Detecting Water in Dry Storage Canisters for Used Fuel


As the United States transitions to storing used fuel from commercial nuclear reactors in interim dry storage facilities for extended periods, utilities must be prepared to technically justify the viability of extended storage to the regulating authority. This requires an analysis of credible aging degradation mechanisms that could contribute to a release of radiological material into the environment. In many cases, the technical data needed to resolve the significance of potential aging degradation mechanisms my not yet exist. Thus, one of the key objectives of the Used Fuel Disposition Campaign (UFDC) within the Fuel Cycle Research and Development (FCR&D) program is to acquire the technical data necessary to support eventual licensing for extended periods. This work focuses specifically on methods for detecting and measuring water inside of dry cask storage systems (DCSSs) via sensors mounted exterior to the confinement boundary and requiring no physical penetration of the confinement boundary. Ideally, the environment inside of a DCSS confinement is inert and free of water to prevent potential corrosion of used fuel cladding or other internal hardware. However, there is some uncertainty about the amount of residual water potentially left behind in a DCSS as a result of drying processes. Considering the complex spatial and time dependent temperature profiles in dry storage casks, water may be in liquid or gas phase depending on where it is located in the cask and how long the cask has been in storage. A review of drying specifications by several vendors concludes that if the specifications are followed correctly, the residual moisture left behind in DCSSs should present an insignificant risk to cladding degradation. A more recent analysis has concluded that much larger quantities of residual water could remain in DCSSs, but the amount would still not be expected to lead to significant corrosion of fuel cladding or other internal components. However, assumptions about the possible quantities of residual water or their potential significance have not yet been corroborated with field experience for periods of extended storage. The measurement techniques described here can facilitate the direct observation of residual water in the field and, thus, help establish operational data that can inform operating and licensing decisions for extended periods of storage. The proposed measurement techniques are based on the application of ultrasonic signals propagated through the confinement boundary with sensors mounted on the exterior of the confinement boundary. This paper will consider detection of liquid water inside of vertically and horizontally oriented DCSSs and the detection of gas-phase water.

International High-Level Radioactive Waste Management