Overview of low temperature sensitization
Lawrence Livermore National Laboratory (LLNL) is responsible for high-level nuclear waste package development as part of the Neveia Nuclear Waste Storage Investigations (NNWSI) Project. This project is part of tne Department of Energy's Civilian Radioactive Waste Management (CRWM) Program, and is investigating the suitability of tuffaceous rocks at Yucca Mountain, Nevada Test Site for high-level radioactive waste disposal. The waste package effort at LLNL is developing multibarriered packages
... arriered packages for safe, permanent disposal in a repository such as the one being considered at Yucca Mountain. The physical, mechanical, and chemical stability of a metal barrier to survive the 300 -1000 year containment objective is the paramount technical issue in selecting a suitable container material for geological disposal of high-level nuclear waste. Austenitic stainless steels serve ai the reference container materials in the conceptual design for nuclear waste packages for a contemplated geological repository in tuff located in Yucca Mountain at the Nevada Test Site. The corrosion resistance of candidate container materials in the anticipated repository environment is the focus of an experimental program to establish a data base on which the final arterial selection will be made and from which models to project the long-range corrosion performance will be developed. One major proQlem in use of austenitic stainless steels is susceptibility to developing a sensitized microstructure when exposed to relatively high process temperatures for short periods of time. Chromium-rich carbide phases precipitate largely in the grain boundary region and impoverish the local area of chromium. The resulting chromium-depleted area is then more susceptible to localized attack, because the steel in this local area does not contain sufficient chromium to maintain a stable, protective, passive film. The low carbon grades of stain'ess steel (such as 304L) were developed to resist sensitization by tolerating a much longer time at a given temperature before carbide formation occurs. A particular concern in geological disposal of nuclear waste packages is development of a sensitized microstructure over the long containment period (100's of years) at modest temperatures (100-300%) which are produced in the container by decay of fission products in nuclear waste. Dr. Michael Fox, an independent consultant, was retained to compose the attached report to assess the possibility of the occurrence of a sensitized microstructure in 304L stainless steel containers. As sensitization effects may accumulate from previous high temperature processes, Dr. Fox was asked to consider the influence of fabrication and welding on the possible subsequent development of sensitization during geological storage at lower temperatures. Additional potential sources of sensitization are the casting of vitrified reprocessed waste forms in stainless steel canisters (Defense and Commercial High Level Waste -DHLW and CHLW). During his previous employment at General Electric and at the Electric Power Research Institute (EPRI), Dr. Fox published papers on low temperature sensitization, particularly as the phenomenon affects the stability of Type 304 stainless steel in the Boiling Water fteactor (BrJR) coolant environment. Type 304 stainless steel is used for piping carrying high-temperature, pressurized water (ca. 290°C) and steam in the BUR. Sensitization effects in the heat-affected-zones around the welds in the piping have led to intergranular stress corrosion cracking (IGSCC) problems which mandated shut-downs for inspection of crack development. This has been a costly problem for the utilities owning BWRs and much work has been sponsored by EPRI in this country and by similar organizations in other countries. Much of the work has centered on understanding different aspects of the sensitization phenomenon and on developing remedial measures. Dr. Fox's access to this information -much of which is not yet published in the open literature -was most helpful. A good deal of the EPRI-sponsored work concerns alternative materials to 304 stainless steel for replacement of the piping in some existing BWRs and for construction of new generation BWRs. These alternative material! include -2 -2. R.