Analysis of tests performed on materials of VVER-1000 vessels for impact toughness according to the standard control-sample program

Abstract

An additional assessment and analysis of the degree of radiation embrittlement of the materials used in the VVER-1000 vessels of nuclear power plants in Ukraine are performed according to the standard (regular) program of irradiation and investigation of control samples. New results for samples tested for impact toughness are presented and compared with the results obtained using reconstitutive procedures. It is shown that the results of control-sample tests performed according to the standard program (with a fluence deviation of ±15% above the norm in a group of 12 samples) can be used to assess the state of the material and preliminary substantiation of the serviceability of vessels before obtaining more accurate data on embrittlement using reconstituted samples.The most crucial structure of a nuclear power plant with VVER-1000 is the vessel, which is exposed to substantial mechanical and thermal loads (constant and cyclic) during operation and cannot be replaced before the service life is exhausted. In addition, its wall is irradiated by a flux of fast neutrons, which results in radiation embrittlement, which is characterized by a gradual decrease of the fracture toughness of the materials. To avoid uncontrollable fracture of the vessel during emergency cool-down of a power-generating unit, the normative documentation which regulates the safe operation of nuclear power plants specifies observation of the state of the vessel metal over the entire service life [1].A complex of measures (periodic nondestructive monitoring, monitoring of the conditions of irradiation of the critical elements of the vessel), including monitoring of the change of the properties of vessel materials with the aid of control samples, to assess the state of the material in the vessel in a nuclear power plant has been planned and is being implemented. Since there are drawbacks in the regular control-sample program because of different circumstances, one such drawback being an unfortunate design of the container assemblies and the associated relatively large variance of the neutron fluence for groups of Charpy samples, which are used to determine the shift of the critical brittleness temperature ∆T F . The large variance of the fluence (i.e., with a deviation from the requirements of [2]) for regular control samples has presented difficulties in obtaining reliable values and objective interpretation of the data when estimating the coefficients of radiation embrittlement A F of vessel materials.Specialists at the Russian Science Center Kurchatov Institute proposed a method of reconstituting tested control samples in order to obtain representative data on the degree of embrittlement of reactor-vessel materials. This method makes