Degradation mechanisms in overpack concrete of spent nuclear fuel dry storage systems: A review

Dry storage systems are successfully used for interim storage of commercial spent nuclear fuel (SNF). Many of these systems consist of sealed, welded canisters placed inside concrete storage modules. Associated with the need for extending the operational periods of some of these systems, there are concerns that their concrete overpacks could degrade over time. This could lead to a loss of strength and durability. Potential degradation mechanisms include chemical attacks, concrete cancer, or freeze-thaw (F-T) cycles, making them prone to failure during extended periods of dry storage, especially when stressed during a hazardous event, such as an earthquake. The concrete mixtures that have been used in dry storage systems have not changed significantly since their introduction, and available mixtures often have the same ingredients used in conventional concrete. With the latest evolutions in material science, new concrete composites with ultra-high strength and outstanding thermal and mechanical properties can be utilized in such structures. Within a collaboration between the University of Idaho and the Idaho National Laboratory, enabled by funding provided by the Center of Advanced Energy Systems (CAES), a thorough review of SNF dry storage system overpack degradation observations were conducted. Further, the latest technologies in concrete composite materials with extraordinary strength, outstanding durability were identified. This paper presents and summarizes the most relevant mechanism of concrete deterioration in SNF systems, and identifies new materials to be used in future concrete mixtures to increase the service life of SNF dry storage systems and to lengthen inspection intervals.