Thermal annealing is a proven material treatment method applicable to WWER type reactor pressure vessels with the aim of prolonging their operational lifetime. However, reactor pressure vessel internals of nuclear power plants are exposed to more severe operating conditions (high pressure and temperatures, higher neutron flux, corrosion environment), which cause gradual degradation of their mechanical properties. Thermal annealing can be one of the possible solutions to re-establish initial mechanical properties of reactor pressure vessel internals materials and thus can contribute to long term operation of nuclear power plants. This paper presents results of mechanical properties evaluation (especially hardness) of non-annealed and annealed state of material of WWER 440 – type reactor internals in initial un-irradiated conditions. Within the project an extensive program of experimental activities realized on un-irradiated materials in initial, work hardened, and annealed state was performed to create basis for further analyses on irradiated structural materials. As an integral part of experimental activities microstructural analyses were also incorporated to study of structural changes of non-annealed and annealed material. This paper includes results obtained within the project TH02020565 “Assurance of Safe and Long-Term Operation of Nuclear Pressure Vessel Internals” which is realized by ÚJV Řež, a. s. in cooperation with Czech Technical University in Prague, FNSPE, in the period from 2017 to 2020 with the support of Technology Agency of the Czech Republic.Keywords: Thermal annealing, mechanical properties, WWER 440 type reactor internals
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