9 to 4.3⋅10–8 dpa/s at temperatures of 370 to 440°C. The elasticity characteristics were measured on samples taken from such portions using a resonance ultrasonic-type test. The swelling calculated from the block diagrams of the porosity distribution depending on the pore size has been shown to have the maximum value at ~ 415°C and to reach values of ~ 3% after 33 years of irradiation. Long-term variations of Young’s modulus have a non-monotonous dependence on damaging dose. The maximum relative variation of Young’s modulus after 22 and 33 years of operation does not exceed respectively 2% and 6% of the initial state values. Apart from the irradiation-induced swelling, the changes in the physical properties in the process of irradiation were shown to be also affected by other structural changes, specifically by the secondary phase formation. As shown by the results of the studies, the BN-600 reactor internals of the Cr18Ni9-grade steel can be operated beyond 33 years of service. The comparison of the results obtained for the material after 22 and 33 years of operation contain information required to describe further changes in the structure and properties of the Cr18Ni9 internals. The available results can be used to predict the reactor ultimate life as part of both existing and developed models
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