Acoustic method for monitoring the state of VVER vessels

Abstract

The results of an investigation of the possibility of using acoustic characteristics to evaluate the state of the VVER vessel material are presented. The investigations were performed on template-samples cut from the VVER-440 vessel material and on control samples from VVER-1000 vessels. It is shown that the characteristics of elastic waves depend on the fluence and the radiation embrittlement of VVER vessel materials.A VVER vessel is the most important component of the third safety barrier of a reactor facility, whose service life determines the maximum operating time of a nuclear power plant with such a reactor. Neutron irradiation and high temperatures substantially change the microstructure and mechanical properties of the materials of reactor vessels made of low-alloy structural steel (15Kh2MFA, 15Kh2MFAA steel, Sv-10KhMFT welding wire) [1-3]. A decrease of plasticity and shift of the temperature of the transition from plastic to brittle failure in the operating temperature range increase the probability of brittle fracture. However, the mechanism of radiation embrittlement is still not completely understood, which creates difficulties for long-term prediction of the behavior of a metal and validating the service life of reactor vessels.The effect of irradiation on VVER-1000 vessel materials and the monitoring of the change of the state these materials during operation are evaluated using different types of control samples which are placed inside the vessel space near the core. Such samples are used to determine periodically the change of the mechanical properties (yield stress, ultimate strength, relative total elongation and relative contraction), characteristics of the resistance to brittle fracture (critical brittleness temperature, fracture toughness or critical aperture of cracks) and low-cycle fatigue. Adhering to all technologies used for vessel fabrication, control samples are cut from the main material of the sample ring of the reactor vessel shell, the weld-seam material, and the thermal-influence zone. Unfortunately, a system of control samples was not provided for first-generation VVER-440 with a vessel without corrosion surfacing. Consequently, to evaluate the change of the properties of the material, templates of weld-seam metal and the main metal were cut from the inner surface of the reactor vessel. Small 5 × 5 × 27.5 mm Charpy mini-samples were cut from the templates and were studied by the acoustic method.In the present work, the possibility of using the acoustic method to evaluate the state of VVER vessel material was investigated. The acoustic studies were performed on control samples and template samples (Charpy type) used for studying fracture toughness and cut from the weld-seam metal of the reactor vessels in the No. 1 and 2 units of the Kola, No. 3 and 4 units of the Novovoronezh, and No. 1 unit of the Khmelnitskaya nuclear power plants with different irradiation conditions. The chemical composition of the weld-seam material in the reactors in the Kola and the control sample materia...