Over 2,000 canisters are being used for dry storage of commercial spent nuclear fuel (SNF) in the United States, and each year, between 150–200 new canisters are loaded. There is evidence that these welded stainless steel canisters are susceptible to stress corrosion cracking (SCC) under certain conditions (1). Undetected crack development may result in a loss of confinement. SCC is very difficult to predict with crack opening displacements of 15–30 ?m, which are much smaller than what can be detected with current visual inspections (~100 ?m) (2, 3). The lack of initial cracks does not preclude formation of cracks in the future. This observation is particularly critical for SCC, which is characterized by a long incubation period, after which crack initiation and growth evolution is depth dependent. If crack growth is rapid, SCC may not be detected in time to prevent the loss of canister confinement without frequent nondestructive examination (NDE) inspections (4).
Proposed NDE techniques include periodic inspections using eddy currents, bulk ultrasonic waves, guided and surface waves, as well as continuous noncontact monitoring methods such as passive acoustic emission. To develop monitoring systems for SCC, sensor requirements must be carefully considered and evaluated with respect to radiation resistance, size, power consumption, defect sensitivity, axial and lateral resolution, signal-to-noise ratio, and scanning time. This paper examines monitoring requirements, and a variety of sensor types are considered and compared against these metrics. This work focuses on detection and characterization of SCC in welded stainless steel canisters placed within concrete overpacks. Potential compromises, advantages, disadvantages, and compatibility with other state-of-the-art and complementary monitoring techniques such as thermographic phosphors are discussed.