Study of the dependence of the cladding – fuel pellet gap conductance coefficient on the fuel burn-up and the effects on the neutronic characteristics of the reactor core
This paper presents the results of the research to study the dependence of the VVER-1000 (1200) reactor core neutronic characteristics on the cladding – fuel pellet gap conductance coefficient in the process of the fuel burn-up. The purpose of the study was to determine more accurately the dependence of the cladding – fuel pellet gap conductance coefficient on the fuel burn-up as shown in the Final Safety Report for the Bushehr NPP and to determine the extent of the effects this dependence had on the spatial distribution of the neutron field, on the xenon accumulation rate, and on the kinetic and dynamic behavior of the reactor facility. The paper presents the results of calculating the parameters using which the heat engineering safety of the reactor core is monitored in the process of the fuel burn-up (for a generalized fuel load of a VVER-1000 reactor) during the transition to an 18-month nuclear fuel cycle. This paper also includes the results of a numerical research to determine the cladding – fuel gap conductance coefficient depending on the fuel burn-up. These results have shown that, in reality, the gap conductance coefficient dependence on the burn-up does not affect greatly the steady-state characteristics. At the same time, it affects to rather a great extent the xenon accumulation rate, specifically in the event of an extended fuel life. In conditions of maneuvering (load following) modes accompanied by the xenon processes in the reactor core, it proves to be very important and timely to take into account this dependence in the engineering codes used to support the operation of the VVER-1000 (1200) NPP equipment and full-scale simulator.
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