Ginna Station steam generator U-bend tube analysis for chemical cleaning data

This report discusses a potential cause of the decrease in steam pressure affecting many nuclear plant recirculating steam generators which is the buildup of deposit on the outside surfaces of the tubing. This deposit layer, if thick enough, acts to ''insulate'' the tube, resulting in decreased heat transfer from the reactor coolant to the secondary fluid. At some point in time, fouling may thus degrade thermal performance to a level that warrants chemical cleaning of the steam generators. To help in evaluating the extent and effect of deposit buildup in Rochester Gas ampersand Electric's (RGE) Ginna Plant steam generators, sections of one outer row Unbend tube were removed. The tube specimens were examined and tested to: (1) determine the deposit morphology (density, thickness, composition); (2) determine the effect of the deposit on the overall heat transfer coefficients; and (3) obtain the basic solubility and dissolution data needed to chemically clean the tube sections. Results of the laboratory examination and test program indicate that the deposit in the U-bend area is a duplex scale approximately 8--9 mils thick. The inner layer (∼2 mils thick) is relatively non-porous and is composed primarily of calcium, phosphorous, and oxygen resulting from the plant's initial use of a phosphate based water treatment program. The outer layer is primarily magnetite with copper particles dispersed throughout. Bench scale tests confirmed that the solubility of the deposit in the EPRI/SGOG solvents is high (∼98 %) with excellent dissolution kinetics. Heat transfer tests performed at prototypical steam generator pressures and temperatures and at nucleate boiling conditions indicate that the overall heat transfer coefficient of the tube specimens increased by ∼34 % when the deposit was removed with the EPRI/SGOG chemical cleaning process