Substantiation of thermal engineering reliability of VVER-1200 with mixing grids installed in fuel assemblies

One of the most important issues when it comes to the safety of a nuclear power plant is the need to ensure it not only during operation, but also at the design stage. To accomplish this task, it is important to correctly organize and accurately establish the conditions of heat transfer. This is necessary for the performance of a nuclear power plant. The work was focused on substantiating the thermal reliability of the VVER-1200 core with improved fuel assemblies. In this assembly, the design solution was the inclusion of mixing grates (MR) of the «vortex» and «running» types. As a result, it was found that this design feature makes it possible to increase the amount of electricity generation by increasing the critical power of fuel assemblies for the VVER reactor. The subject of the study were: the phenomenon of the crisis of heat transfer of the first kind, as well as thermal-hydraulic processes in the core. In accordance with the intended purpose of the study, a solution technique was determined, for which the whole work was divided into two stages. During the first stage, a thermal-hydraulic calculation was carried out, the temperature and heat flux distributions were obtained in the cross section and along the height of the fuel element, in which it is maximally loaded, taking into account the surface boiling zone. As a result of the second stage, it was found: the safety factor before the heat transfer crisis. Thus, this technique made it possible to obtain a result on the assessment of the heat engineering reliability of the reactor plant, based on the criteria found. As a final result, the following factors were determined: the reserve before the heat transfer crisis, pressure losses in the reactor plant in the near-wall boiling zone, flow parameters in a two-phase state, and uneven heat flows. Based on the results, it was proved and shown that the improvement of the FA design by adding four «vortex» and «running» gratings made it possible to increase the thermal reliability of the reactor core. This, in turn, makes it possible to operate the power unit at an increased power level, which will lead to an increase in the energy efficiency of the NPP. The significance of this work is: determination of indicators of thermal reliability of the AZ, responsible for the safety of the reactor plant.

Authors: I. A. Lazareva, I. L. Paramonova

Direction: Physics

Keywords: thermal reliability, fuel assembly, critical power, energy efficiency, heat transfer crisis, reserve before the crisis, mixing grids, pressurized water reactor, spacer grid, surface boiling

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