EXPERIMENTAL ASSESSMENT OF THE EFFECTIVENESS OF RECOVERY ANNEALING OF VVER-1000 VESSELS

An experimental assessment of the effectiveness of recovery annealing of VVER-1000 vessels on the basis of mechanical tests and structural studies of control samples of the main metal and the weld-seam metal with high nickel content (>1.65%) in the No. 1 unit of the Balakovo nuclear power plant in states after primary irradiation in the control sample channels, annealing in a chosen regime, and repeat accelerated irradiation in the IR-8 research reactor to fluence corresponding to extended service to 60 yr and longer is presented. The mechanical tests and structural studies showed a high degree of recovery of the structure and properties for the chosen temperature‐time regime of annealing as well as a lower rate of repeated after the recovery annealing embrittlement as compared with the rate of embrittlement with primary irradiation. Extended operation of reactors at elevated temperature and in ionizing radiation degrades the properties of vessel materials. This is expressed as a shift of the viscobrittle transition temperature. There exist several mechanism of radiation embrittlement of vessel steel ‐ hardening as a result of radiation defects and radiation-induced precipitates, forming grainboundary segregations of phosphorus (development of reversible temper brittleness [1]). For VVER-440 vessels, copper and phosphorus are the main elements affecting embrittlement. The highest rate of embrittlement of weld-seam metal with a superhigh content of nickel and manganese has been found in VVER-1000. The rate of radiation embrittlement of such weld seams is higher than for VVER-440 vessel materials, and it is a result of the density of radiation-induced precipitates enriched with nickel, manganese, and silicon continuously increasing under irradiation as well as intensification of segregation processes. As a result, radiation embrittlement of control samples of weld-seam metal in VVER-1000 vessels with high nickel content (>1.65%) was found to be higher than the normative value [2]. For this reason such weld seams in operating reactors are elements that limit the service life of the vessel and the facility as a whole. The problem of extending the service life of reactor vessels to 60 years and longer has now arrived. This result can be attained by two methods. For VVER-1000 with low nickel content in the weld seams, service life extension can be validated by recertifying the vessel materials (re-examining the degree of embrittlement on the basis of new data on control samples from VVER-1000) for higher fast-neutron fluence. For some reactor vessels with high nickel content (>1.65%), together with structural upgrading it is necessary to perform recovery annealing of the metal in weld seams located opposite the center of the core and as a result exposed to reactor radiation. Such a method of recovering the properties of a metal was successfully used for VVER-440 vessels, but the differences in the chemical composition, construction, and conditions of operation require for VVER-1000 vessels choosing other temperature-time regimes for recovery annealing.