Termination of the Yucca Mountain repository program implies that U.S. spent nuclear fuel (SNF) and high-level radioactive waste may be stored for very long periods, i.e., more than 100 years. Many of the storage systems for commercial SNF are large, cylindrical, welded, stainless-steel canisters containing 24-80 SNF assemblies in an inert gas (helium) environment. Each canister is contained in a massive reinforced concrete structure for shielding, safety, and security reasons. Airflow in the annular space between the canister and the concrete structure removes SNF decay heat by natural convection.
Undetected defects in the canister-lid welds and stress corrosion cracking (SCC) in the canister-lid weld area may combine to form pathways for slow escape of the inert fill gas from the canister. SCC may not occur until many decades after SNF is loaded into the canister because SNF decay heat initially would keep the canister surface temperatures above the temperature where liquid water, a prerequisite for SCC, could condense. The period required for inert gas to escape from the canister once pathways have formed and for air subsequently to enter the canister may also be measured in decades, depending on the size and shape of the pathways.
In this paper, the authors explore the limits of weld-flaw detectability; when necessary conditions for SCC could occur in the canister-lid weld area; the period for loss of inert fill gas if through-weld cracks occur; and the possible consequences of fill gas loss. The authors also develop a list of research and study needs to address the possibility of SCC resulting in the loss of fill gas.