In Loviisa VVER-440-type NPP the coolant outlet temperature of the hot subchannel is constantly monitored during the operation. According to the authority requirement the maximum subchannel outlet temperature must not exceed the saturation temperature. Coolant temperature distribution inside the fuel assembly is affected by the efficiency of the coolant mixing. In order to enhance the coolant mixing the fuel manufacturer is introducing the additional mixing vanes on the fuel bundle spacer grids. In the paper the effect of the different mixing vane modifications is studied with computational fluid dynamics (CFD) simulation. Goal of the modelling is to find vane modifications with which sufficient mixing is reached with acceptable increase in the spacer grid pressure loss. The results of the studies are discussed in the paper.
Recently, an initiative has been made to improve the fuel economy of VVER-440 reactors by implementing a modification on the geometry of the current fuel assembly design. The proposed modification involves reduction of the fuel rod outer diameter from 0.91 cm to 0.89 cm by using 0.01 cm thinner cladding tubes than earlier. The design improvement would shift the neutronics of an under-moderated system slightly towards optimum moderation and, therefore, increase the reactivity of the assembly. In this paper, a neutronics feasibility study on utilization of the proposed new fuel design at Loviisa NPP is carried out. The study involves a comprehensive comparison of two individual equilibrium fuel cycles: one applying current TVEL 2nd generation fuel design and another one where the new fuel design is used. In addition to equilibrium cycle characteristics, also cycle economics as well as back-end effects are considered. The study concludes that the proposed fuel design modification enables to improve the fuel economy of Loviisa NPP.
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