U.S. experience with hydrazine and flow-accelerated corrosion

Flow-accelerated corrosion (FAC) has been studied intensely for the last 30 years. In most regards, the parametric behavior of FAC has been well understood since the eighties. However, recent experience showing the impact of hydrazine has challenged this understanding. Until the early 1990's, hydrazine levels at most U.S. PWRs had been low, typically around 20 ppb. Since the mid 1990's the U.S. trend has been to increase hydrazine concentration (> 100 ppb) to provide further protection the steam generators tubes from stress corrosion cracking. In the last five years, high concentrations of hydrazine have been shown in the laboratory to affect the rate of FAC. These experiments have demonstrated this effect although there remains considerable scatter in the data supporting this conclusion. It is believed that the impact of hydrazine is due to the change in the oxidizing-reducing potential of the solution, but there is no generally accepted theory to completely describe its behavior. Recently, plant experience has begun to support the laboratory data that high hydrazine does increase the rate of FAC. It should be noted that there are several factors affecting the rate of FAC. Since most plants have changed their water chemistry in recent years, it has been difficult to ''back-out'' the influence of hydrazine. But now, plant experience has clearly shown the impact of high hydrazine especially in the high temperature portions of the feedwater piping. This has been seen at two PWRs, discussed below, and perhaps at several others. This accelerated thinning of the feedwater piping is especially significant since feedwater piping is particularly expensive to inspect and to replace. In addition to the experience with piping, there has also been degradation reported in steam generators, particularly at Gravelines in France, possibly linked to high hydrazine. In order to deal with this issue, the EPRI computer program CHECWORKS was modified in 1998 to determine the local hydrazine concentration in the liquid phase throughout the secondary system. Also, a model to predict the impact of hydrazine on FAC was developed and included in the CHECWORKS corrosion rate formulation. These are discussed below as is future potential changes to the FAC model in CHECWORKS to reflect the most recent laboratory data and plant experience. (authors)