Role of Irradiation Embrittlement in RPV Lifetime Assessment
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Role of Irradiation Embrittlement in RPV Lifetime Assessment Milan Brumovský Research Centre Řež, 250 68 Řež, Czech Republic. ABSTRACT Integrity and lifetime of reactor pressure vessels are practically determined by their material resistance against fast/non-ductile failure and consequently by their radiation damage resulting in irradiation embrittlement and hardening. Mechanism of irradiation embrittlement of RPV materials depends on selected materials and operation conditions but their values depend in a large extent on reactor design, i.e. on neutron flux/fluence depending on RPV wall. Generally, new RPV design allows smaller neutron fluences but absolute value of irradiation embrittlement still depends on a choice of RPV material. Even though radiation damage (especially irradiation embrittlement) is important for RPV behavior, integrity and lifetime depends, in principle, on final value of applied fracture mechanics parameter –transition temperature. Thus, its initial value as well as its shift due to irradiation embrittlement is of interest but only the embrittlement can be affected during operation. INTRODUCTION Reactor pressure vessel (RPV) is a unique component, as: - nuclear reaction is realized inside RPV, - RPV contains whole nuclear fission materials, - RPV contains practically all radioactively induced materials, RPV cannot be practically cooled down if it ruptured, then non-controlled core melting will take place. Due to a large volume of reactor active core to produce required heat output and due to high operation conditions: - RPV is a large and heavy component and thus it is practically non-replaceable, - RPV must be safe during whole lifetime, - RPV must fulfil the most severe requirements to materials quality. - RPV must ensure long term and safe operation under conditions of high pressure, temperature and radiation. - RPV must withstand effect of several stressors resulting from operating conditions: - Pressure between 12 and 18 MPa, - Temperature between 270 and 325 °C, - Neutron fluence between: 1022– 2x1024 m-2 (En > 1 MeV) INTEGRITY AND LIFETIME ASSESSMENT Integrity assessment of RPVs are nowadays performed using fracture mechanics approach with characteristic transition temperatures of RPV materials. PWR RPVs (based on ASME, RCC-M, KTA, JSME Codes): ART = RTNDT + ΔT Where ART= Adjusted Reference Temperature
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RTNDT = Initial NDT Reference Temperature, based on Drop Weight Test and Charpy V-notch Impact Test results, ΔT= Shift of RTNDT due to irradiation. VVER (PWR designed according to Russian Codes PNAEG): TkF = Tk0 + ΔTF + ΔTT + ΔTN (2) Where TkF = critical temperature of brittleness, Tk0 = initial critical temperature of brittleness, based on Charpy V-notch Impact Test results, only, ΔTF = shift of critical temperature of brittleness due to irradiation, ΔTN = shift of critical temperature of brittleness due to thermal ageing, ΔTF = shift of critical temperature of brittleness due to cyclic damage. Regarding the amount of individual degradation effects, the most important is, of cour
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