Expanded Materials Degradation Assessment (EMDA): Aging of Concrete andCivil Structures (NUREG/CR-7153, Volume 4)
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Manuscript Completed: October 2013
Date Published: October 2014
Prepared by Expert Panel:
Herman Graves, U.S. Nuclear Regulatory Commission;
Yann Le Pape, Electricite de France and Oak Ridge
National Laboratory; Dan Naus, Oak Ridge National
Laboratory; Joseph Rashid, Anatech; Victor Saouma,
University of Colorado-Boulder; Abdul Sheikh,
U.S. Nuclear Regulatory Commission; James Wall,
Electric Power Research Institute
On behalf of:
Oak Ridge National Laboratory
Managed by UT-Battelle, LLC
J. T. Busby, DOE-NE LWRS EMDA Lead.
P. G. Oberson and C. E. Carpenter, NRC Project Managers
M. Srinivasan, NRC Technical Monitor
Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001
In NUREG/CR-6923, "Expert Panel Report on Proactive Materials Degradation Assessment," referred to as the PMDA report, NRC conducted a comprehensive evaluation of potential aging-related degradation modes for core internal components, as well as primary, secondary, and some tertiary piping systems, considering operation up to 40 years. This document has been a very valuable resource, supporting NRC staff evaluations of licensees' aging management programs and allowing for prioritization of research needs.
This report describes an expanded materials degradation assessment (EMDA), which significantly broadens the scope of the PMDA report. The analytical timeframe is expanded to 80 years to encompass a potential second 20-year license-renewal operating-period, beyond the initial 40-year licensing term and a first 20-year license renewal. Further, a broader range of structures, systems, and components (SSCs) was evaluated, including core internals, piping systems, the reactor pressure vessel (RPV), electrical cables, and concrete and civil structures. The EMDA uses the approach of the phenomena identification and ranking table (PIRT), wherein an expert panel is convened to rank potential degradation scenarios according to their judgment of susceptibility and current state of knowledge. The PIRT approach used in the PMDA and EMDA has provided the following benefits:
- Captured the status of current knowledge base and updated PMDA information,
- Identified gaps in knowledge for a SSC or material that need future research,
- Identified potential new forms of degradation, and
- Identified and prioritized research needs.
As part of the EMDA activity, four separate expert panels were assembled to assess four main component groups, each of which is the subject of a volume of this report.
- Core internals and piping systems (i.e., materials examined in the PMDA report) – Volume 2
- Reactor pressure vessel steels (RPV) – Volume 3
- Concrete civil structures – Volume 4
- Electrical power and instrumentation and control (I&C) cabling and insulation – Volume 5
This volume provides background information on nuclear power plant safety-related concrete structures, their materials of construction, and durability mechanisms and processes that could potentially effect the functional and performance requirements of these structures. This volume also summarizes the results of an expert-panel assessment of the aging and degradation of concrete materials and structures in nuclear power plants. The main objective of the work described herein was to evaluate concrete structures and components in nuclear power plants in which, based on specific operating environments, degradation is likely to occur, or may have occurred; to define relevant aging and degradation modes and mechanisms; and to perform systematic assessment of the effects of these aging-related degradation mechanisms on the future life of those materials and structures. This was accomplished by drawing on the knowledge and expertise of the above-cited expert panel. The approach utilized by the expert panel was based on the Phenomena Identification and Ranking Table (PIRT) process utilized in the original PMDA report in order to identify safety-relevant phenomena and assess their importance as well as identify and prioritize research needs. The objectives of this report are to determine the degradation mechanisms known for concrete materials and structures, specifically listing the current knowledge on aging degradation of the concrete materials and structures and the confidence level of this knowledge. For areas where there is a lack of knowledge, this report will evaluate the technical gaps in knowledge to identify potential research areas, and prioritize them using the PIRT process.
The PIRT identified a number of mechanisms and degradation modes that may affect the safety function of the concrete and civil structures and components. The highest-ranked mechanism is associated with the corrosion of the reinforcement of cooling towers. This high ranking is the result of significant evidence that the phenomenon will affect many towers beyond 60 years of operation. Though corrosion of reinforced and pre-tensioned concrete elements is well understood and documented, there remain significant knowledge gaps related to the evaluation of the actual state of degradation (inspection) and the evaluation of the structural integrity.
Three of the five high-ranked degradation modes potentially affect the concrete containment, which is the safety-related structure of primary interest.
- The first identified mode is the creep of the post-tensioned concrete containment. Creep is a long-term process associated with sustained loading and moisture transport that affects the internal stress state and, because it adds to tendon relaxation in causing gradual loss of prestress, which is usually restored by periodic re-tensioning thereby introducing a form of cyclic activation of primary creep, can potentially damage the concrete and lead to tertiary (creep-fracture interaction) under accidental loading.
- Related to the creep mode identified above is the interaction between creep and cracking in post-tensioned containments subjected to repair involving prestress modification during the operational life of the containment. While concrete racking is a well understood behavior characteristic of concrete structures in general, and is accounted for in the usual manner in the structural design of reinforced containments, it plays a unique role, (usually unaccounted for in design), in post-tensioned containments. Depending upon the position of the tendons relative to the surface of the containment wall, radially-oriented dilation damage, eventually leading to discrete split cracking, can form on a lamellar surface parallel to the wall surface, which evolves with time as a creep-cracking interaction mechanism. This mode of cracking can potentially occur during initial pre-stressing, during re-tensioning to repair loss of prestress due to concrete creep and tendon relaxation, or during de-tensioning and retensioning operations which may be undertaken as part of life extension re-construction work. This type of split cracking can be controlled by radial reinforcement, which generally is not part of the initial design, and because such cracking configuration is internal and is not visible on the surface, it can potentially evolve into an undetectable degradation mode.
- The second mode is the irradiation of concrete. This is due to a lack of sufficient test data to support a clear evaluation of the significance of such mechanism for long-term operations. As a reminder here, the term "concrete containment" is used in a generic sense to describe any concrete part within the containment building. It is obvious here that radiation mainly affects the reactor cavity and the biological shield.
- The third identified mode is the alkali-silica reaction. Though this degradation is well documented by the operating experience (for bridges and dams in particular) and scientific literature, its high ranking in this EMDA analysis describes the need to assess its potential consequences on the structural integrity of the containment.
The fifth identified mechanism is related to boric acid attack of concrete in the spent fuel pool. The knowledge gaps are essentially related to the kinetics and the extent of the attack (role of the concrete mix design) and their consequences on the structural integrity.
The steel components within concrete and civil structures and components were also examined. The two degradation modes of highest priority identified in the PIRT processes for the steel component of the containment are
- the corrosion and stress corrosion cracking of the tendons and
- the corrosion of the inaccessible side of the liner. The lack of knowledge here is associated with the absence of a current in-service inspection technique.
These degradation modes and mechanisms have been identified as having the greatest potential effect on the ability of the concrete and civil structures and components to fulfill their safety related functions during long-term NPP operation. This potential effect may be mitigated by (1) improving the overall level of knowledge about the identified degradation modes in order to better predict and mitigate possible consequences and/or by (2) identifying and implementing acceptable mitigation strategies (replacement, treatments, etc.). Research will be required in either case, and these topics should be the highest priorities for research for concrete and civil structures and components.
The work was conducted via a partnership between the U.S. Nuclear Regulatory Commission's (NRC's) Office of Nuclear Regulatory Research (RES), and the U.S. Department of Energy's (DOE's) Light Water Reactor Sustainability (LWRS) program to extend the NRC's original PMDA report in both time span and scope.
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