ABSTRACT

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.

INTRODUCTION

At operating and decommissioned reactors in the United States, spent nuclear fuel (SNF) is stored in sealed dry storage systems (DSSs). The majority of DSS designs are canister-based systems in which assemblies are loaded into a welded canister that is placed in a storage overpack or module such as the TN NUHOMS System®.1 The overpack or module typically consists of reinforced concrete and steel, which provide structural support and radiation shielding.

Specific licenses and Certificates of Compliance (CoCs) for storage systems have been issued by the U.S. Nuclear Regulatory Commission(1) (NRC) for an initial term of 20 years. The license or CoC may be renewed for additional terms of up to 40 years in accordance with 10 CFR 72.42 or 72.240, respectively. NRC is evaluating aging-related mechanisms of DSSs to ensure that structures, systems, and components (SSCs) important to safety continue to function as intended for the period of the initial term and extended operation. Overpacks and support pads include various structural subcomponents that are constructed of concrete and reinforcing steel. These subcomponents are commonly exposed to outdoor air and groundwater or soil environments. Here, outdoor air refers to direct exposure to weather, including precipitation and wind; possibly salt laden. Below-grade concrete structures are assumed to be partially exposed to a groundwater or soil environment. The environment may also include (i) elevated temperatures due to heat released by the SNF and (ii) radiation, with dose rates depending on the SNF characteristics (e.g., burnup and age of fuel), exposure time, and location of the subcomponent. Potential aging mechanisms were identified from reviews of gap assessments of DSSs, relevant technical literature, American Concrete Institute (ACI) guides and reports, and operating experience from nuclear and nonnuclear applications.2-5 Additional mechanisms were identified during a relatively recent NRC concrete expert panel workshop.6 The potential environmental, thermal, mechanical, and irradiation-induced aging mechanisms important to safety of SSCs are considered to be 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.

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