Peculiarities of Changes in the Isotopic Composition of Pilot Fuel Elements of a VVER-SKD Reactor under Successive Irradiation in a Fast and Thermal Neutron Spectrum

The paper considers the methodological aspects of experiments planning on the irradiation of experimental fuel rods of a VVER-SKD reactor and analyzes the features of the concept of sequential irradiation in fast and thermal reactors. It is shown that the 149 Sm accumulated at the irradiation in the BOR-60 plays the role of a burnable absorber at the initial stage of irradiation in the reflector cell of the IR-8 reactor. This absorber quickly burns out owing to large absorption cross section and its concentration gradually comes to equilibrium. Burnup of 149 Sm leads to a gradual increase in the linear load of the irradiated experimental fuel rod, followed by a decrease and reaching the equilibrium irradiation regime. This effect must be considered when planning the irradiation regime for experimental fuel elements of the VVER-SKD reactor.