Primary Side Crud Sampling and Characterization: How It May Help to Better Maintain CANDU Reactors

Abstract

The paper reports a corrosion-product transport study in the primary-side of CANDU reactors. The study is based on systematic crud sampling examinations at Darlington NOS. Emphasis is placed on the corrosion-product transport and oxidation state as a monitor of primary water chemistry state, and as a monitor of system corrosion effects. The D2O was sampled at reactor outlet headers and the deposits collected on 0.45 um membrane filters were analyzed by using y-ray spectroscopy, X-ray fluorescence and Mossbauer effect techniques. In the beginning of this study in 1994 and 1995, the steady state crud concentration used to be as high as 20 to 30 ug/kg D2O, and the crud contained large fractions of highly oxidized species (40 to 60% Fe), in the form offerrihydrite, lepidocrocite, goethite and hematite. During startup operations, the crud concentrations were as high as 8 mg/kg D2O, and contained 70 to 80% Fe in the form of oxidized iron species. However, as a result of various improvements, and especially because of better oxygen control during lay-ups (nitrogen blanketing) and better pH control (lower and more stable values), in recent steady-state runs, the crud concentrations were found to be lower than 3 to 5 ug/kg D20. At present, the steady-state crud consists mostly of Fe, largely in the form of magnetite (60 to 80% Fe), and hematite (20 to 40% Fe). The steady-state concentration of crud was found to increase markedly with pH. in the range from 10.3 to 10.7. This seems to be in accord with the trend predicted by flow-assisted solubility of magnetite in lithiated water at -300 degrees C, and is generally in accord with the model of dissolution, transport, and redeposition of iron in the primary heat transport system of CANDU reactors. The crud contains also significant amounts of Zr (-5 to 30 wt.% Fe), apparently in the form of zirconium oxide. Zirconium oxide particles may originate largely from the wear of fuel bundle pads and pressure tube fretting; which should be minimized.