Fiber Optic Sensing for Surveillance of In-ground Long-term Storage (FOSSILS)

Long-term surveillance of materials in deep geological repositories is often considered limited to sealing and monitoring entrances and exits once they are closed because alternatives would need to operate for tens or hundreds of years without access for maintenance. A collaborative research project by Oak Ridge National Laboratory and Sandia National Laboratories is working to develop and demonstrate fiber-optic sensing technologies that wouldn’t require underground access after installation and could detect disturbance, diversion, and leakage of underground nuclear materials. This system is designed to use signal processing equipment only at the surface outside the repository and alert users to detected events at individual packages through distributed acoustic sensing (DAS) and radiation detection elements. DAS is a commercially available technology that uses 10+ km of optical fiber as a chain of back-to-back vibration and thermal sensors spaced a few meters apart, enabling awareness of thousands of locations throughout a repository. This research project is focusing on developing smart algorithms to distinguish localized package-level disturbances from other vibration sources in the environment, such as natural seismic activity or distant excavations. The goal is that this potential safeguards measure will provide users with a relevant contextual summary of detected activities. Radiation detection research for this project at Sandia has focused on transmitting the scintillation light from ? particle detectors through long optical fibers as a passive underground radiation sensor. The ultraviolet light generated by typical radiation detection scintillators is severely attenuated by optical fibers greater than 1 km in length; however, red or infrared light can be transmitted over much greater lengths. To demonstrate and test an integrated system, we are constructing a small scale testbed at Oak Ridge National Laboratory with simulated used fuel canisters to assess the sensing performance of DAS and passive ? particle detectors modes. This testbed contains more than 70 surrogate used fuel canisters at 1:40 scale and will be used to evaluate algorithm performance in localizing disturbances, as well as the integration with radiation-sensing elements in a custom designed package attachment mechanism.