2008 Volume 2 Issue 2 Pages 775-789
A power uprate of Forsmark's Unit 3 from 109 % to 125 % will be implemented during the 2010 refuelling outage. This implies an increase in gamma heating of the core shroud which could lead to temperatures higher than the design thickness-mean temperature, 300°C, according to ASME regulations. The CFX 5 model to estimate the core shroud temperature distribution consists of the core bypass, from the lower core support plate up to the core grid, and the upper plenum, limited from above by the core shroud cover with steam separator inlets. The conjugate heat transfer at the core shroud inner wall comprises gamma heating from the core, considered as volume distributed heat sources, and subcooled boiling of the bypass flow. The effect of subcooled boiling has been included by using the model by Kurul and Podowski. Using a conservative gamma heat source distribution, leads a maximum thickness-mean temperature that exceeds the temperature limit by approximately 4°C. If a higher temperature limit is accepted, the ASME regulations are not fulfilled, but the consequence is a minor change in the design stress intensity value, Sm. Using a somewhat realistic gamma heat source distribution, the maximum thickness-mean temperature is well below the design temperature. Decreasing the core inlet temperature from 275.4°C to 272.85°C, i.e. the core inlet temperature used today, leads to no subcooled boiling at the core shroud inner wall and to a different location of the maximum temperature.
