More accurate modeling of the radiation embrittlement of a calibration metal on the basis of the shift criteria and envelopes of the dose-time dependences experimentally confirms that the modeling error corresponds to the variance band limits of the results of tests performed in control samples and reflects the structural nonuniformity of the metal of large-size blanks. It is recommended that high-quality VVER-1000 vessel steel with critical brittleness temperature from -44 to -78°C and from -78 to -121°C be used to eliminate the risk of brittle fracture of the core shells of GEN-IV VVER vessels.To eliminate the risk of brittle fracture of the core shells of GEN-IV VVER vessels, it is necessary to develop vessel steel that decreases the critical brittleness temperature admissible during operation to zero. This requires reducing to a minimum the conservatism employed when determining the guaranteed values, the error in modeling the dose-time dependence of T cr , and the effect of the chemical and structural nonuniformity on the change of the critical brittleness temperature when fabricating and using large-size shells.Error in Dose-Time Dependence Modeling and the Conservative Approach to Evaluating T cr . During the manufacture and use of VVER vessels, depending on the time, temperature of heating and cooling, deformation, irradiation, and other technological and operational factors reversible and irreversible brittleness processes, which are modeled by the time, dose, and dose-time dependence of T cr , occur in the vessel steel [1][2][3]. The main characteristic of resistance to brittle fracture is the critical temperature described by the very simple mathematical model:where T crA is the maximum admissible critical brittleness temperature at the critical point; T cr is the critical brittleness temperature which depends on the operating time of the vessel and is determined by the envelope of the fitted curve reflecting the dose-time dependence; T cr0 is a conservative estimate of the critical brittleness temperature in the initial state, independent of the operating time of the vessel; ΔT[F(t)] = A F [F(t)/F 0 ] 1/3 is a conservative estimate of the shift of the critical brittleness temperature, which depends on the operating time of the vessel and is determined by the envelope fitting the

The high conservativeness of the evaluation of the service life of VVER-1000 vessels is due to the operational reliability margin and the high error in determining the initial computational data. A structural analysis of the error in critical brittleness temperature investigations is presented and the factors which have a large effect on the error in constructing the dose-time dependences of the critical brittleness temperature are determined. In constructing dose-time curves, the use of the critical temperature of a calibration metal and heat-treatment regimes of calibration samples makes available and permits checking a wide range of structures that is characteristic for large blanks, makes it possible to take account of radiation embrittlement effects and radiation-stimulated reversible brittleness, decrease the error in determining the guaranteed values to T cr ± 5°C, and eliminate the large effect of structural nonuniformity on objective predictions made for a reactor vessel.Reliable prediction of the service life of VVER-1000 vessels with computational validation of the strength and reliability of the materials used for the structural elements of the reactor facility is a topical problem of reactor materials engineering [1]. Computational reliability is determined by the objectiveness of the mathematical model of the method and initial data, whose conservativeness is affected by the model error for the processes occurring in the vessel steel over a prolonged period of operation. Thus, in calculations of the brittle fracture resistance of the equipment in a nuclear power facility (vessel steel 12Kh2NMFA-A) according to PNAE G-7-002-86 the critical coefficient of stress intensity K 1C depends on the reduced temperature T red = T -T cr , due to the neutron irradiation of the material, including reversible brittleness, cyclic damage, and other factors.The effect of the error in determining the reduced temperature on the critical stress intensity coefficient can be illustrated on the following examples: the stress intensity coefficients are 74.7-118.4 and 54.5-212.9 MPa·m 1/2 for errors 60 ± 10°C and 60 ± 30°C, respectively (Fig. 1). Thus, the critical stress intensity coefficient increases considerably with increasing reduced temperature and its error of determination.

Brittle fracture resistance of RPV 15H2NMFA grade 1 steel is investigated. Sets of small-sized testing samples and a set of standard-sizes samples were used in the study. It was demonstrated that application of sets of small-sized specimens in mechanical tests for determining the brittle fracture resistance of RPV 15H2NMFA grade 1 steel makes possible the following: increasing the volume of tests in each batch by 8 times without significant changes in the design of irradiation facility thus ensuring maintaining the initial irradiation parameters during testing; substantially expanding the database of test results for statistical processing. The need for large-scale modeling of process conditions arising in weld joint zones inaccessible for direct testing, such as: (1) the welding zone between the base metal and the corrosion-resistant coating metal, (2) the welding area between the weld metal and the corrosion-resistant coating metal, and (3) the fusion area between the base metal, the weld metal, and the anticorrosive cladding metal, is demonstrated in the paper. Process modeling of the metal in fusion areas up to 0.5 mm wide (each is 100 μm in size) with an experimental electroslag refined (ESR) ingot of up to 300 mm long with similar microstructure and variable chemical composition allows the following: (1) examining not less than 1000 small-sized impact testing samples with continuous distribution of concentrations of chemical elements in accordance with a certain law; and (2) testing these samples and identifying brittle fracture dangerous zones across fusion areas between the base metal and the anti-corrosive padding metal in the initial state or after subsequent irradiation at a given fluence rate and temperature.

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