THE ROLE OF NICKEL IN FORMING A STRUCTURE PROVIDING INCREASED SERVICE PROPERTIES OF REACTOR STRUCTURAL MATERIALS

Abstract

Nickel is an essential alloying element in steels used as structural materials in the most common nuclear power reactors of the VVER type. The paper considers the results of structural studies of traditional and advanced materials of the vessel and internals of VVER-type reactors with a high content of nickel in the composition. It is shown that an increased content (up to 5 wt.%) of nickel in the steels of VVER reactor vessels contributes to the formation of a more dispersed structure with a smaller size of substructural elements and an increased density of dislocations, as well as a higher volume density of carbide phases. The revealed features of the structure of the reactor vessel steel with a high content of nickel have the prerequisites for improving the strength and viscoplastic properties by increasing the number of barriers both for the movement of dislocations and for the propagation of a brittle crack. On the example of materials of VVER internals, it is shown that the nickel content increased in them up to 25 wt.% contributes to an increase in the volume density of radiation defects (dislocation loops of various types) and radiation-induced phase precipitates (G-phase). As nickel increases from 10 to 25 wt.%, there is a tendency to reduce swelling, which contributes to less shape change of the components of the internal devices. At the same time, in steel with the highest nickel content, the highest nickel content was found in the near-boundary regions of the matrix, which contributes to greater austenite stability and a lower probability of the formation of an embrittling α-phase. The data obtained in the work on the effect of nickel alloying on the structural phase state and service characteristics of steels were used in the development of new materials for vessels and internals of advanced reactors.