Secondary system chemistry control and sludge management

Since 1975 All Volatile Treatment (AVT) has been the preferred method for controlling the secondary system operating environment in Westinghouse PWR plants. However, since AVT provides no buffering action against corrodent species present in the water as trace contaminants, utilities have initiated programs which combine control of contaminant ingress with total steam generator sludge management. The earliest applications of boric acid, to control denting type corrosion, began in 1978 and have continued up to the present time. Boric Acid has also been added as an inhibitor for SCC/IGA type corrosion since 1985. There are now approximately 30 plants operating with boric acid or boric acid/morpholine chemistry in the secondary system, and a growing number of plants where morpholine is being used as an additive to maintain pH and reduce iron transport attributable to erosion/corrosion. The impact of these modifications in the operating chemistry environment are examined for their potential effect on secondary system corrosion and sludge control. In addition to these changes in chemistry control, advances in sludge lancing, together with bundle and crevice flushing, has helped utilities to be more successful in removing sludge. However, other techniques are available and compliment the normal sludge removal methods. Pressure pulse cleaning has been used as a mechanical method for removing sludge from the secondary side of steam generators, including the tube bundle and support plates. Field data indicate that up to 100 % more sludge can be removed using this technique, and areas not readily accessible using other techniques are also cleaned. Chemical cleaning is another part of the removal program. However, plant specific qualification testing needs to be completed and detailed process implementation and control procedures developed before a chemical cleaning campaign is initiated. (author)