Grids with Mixing Elements for VVER Fuel Assemblies
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GRIDS WITH MIXING ELEMENTS FOR VVER FUEL ASSEMBLIES N. I. Perepelitsa
UDC 621.039.524:536.24
Grid designs with mixing elements for VVER FA and experimental studies of their thermohydraulic and mass transfer characteristics are analyzed. The Novosibirsk Chemical Concentrates Plant and the Machine Building Plant manufacture the grids. Comparative assessments of the coolant mixing intensity, increase of the critical heat flux and critical power, and coefficient of hydraulic resistance are made on the basis of studies performed on VVER FA models with mixing grids as well as with and without mixing spacer grids. It is shown in FA models that mixing grids used in the advanced TVS-2M and TVSA assemblies increases the critical power for operating parameters close to the nominal VVER-1000 FA by 5.1–14% and 10–12%, respectively.
Basically, mixing grids are required to increase the power of VVER FA. With their aid the coolant is mixed and swirled, which equalizes the temperature over the cells of the FA and increases the critical power and the critical heat flux. Three types of transverse coolant flows can be formed by means of the mixing elements located in these grids: predominantly twisted flows around a fuel rod or between fuel rods, flows with a specified direction in the transverse section of a channel within one or two cells, and flows with a quite long route relative to chosen landmarks (row of fuel rods, FA axis, and others). Primarily the first type of flow is used in channels with FA in light-water nuclear reactors with a triangular packing of the fuel rods, similar to that in VVER FA. Twisted flows around each fuel rod are formed with the aid of vanes placed on grid strips and bent back, helical deformations of cell walls, and inclined centering corrugations and plates arranged at an angle to the longitudinal axis of the FA. For coolant flow directed toward the fuel rods, the vanes can also be arranged on the rim of a grid. Twisted flows between fuel rods are formed by means of two-tier grids whose top tier is formed from strips with the presence of local vane swirlers. The present article is devoted to an analysis of effective grid designs with mixing elements for the TVS-2M and TVSA advanced fuel assemblies developed by OKB Gidropress and OKBM Afrikantov, respectively. Three groups can be singled out from the existing designs of spacer grids for VVER FA: tubular cells of the honeycomb type, strips with different configuration, and one-piece sheet structures. Grids made from tubular cells (sleeves). A specified deformation of comb cells was used at OKB Gidropress and NZKhK to develop mixing grids of the type ‘cyclone,’ sector,’ ‘vortex,’ and ‘pass-through’ as well as at the Machine Building Plant (MSZ) – comb mixing spacer grid [1–6]. A common feature of cyclone and sector grids is the presence of inclined channels between the cell walls, where the inclination is created by changing the width of the cell faces from bottom to top [1, 2]. Channels in cyclone grids create a rotational motion of the coolant around
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