Modeling the Graphite Stack of RBMK-1000 Reactors Taking into Account Certain Features of Graphite Microstructure

Abstract

Eleven RBMK-1000 reactors (the RBMK-1000 denotes a large-capacity pressure tube nuclear reactor for a power unit with a 1000 MW electric power output) are presently in operation in the Russian Federation. At the time when these reactors were only commissioned, their service life was assigned to be 30 years. However, the replacement capacities had not been commissioned, and studies of the state of reactor elements revealed that the reactor service life was far from having been exhausted. Therefore, it was decided to extend the operation of these reactors beyond their initially assigned lifecycle. One of the measures used to justify that decision was numerical simulation of the graphite stack behavior, because this element determines the reactor service life in view of impossibility to repair, anneal it, etc. In turn, to achieve the required accuracy of graphite stack numerical analysis, it is necessary to realistically calculate the state of individual graphite blocks from which it is assembled. This is a rather intricate problem because it involves the need to carry out combined thermal, neutronic, and strength calculations. In addition, the analysis must take into account that the graphite used in the RBMK reactor is an anisotropic material. In addition, there was a need to solve a very complicated problem of taking into account the two-phase structure of the graphite, because it consists of a binder and filler. The article presents a study of the effect the scale factor has on the reactor graphite behavior, which is taken into account by means of a phenomenological model. It is assumed that the microstresses arising in a graphite item have a significant influence on its behavior. The calculations were carried out, taking the RBMK reactor graphite block as an example, using a 3D finite element technique and a computer program that takes into account the anisotropy of the reactor graphite behavior under the effect of thermal and radiation factors, its creep, and possible occurrence of cracks.