Dry storage systems (DSSs) store spent nuclear fuel at many operating and decommissioned power reactor sites in the United States. Overpacks and support pads include various structural subcomponents that are constructed of concrete and reinforcing steel. These concrete components are commonly exposed to outdoor air and groundwater or soil environments in which the materials may be susceptible to degradation. Potential environmental, thermal, mechanical, and irradiation-induced aging mechanisms include freeze and thaw, creep, reaction with aggregates, aggressive chemical attack, corrosion of reinforcing steel, differential settlement, shrinkage, leaching of calcium hydroxide, radiation damage, fatigue, dehydration at high temperature, microbiological degradation, delayed ettringite formation, and salt scaling. This paper presents an assessment of degradation mechanisms based on reviews of literature and operating experience from nuclear and nonnuclear applications, considering the long-term effects of these mechanisms on the integrity of DSSs.
The results of this study indicate that the following mechanisms could cause degradation of concrete subcomponents in DSSs during a 60-year timeframe: (i) freeze and thaw, (ii) reaction with aggregates, (iii) aggressive chemical attack, (iv) corrosion of reinforcing steel, (v) differential settlement, (vi) leaching of calcium hydroxide, (vii) microbiological degradation, and (viii) salt scaling. The results of this work are being used to inform recommendations for monitoring, inspection, and other preventive or mitigative activities to manage the aging of DSSs.
Key words: concrete, reinforcing steel, outdoor air, groundwater, soil