Transition to four batch loading scheme in Loviisa NPP

The WWER-440 reactors of Loviisa NPP are operated with 1500 MWth power and reduced core. During recent years a 3-batch loading scheme has been used. Loviisa-1 is currently running with BNFL fuel equilibrium cycle and Loviisa-2 with TVEL fuel equilibrium cycle. Our goal is to move to a 4-batch scheme with TVEL fuel for both reactors. To achieve this goal the U-235 enrichment has to be increased from the current designs used. The fuel to be used in the near future is 4.37 % enriched fuel with six Gd2O3 doped rods. The characteristics and consequences of the core consisting of Gd-fuel are discussed based on our target equilibrium loading pattern. With the 4-batch loading scheme the discharge burnups exceed the current assembly burnup limit with a clear margin. Thus, we also have to prepare an application for the safety authority to increase the assembly average burnup limit from the current 45 MWd/kgU to about 56 MWd/kgU. First Gd-fuel assemblies are loaded into Loviisa-1 core in September 2009 and into Loviisa-2 core in October 2010. The reload batch of Loviisa-1 in 2009 consists of 60 Gd-assemblies and 24 non-Gd-assemblies plus 12 followers. In this paper some results are presented from Loviisa-1 zero power startup experiments and the first days of power operation. Among other aspects the six Gd2O3 doped pins used in the assembly have an effect on the pin power profile of the assembly during the first half of the cycle. The influence of the changing pin power profile on the outlet temperature measurements is briefly discussed based on expected effect and measurement (Authors)