Materials issues are a key concern for the existing nuclear reactor fleet as material degradation can lead to increased maintenance, increased downtown, and increased risk. Extending reactor life to 60 years and beyond will likely increase susceptibility and severity of known forms of degradation. Additionally, new mechanisms of materials degradation are also possible. The purpose of the
US Department of Energy Office of Nuclear Energy’s Light Water Reactor Sustainability (LWRS) Program is to develop technologies and other solutions that can improve the reliability, sustain the safety, and extend the operating lifetimes of nuclear power plants beyond 60 years.
The intent of this LWRS Concrete nondestructive evaluation (NDE) roadmap is to define Research & Development (R&D) actions to address gaps between available NDE concrete techniques and the technology needed to make quantitative measurements to determine the durability and performance of concrete structures on our current NPP fleet. To assist in the development of this roadmap, a workshop (See Appendix B for Workshop Agenda) was held on July 31, 2012, at the Oak Ridge National Laboratory (ORNL) Conference Center with 28 attendees: three representatives from the Electrical Power Research Institute (EPRI), one representative from Electricité de France, two representatives from the US Nuclear Regulatory Commission (NRC), six representatives from universities (University of Tennessee-Knoxville, Georgia Tech, Iowa State University, University of Illinois, and University of Houston), one representative from industry (WesDyne), one representative from a non-USA nonprofit organization (Swiss Association for Technical Inspection), and 14 representatives from DOE national laboratories (ORNL, Pacific Northwest National Laboratory, Idaho National Laboratory, and Argonne National Laboratory).
A multitude of concrete-based structures are typically part of a light water reactor (LWR) plant to provide foundation, support, shielding, and containment functions. Concrete has been used in the construction of nuclear power plants (NPPs) because of three primary properties—its inexpensiveness, its structural strength, and its ability to shield radiation. Examples of concrete structures important to the safety of LWR plants include the containment building, spent fuel pool, and cooling towers. This use has made its long-term performance crucial for the safe operation of commercial NPPs.
With respect to the concrete structures, age-related degradation may affect engineering properties, structural resistance/capacity, failure mode, and location of failure initiation that in turn may affect the ability of a structure to withstand challenges in service. In order to ensure the safe operation of NPPs, it is essential that the effects of potential degradation of the plant structures, as well as systems and components, be assessed and managed during both the current operating license period as well as subsequent license renewal periods. In contrast to many mechanical and electrical components, replacement of many concrete structures is impractical. Therefore it is necessary that safety issues related to plant aging and continued service of the concrete structures are resolved through sound scientific and engineering understanding.
Unlike most metallic materials, reinforced concrete is a nonhomogeneous material, a composite with low-density matrix, a mixture of cement, sand, aggregate and water, and a high-density reinforcement (typically 5% in NPP containment structures), made up of steel rebar or tendons. Plants have been typically built with local cement and aggregate fulfilling the design specification regarding strength, workability, and durability, but as a consequence, each plant’s concrete composition is unique and complex. In addition, NPP concrete structures are often inaccessible and contain large volumes of massively thick concrete. These structures are exposed to different environments (moisture, temperature) and a diversity of degradation mechanisms (high temperatures, radiation exposure, chemical reactions) at different plant sites, all of which adds to the complexity of determining the integrity/quality of the concrete.
After an introduction to the challenges of making NDE measurements on thick sections of concrete as seen in NPPs, the focus of the workshop was on defining R&D actions to address gaps between available techniques and the technology needed to make these measurements on our current NPP fleet. Several important themes were formulated.
1) Need to survey available samples Comparative testing on the various NDE concrete measurement techniques will require concrete samples with known material properties, voids, internal microstructure flaws and reinforcement locations. These samples can be artificially created under laboratory conditions where the various properties can be controlled. In addition, concrete samples that have been removed from the field and exposed to known degradation mechanisms (different levels of radiation/temperature/chemical reaction) provide the most realistic concrete aging specimens.
2) Technique(s) to perform volumetric imaging on thick reinforced concrete sections A technique or a combination of techniques that could reliably and quickly generate an image of the volume of thick concrete structures will significantly enhance the interpretability of the outcome of the various NDE measurement methodologies and is greatly desired.
3) Determine physical and chemical properties as a function of depth Knowledge of the physical and chemical properties of a concrete structure, especially as a function of depth, will provide highly relevant information on its structural integrity.
4) Techniques to examine interfaces between concrete and other materials In some cases, the structural concrete to be inspected is covered by a steel liner. Presently there are no techniques designed for inspecting concrete through steel.
5) Development of acceptance criteria – model and validation Through modeling and validation, an acceptance criterion needs to be developed to determine that a concrete structure is “good enough.” For each NDE concrete measurement metric (void size, crack size, reinforcement degradation, physical properties), an upper and lower acceptance boundary needs to be determined.
6) Need for automated scanning system for any of the NDE concrete measurement systems Due to the massively large concrete areas to be surveyed, an automated scanning system for any NDE concrete measurements is greatly desired.
This report is organized into eleven chapters. Chapter 1 explains the importance of safely extending the lifespan of existing NPPs and provides an introduction to concrete structures. Chapter 2 describes concrete structures of interest. Chapter 3 details the general characteristics of concrete in NPPs. As part of the license renewal process, Chapter 4 describes the NRC’s aging management review that includes an assessment of the applicant’s proposed programs to manage aging of structures and components. Chapter 5 describes the various degradations of concrete. Chapter 6 describes in-service inspection testing as required by the NRC. Chapter 7 describes requirements for augmented NDE as these tests are not typically employed during the in-service inspection testing. Chapter 8 surveys the currently used NDE techniques used to measure concrete degradation. Chapter 9 summarizes the results from the LWRS Concrete NDE workshop held on July 31, 2012. Chapter 10 summarizes the main points of this report. Chapter 11 lists the references.