Occurrence of fluorides in the secondary circuit of all units in the Dukovany NPP

The concentration of fluorides in the feedwater was below a detection limit (i.e. below 0,1 μg/l) before replacement of brass condensers. After start-up of the unit the concentration of fluorides in feedwater reached value by cca 3 μg/l. The increase from the value < 5 μg/l to 10 - 15 μg/l has been identified in the SG blowdown water. During shutdown of unit the values of about 150 μg/l were measured. One electrode (cca 20 g of fluxing agent) contains about 4 mg of fluorides based on the secondary circuit extent and on the measured concentration it was estimated that about 1000 of electrodes have been applied for one unit. Binding of fluorides within slag can explain the slow and long-term leach of fluorides to the secondary circuit coolant. (R.P.)