Dimensional changes in elements of the BN-600 reactor core

The spread of the data concerning dimensional changes of the components of the BN-600 core is typical for most of the construction materials but cannot be explained exclusively by their nonuniform operational parameters. The spread is caused by nonuniformity of the composition, structure, and properties of the materials within the effective specification requirements. The nonuniformity manifests itself from one lot to another and also within a single lot and, possibly, even in individual finished articles. Embrittlement of the materials during their irradiation is still another important factor which makes it impossible to obtain a burnup in excess of 10% of the heavy atoms. Proper functioning of the fuel assemblies is also limited by the stress caused by irradiation during operation as a consequence of the combined deformation of a fuel-element bundle and the hexagonal box. This holds particularly for fuel assemblies having boxes of weakly swelling ferritic-martensitic steel and fuel cladding tubes made from austenitic steel. It seems that this is the main reason for the fact that proper functioning of standard fuel assemblies with fuel cladding tubes made from cold-worked steel and boxes which is restricted to a burnup of 11-13% of the heavy atoms. When the combined deformation occurs, 05Kh12N2M is the material to be preferred for the boxes. Increasing the service life of the core components of fast reactors must involve work on improving the materials to obtain well-reproducible properties as these determine the initial state of the steel