PNNL

Abstract

Safe storage of spent nuclear fuel (SNF) in dry cask storage systems (DCSSs) is critical to the nuclear fuel cycle and the future of nuclear energy. A crucial component of DCSSs is the SNF canister, which is a sealed stainless structure vacuumed and backfilled with helium. The structural deterioration within a canister can be monitored through its internal gas properties. This serves as the driving force behind the proposed non-invasive ultrasonic sensing approach in this paper. A major challenge in collecting gas-borne signals is the impedance mismatch between the stainless-steel canister and the helium gas inside. Only a small fraction of the ultrasonic signal can be transmitted to the gas medium through the outside surface of the canister. In this paper, experimental studies on a full-scale canister mock-up were carried out to capture the gas-borne signals. Damping materials were pasted on the outside and blocking and unblocking tests were conducted to identify the gas-borne signal. The research results showed that the excitation frequency played an important role in maximizing the gas-borne signals. The gas-borne signal was successfully detected at around the theoretical time-of-flight (TOF) at 225 kHz. A high signal-to-noise ratio (SNR) was achieved in the measurements. Next, the acoustic impedance matching (AIM) layers were added, and the gas signal energy was improved by 160.4% compared with no AIM layers. Subsequently, the relative humidity (RH) level and temperature of the gas were varied to simulate abnormal internal conditions inside of the canister. The non-invasive testing system demonstrated both reliability and sensitivity in detecting gas temperature and RH variations. Theoretical calculations demonstrated the potential for detecting low-level xenon and air within an actual SNF canister filled with helium. Last, an active noise cancellation (ANC) method was verified on the canister mock-up for the first time. The results showed that the SNR of the gas signal was improved by 213.6% compared with no active noise cancellation.