Prediction of External Corrosion for Steel Cylinders 2001 Report

The United States Department of Energy (DOE) currently manages depleted uranium hexafluoride that is stored in approximately 50,000 carbon steel cylinders located at three DOE sites. Using either a hand-held ultrasonic transducer or an automated scanner, wall thickness and corrosion pit depth data have been collected for several subpopulations of cylinders. In this report, the most recently collected wall thickness data were used, along with previously collected data, to make projections about the condition of the cylinders located at ETTP, PGDP, and PORTS. The results presented here are intended to supercede and enlarge the scope of those presented previously (Lyon 1995,1996, 1997, 1998, 2000). In particular, projections are made for thin-walled cylinders (nominal initial thickness 312.5 mils), thick-walled cylinders (nominal initial thickness 625 mils), and model30A cylinders (nominal initial thickness 500 mils). In addition, an analysis is conducted for the minimum thickness at the head/skirt interface for skirted cylinders. The most recently collected data, which were not available for the previous report (Lyon 2000), consisted of evaluations of the wall loss for additional cylinders. In some cases the more recent data generally suggest a more ''optimistic'' view of the current minimum point wall thickness for the cylinder populations, although in some cases there is a wider spread (i.e., variance) in the newer data that can impact the predictions for rare events such as breaches. In some cases (e.g., Paducah G-yard bottom cylinders), this is due to relatively recent changes in cylinder storage conditions. However, some of the differences are also due to differences between the manual UT and PSCAN measurement methods, differences for which further investigation is probably warranted. On a percentage basis, most of the thin-walled cylinders predicted to have a minimum point thickness less than 250 mils in 2001 are located in the bottom rows of C-745-G yard at PGDP and K-1066-K yard at ETTP. Some of these cylinders are also expected to fall below 62.5 mils and some all the way to 0 mils (breach). Cylinders in K-1066-K yards population, all PGDP populations (including 30As), and Portsmouth thin skirted bottom populations are all predicted to have one or more cylinders in the breach range. These predictions almost surely reflect the conservative nature of the models on which they are based. Note that both the variance and location estimates in the fitted models affect the predictions for the thickness extremes.