Guided Wave Phased Array for Spent Fuel Dry Storage Canister Evaluation

Problem Statement: There is currently no long-term storage solution for the final disposition of the more than 80,000 metric tons of spent nuclear fuel (SNF) in the United States, which is predominately stored in in dry cask storage systems (DCSSs) comprised of welded 304/316-stainless steel canisters placed in concrete/metal overpacks. Initially, these DCSSs were licensed to be stored on-site at nuclear power plants for a period of 20 years with a 20-year license renewal option However, many of these DCSSs are already 20-40 years old and it is now anticipated that they may need to remain in service for another 40 to 60 years. Due to this long-term service requirement, nondestructive evaluation (NDE) of these canisters is imperative to ensure their integrity during storage and to confirm that they are safe to transport for to a final repository at some point in the future; the primary failure mechanism or concern is chloride-induced stress corrosion cracking (SCC). If cracks were to develop during storage or propagate during transport, a potential pathway would exist for gaseous materials in the canister to escape. In response to this long-term concern, the DOE Office of Nuclear Energy, Spent Fuel and Waste Science and Technology program has identified external NDE monitoring of DCSS components, along with developing a better understanding of the SCC mechanisms and risks associated with these structures, as one of the highest-priority areas for R&D. Proposed Solution: It is proposed to utilize an ROV-deployed, dry-coupled, large-area guided wave scanning system to rapidly image dry storage canisters for SCC and other damage mechanisms using one or two line scans along the height of the canister. Two advanced GW scanning methods will be evaluated as part of the Phase I effort; both of these technologies are relatively mature and have been successfully deployed on robotic platforms for other applications. The first of these technologies is a patented guided wave phased array (GWPA) scanning technology for large-area plate inspection, and the second is a patented dry-coupled magnetostrictive scanning technology (DCMS). Both techniques can provide detailed inspection images of large-area structures from a very limited set of scans by leveraging the long-range detection capabilities of guided wave testing with advanced sensor design and innovative signal processing; they also both utilize the same ultrasonic guided wave SH0 that was successfully used for SCC detection during earlier DOE-funded R&D efforts. The primary difference is the implementation of this guided interrogation with innovative piezoelectric (in the case of GWPA) and magnetostrictive (in the case of DCMS) sensing systems as opposed to the lower-power EMATs used in previous efforts. The technologies can both be deployed using the same robotics platform, as this has already been accomplished for other applications. Phase I Objectives: The Phase I effort will be focused on demonstrating the feasibility of utilizing robotically-deployed, dry-coupled GWPA and DCMS sensors for efficient and reliable inspection of spent fuel dry storage canisters. Guidedwave will demonstrate the sensitivity of the GWPA and DCMS technologies to critical storage canister SCC flaws, which will be accomplished by a combination of experimental and numerical modeling. The Phase I final system demonstration will show that the probe can be robotically-deployed in a representative DCSS mock-up. Based on the preliminary results in Phase I, one or both scanning technologies will be moved into the next phase of technology maturation in Phase II and Phase III. Commercial Applications & Other Benefits: The temporary dry storage of SNF for the next several decades, and the eventual transportation that will be required for long-term disposition of this fuel, require high confidence in the structural integrity of the stainless steel dry storage canisters that house this material. The environmental, health, and financial risks associated with a potential breach or structural failure of the DCSS canisters are unacceptable. The currently-accepted method of inspection (visual testing) may be insufficient. However, the proposed guided wave technology would provide an economical, efficient solution for screening horizontal and vertical DCSS canisters for SCC. Beyond the DCSS application, both the GWPA and DCMS technologies have already shown substantial public benefits. In particular, the GWPA technology lies at the core of the critical Hanford DST inspection plan.