Measurements obtained from LaSalle spent fuel indicates that the liftoff [corrosion film and coolant system crud deposit thickness] on LaSalle fuel may be greater than assumed in thermal-hydraulic calculations for
MCPR (Minimum Critical Power Ratio) and
LHGR (Linear Heat Generation Rate). Thickness measurements of liftoff on ATRIUM-10 fuel at LaSalle are as high as 93 micrometers whereas the maximum value observed for other facilities is less than 60 micrometers. The greater than expected liftoff present on LaSalle fuel has been attributed to the operation with zinc levels that exceeded
EPRI guidelines for reactor water chemistry. AREVA thermal-hydraulic analyses for LaSalle did not account for the higher liftoff.
Following discovery of the deviation, AREVA supplied an operability assessment to Exelon that identified limitations on the operating limit MCPR and rod average exposure. Revised operating limits are being provided to Exelon based on liftoff values that are lower than used in the original operability assessments. These limits are scheduled to be provided by July 31, 2009. Actions have been identified where AREVA will obtain information from the licensees on the anticipated water chemistry environment to be implemented for the next operating cycle, in advance of AREVA core design efforts supporting that cycle. This will assure that AREVA can adequately project the impact of this chemistry environment on liftoff, and to account for it during the design activities.
AREVA states that this defect affects only LaSalle Units 1 and 2. Data from other plants currently operating with fuel licensed using AREVA methods indicate that the liftoff is low and within historical levels.