Managing Aging Effects on Dry Cask Storage Systems for Extended Long-Term Storage and Transporation of Used Fuel Rev. 1

Because there is currently no designated disposal site for used nuclear fuel in the United States, the nation faces the prospect of extended long‐term storage (i.e., >60 years) and deferred transportation of used fuel at operating and decommissioned nuclear power plant sites. Under U.S. federal regulations contained in Title 10 of the Code of Federal Regulations (CFR) 72.42, the initial license term for an Independent Spent Fuel Storage Installation (ISFSI) must not exceed 40 years from the date of issuance. Licenses may be renewed by the U.S. Nuclear Regulatory Commission (NRC) at the expiration of the license term upon application by the licensee for a period not to exceed 40 years. Applications for ISFSI license renewals must include the following:

1. Time‐limited aging analyses (TLAAs) that demonstrate that structures, systems, and components (SSCs) important to safety will continue to perform their intended function for the requested period of extended operation, and

2. A description of the aging management program (AMP) for management of issues associated with aging that could adversely affect SSCs important to safety.

In addition, the application must include design basis information as documented in the most recently updated final safety analysis report, as required by 10 CFR 72.70. Information contained in previous applications, statements, or reports filed with the Commission under the license may be incorporated by reference, provided that these references are clear and specific.

The NRC has recently issued the “Standard Review Plan for Renewal of Spent Fuel Dry Cask Storage System Licenses and Certificates of Compliance,” NUREG‐1927, under which NRC may renew a specific license or a Certificate of Compliance (CoC) for a term not to exceed 40 years. Both the license and the CoC renewal applications must contain revised technical requirements and operating conditions (fuel storage, surveillance and maintenance, and other requirements) for the ISFSI and dry cask storage system (DCSS) that address aging effects that could affect the safe storage of the used fuel. The information contained in the license and CoC renewal applications will require NRC review to verify that the aging effects on the SSCs in DCSSs/ISFSIs are adequately managed for the period of extended operation. To date, all of the ISFSIs across the United States with more than
1,500 dry casks loaded with used fuel have initial license terms of 20 years; three ISFSIs (Surry, H.B. Robinson, and Oconee) have received their renewed licenses for 40 years, and two other ISFSIs (Calvert Cliffs and Prairie Island) have applied for license renewal for 40 years.

This report examines issues related to managing aging effects on the SSCs in DCSSs/ISFSIs for extended long‐term storage and transportation of used fuels, following an approach similar to that of the “Generic Aging Lessons Learned” report, NUREG‐1801, for the aging management and license renewal of nuclear power plants. The report contains five chapters and an appendix on quality assurance for AMPs for used‐fuel dry cask storage systems.

• Chapter I of the report provides an overview of the ISFSI license renewal process based on 10 CFR 72 and the guidance provided in NUREG‐1927.

• Chapter II contains definitions and terms for structures and components in DCSSs, materials, environments, aging effects, and aging mechanisms.

• Chapter III and Chapter IV contain the TLAAs and AMPs, respectively, that have been developed for managing aging effects on the SSCs important to safety in the DCSS designs described in Chapter V.
• Chapter V contains summary descriptions and tabulations of evaluations of AMPs and TLAAs for the SSCs that are important to safety in the DCSS designs (i.e., NUHOMS; HI‐ STORM/HI‐STAR 100; Transnuclear, Inc. TN metal cask; NAC International S/T storage cask; Ventilated Storage Cask [VSC‐24]; Westinghouse MC‐10 metal dry storage cask;
CASTOR V/21 and X/33 dry storage casks; and W150 FuelSolutions storage system) that have been and continue to be used by utilities across the country for the dry storage of used fuel.

The goal of this report is to help establish the technical basis for extended long‐term storage and transportation of used fuel. Future efforts should include development of additional AMPs and TLAAs that may be deemed necessary, and further evaluation of the adequacy of the generic AMPs and TLAAs that may need augmentation. Industry and site‐specific operating experience from the various DCSSs/ISFSIs located across the country should be periodically examined to (a) ascertain the potential aging effects on the SSCs in the DCSSs, thereby enabling a compilation of existing aging management activities, and (b) assess these activities’ adequacy for extended long‐term storage and transportation of used fuel.

It should be noted that managing aging effects on DCSSs for extended long‐term storage and transportation of used fuel “begins” when the used‐fuel assemblies are loaded into a canister (or cask) under water in the spent‐fuel pool. The canister (or cask) containing the used‐fuel assemblies is then drained, vacuum dried, and back‐filled with helium before the lid is closed, either by welding or by bolted closure. The bolted cask as well as the welded canister (after being placed inside a transfer cask) are moved to an outdoor concrete pad of an ISFSI, where it would stay for 20 or
40 years of the initial license term (and up to another 40 years for a renewal license term), according to 10 CFR 72.42. More than 1,700 dry casks have begun long‐term storage under the initial license terms; some of them have been in storage for over 20 years and are already in the renewed license term for up to 40 years.

Transferring from pool to pad or from wet to dry storage is an abrupt change of environment for the used‐fuel assemblies, and the effects are most pronounced during vacuum drying, especially for high‐burnup fuel, because of the likelihood of cladding radial hydride formation and embrittlement. The likelihood of this phenomenon will diminish only after the cladding temperature has dropped below 200oC because of the decrease of fission‐product decay heat during prolonged cooling, which may occur 20–25 years after the high‐burnup used‐fuel assemblies are placed in dry storage. Preventing and/or minimizing cladding embrittlement by radial hydrides during drying, transfer, and early stages of storage will maintain the configuration of the used fuel in the dry canister (or cask) and ensure retrievability of the used fuel and its transportability after extended long‐term storage.

Management of aging effects on DCSSs for “extended” long‐term storage of used fuel is no different from that required during the “initial” license term. If aging effects on the SSCs important to safety in the DCSS/ISFSI are not adequately managed for the initial license term of storage, an application for a renewal of the license for extended long‐term storage is unlikely to be granted by the regulatory authority. Therefore, the same principles and guidance developed by the NRC in NUREG‐
1927 should be applicable to extended long‐term storage, as the period of operation, or term, reaches 20, 40, 60, 80, or >120 years. The term in the initial or renewal license is important and

indicates a finite period of operation and, although not mentioned specifically in the current regulations, does not rule out license renewal for multiple terms, as long as aging effects are adequately managed.

Managing aging effects on DCSSs for extended long‐term storage and transportation of used fuel requires knowledge and understanding of the various aging degradation mechanisms for the materials of the SSCs and their environmental exposure conditions for the intended period of operation. The operating experience involving the AMPs, including the past corrective actions resulting in program enhancements or additional programs, should provide objective evidence to support a determination that the effects of aging will be adequately managed so that the intended functions of the SSCs will be maintained during the period of extended operation. Compared to nuclear power plants, the operating experience of the DCSSs and ISFSIs is not as extensive; however, evaluations have been performed on the NRC’s requests for additional information (RAIs) on applications for renewal of licenses for ISFSIs and DCSSs, as well as the applicant’s responses to the RAIs, to assess their relevance to the TLAAs and AMPs described in Chapter III and Chapter IV of this report, respectively. Those found relevant have been incorporated into the AMPs and TLAAs.

Managing aging effects on DCSSs for extended long‐term storage and transportation of used fuel depends on AMPs to prevent, mitigate, and detect aging effects on the SSCs early, by means of condition and/or performance monitoring. Detection of aging effects should occur before there is a loss of any structure’s or component’s intended function. Among the important aspects of detection are method or technique (i.e., visual, volumetric, or surface inspection), frequency, sample size, data collection, and timing of new/one‐time inspections to ensure timely detection of aging effects. The challenges in the detection of aging effects will always be the areas that are inaccessible for inspection and monitoring and the frequency of inspection and monitoring (i.e., periodic versus continuous).