Effect of spacer grid mixing vanes on coolant outlet temperature distribution

In the Loviisa 1 and 2 WWER-440 type nuclear power plants, the enthalpy rise margin of the hottest subchannel is monitored during normal operation. The temperature of the coolant in the hottest subchannel of the fuel assembly is limited to the saturation temperature. This limit prevents the coolant from reaching bulk boiling and, in addition, prevents excessive crud build-up caused by subcooled boiling. Coolant temperature inside the fuel assembly is affected by mixing of the flow inside the fuel rod bundle. Striving for higher discharge burn-up via higher enrichment and four batch loading leads to increased reactivity differences between adjacent fuel assemblies in the core. This results in lower safety margins in the hottest fuel assemblies. In order to retain higher margins the mixing needs to be enhanced. This is possible by using mixing vanes on the spacer grid outer strips. In the future Fortum is planning to order the fuel from the supplier with mixing type of spacer grids. Already in September 2011 a dummy assembly (fuel pellets substituted with steel) including mixing vanes on spacer grids was loaded into the core of Loviisa 1. The purpose here is to become convinced with the mechanical endurance of the assembly. The first fuel assemblies with mixing vanes are planned to be loaded into the core during the refueling outage in 2012. The effect of the different mixing vane modifications for the second generation TVEL 4.4 % fuel assembly which includes six fuel rods with burnable poison, gadolinium, is studied with computational fluid dynamics 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. (Authors)