Review of core design and operating experience in Loviisa

T. Lahtinen, M. Antila & S. Saarinen. Fortum Nuclear & Thermal, Finland

20th Symposium of AER on VVER Reactor Physics and Reactor Safety (2010, Hanasaari, Espoo, Finland)
Core Design and Operation


Loviisa NPP is currently in the transition phase from 3-batch to 4-batch loading scheme.
During the 3-batch loading 3.7 ? 4.0 % enriched fuel has been used. By the end of the
transition phase the cores of both NPP units will be consisting purely on TVEL second
generation fuel with average enrichment of 4,37 %. The new fuel assembly type includes six
gadolinium doped rods (3,35 % Gd2O3) that are located next to the corner rods.
Introduction of the new fuel type has an effect on certain reactor core characteristics.
From the point of view of reactor operation under normal conditions the most remarkable
change is seen in the decrease of the hot subchannel enthalpy rise margin. In the worst case
the reduced margin could lead to power limitation for some period of the cycle. In order to
reduce the risk of power limitation special attention is paid on modeling of the coolant mixing
in the fuel bundle and also in the upper part of the assembly. A short study on these modeling
issues is given in the paper.
The first transition cycle with Gd fuel reload batch started in the Loviisa-1 unit in September
2009 and ended in August 2010. In the Loviisa-2 unit the first transition cycle will be starting
in October 2010. The paper gives a discussion on core operating experience in the recent
cycles. Attention is paid on outlet temperature measurement readings for the Gd fuel
assemblies during the first transition cycle. Measured outlet temperatures are expected to
change slightly due to Gd induced change in assemblywise pinpower distribution.

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