The thesis work is devoted to the study of the influence of the core baffle geometry degradation due to the change in the VVER-1000 reactor internals cooling conditions.
The introduction substantiates the choice of the research topic, formulates the purpose and objectives of the study, defines the research methods, highlights the connection with research programs and works, as well as the scientific novelty and practical significance of the results obtained, characterizes the personal contribution of the author, provides the approbation of the dissertation results, its structure and scope.
The first section reviews the existing research related to the subject of the thesis. So the following are analyzed: the root causes and peculiarities of the internals metal degradation processes, the most probable changes in relation to the design geometry - exhaustion of the gaps between the core baffle and reactor core barrel, as well as the appearance of radial flows not provided by the VVER-1000 reactor design. Based on the literature review, the aim, objectives, study subject and object were formulated.
The second section covers to the study of changes in the core baffle cooling conditions during the degradation of its geometry, namely its contact with the reactor core barrel and the baffle rings opening.
A methodology for solving the problem of the change in the internals cooling conditions, based on a gradual analysis, is proposed. In order to carry out this analysis, a universal three-dimensional baffle cooling CFD model was developed. The series of runs was performed for different fuel campaigns of Ukrainian NPP power unit and the choice of a representative campaign for the further analysis was substantiated.
In order to study the changes in the core baffle cooling conditions in the presence of its contact with the reactor core barrel, an approach with a consideration of various possible contact configurations is proposed. In the framework analysis, 8 variants of contact are considered. In addition its influence on the change in the temperature state of the main internals structural elements (baffle, barrel, studs and tie bars) was clarified.
The influence of the baffle rings opening presence and the appearance of radial flows on the change in the internals cooling conditions are investigated. The intensity of radial flows was estimated analytically and by using the developed CFD model. The possible maximum limit of the bypass intensity from the reactor core was obtained and its influence on the baffle temperature state was analyzed. The following inverse influence presence "opening - local cooling intensification - decrease in the intensity of further opening" had been received and the conclusion about the needs to use a coupled approach considering the mutual influence of the three physics type - neutron, thermal-hydraulic and strength analysis was made.
The third section presents a description of the neutron-physics module key components. The developed module consists of three main parts, which include: fuel loading engineering analysis unit, neutron and gamma-ray transport model and parametric analysis. A radiation transport model was built based on the Monte Carlo code MCNPX and allowing the setting of non-uniformity of temperature and material composition. Using the transport model and the developed post-processing subprogram, the synthesis of three-dimensional (r-Θ-z) instantaneous and integral radiation characteristics is implemented, which can be exported to the thermal-hydraulic and strength calculations module.
The fourth section includes the description of the complex coupled multiphysical procedure development for assessing the change in internals elements cooling conditions during the degradation of their geometry, considering the presence of the inverse influence on the cooling and deformation intensity. The following concept, based on the three physical components of the swelling problem - neutron-physical module, thermal-hydraulic analysis and strength assessment module is proposed. The thermal-hydraulic analysis module was modified and divided into 3 sub-models - a 1-D thermal-hydraulic model of the reactor core bypass, a solid model of the internals main elements, and a modified reactor core CFD model. The description of each component is given and effective interfaces for coupling of individual modules are developed.
Applying the developed multiphysics procedure, a calculation analysis of the baffle geometry degradation was carried out. The procedure considered feedback related to the local influence of the radial flows presence on the baffle cooling conditions. As a result of coupled assessment, the values of the rings opening after 60 years of operation, obtained in Section 2 as part of the sequential analysis, were clarified. The resulting values of the gaps between the rings have smaller values, which are on average 22% lower than the results of conservative sequential analysis.
