Herman L. Graves

Summary and conclusions of a program addressing aging of nuclear power plant concrete structures

Research has been conducted by the Oak Ridge National Laboratory to address aging management of nuclear power plant concrete structures. The purpose was to identify potential structural safety issues and acceptance criteria for use in continued service assessments. The focus of this program was on structural integrity rather than on leaktightness or pressure retention of concrete structures. Primary program accomplishments include formulation of a Structural Materials Information Center that contains data and information on the time variation of material properties under the influence of pertinent environmental stressors and aging factors for 144 materials, an aging assessment methodology to identify critical structures and degradation factors that can potentially impact their performance, guidelines and evaluation criteria for use in condition assessments of reinforced concrete structures, and a reliability-based methodology for current condition assessments and estimations of future performance of reinforced concrete nuclear power plant structures. In addition, in-depth evaluations were conducted of several nondestructive evaluation and repair-related technologies to develop guidance on their applicability.

Inspection of nuclear power plant containment structures

Safety-related nuclear power plant (NPP) structures are designed to withstand loadings from a number of low-probability external and interval events, such as earthquakes, tornadoes, and loss-of-coolant accidents. Loadings incurred during normal plant operation therefore generally are not significant enough to cause appreciable degradation. However, these structures are susceptible to aging by various processes depending on the operating environment and service conditions. The effects of these processes may accumulate within these structures over time to cause failure under design conditions, or lead to costly repair. In the late 1980s and early 1990s several occurrences of degradation of NPP structures were discovered at various facilities (e.g., corrosion of pressure boundary components, freeze- thaw damage of concrete, and larger than anticipated loss of prestressing force). Despite these degradation occurrences and a trend for an increasing rate of occurrence, in-service inspection of the safety-related structures continued to be performed in a somewhat cursory manner. Starting in 1991, the U.S. Nuclear Regulatory Commission (USNRC) published the first of several new requirements to help ensure that adequate in-service inspection of these structures is performed. Current regulatory in-service inspection requirements are reviewed and a summary of degradation experience presented. Nondestructive examination techniques commonly used to inspect the NPP steel and concrete structures to identify and quantify the amount of damage present are reviewed. Finally, areas where nondestructive evaluation techniques require development (i.e., inaccessible portions of the containment pressure boundary, and thick heavily reinforced concrete sections are discussed.

Aging management of containment structures in nuclear power plants

Research is being conducted by Oak Ridge National Laboratory under US Nuclear Regulatory commission (USNRC) sponsorship to address aging management of nuclear power plant containment and other safety-related structures. Documentation is being prepared to provide the USNRC with potential structural safety issues and acceptance criteria for use in continued service evaluations of nuclear power plants. Accomplishments include development of a Structural Materials Information Center containing data and information on the time variation of 144 material properties under the influence of pertinent environmental stressors or aging factors, evaluation of models for potential concrete containment degradation factors, development of a procedure to identify critical structures and degradation factors important to aging management, evaluations of non-destructive evaluation techniques, assessments of European and North American repair practices for concrete, review of parameters affecting corrosion of metals embedded in concrete, and development of methodologies for making current condition assessments and service life predictions of new or existing reinforced concrete structures in nuclear power plants.

SEISMIC RESPONSE OF MULTIPLE-ANCHOR CONNECTIONS TO CONCRETE

The aim of the research program discussed here was to study the behavior under seismic loading of multiple-anchor connections to concrete. The static and dynamic behavior, under tension and shear (separately and combined) of single and multiple-anchor connections to concrete are discussed. Effects of cracking and dynamic loading rates were addressed. The paper also deals with the seismic response of multiple-anchor connections to concrete. Its most important conclusion is that multiple-anchor connections designed for ductile behavior in uncracked concrete under static loading will probably still behave in a ductile manner in cracked concrete under dynamic loading.

Dynamic behaviour of anchors in cracked and uncracked concrete: a progress report

Abstract In early 1993, the US Nuclear Regulatory Commission began a research program at The University of Texas at Austin, dealing with the dynamic behavior of anchors in cracked and uncracked concrete. In this paper, the progress of that research program is reviewed. The test program is summarized, and work performed to date is reviewed, with emphasis on the dynamic and static behavior of single tensile anchors in uncracked concrete. General conclusions from that work are discussed, and future plans are presented.

Static Behavior of Anchors under Combinations of Tension and Shear Loading

Under the sponsorship of the US Nuclear Regulatory Commission, a research program was carried out on the dynamic behavior of anchors (fasteners) to concrete. This paper deals with the static behavior of single and multiple undercut and sleeve anchors, placed in uncracked concrete and loaded by combinations of tension and shear. The results are used to draw conclusions regarding force and displacement interaction diagrams for single anchors, and regarding the applicability of elastic and plastic theory to the design of multiple-anchor connections to concrete.

Risk-Informed Assessment of Degraded Containment Vessels

This study couples structural analyses of four typical U.S. nuclear power plant containments with existing probabilistic risk analysis (PRA) models to assess the effects of degradation. This method is used to determine the increase in the early release frequencies (risk) due to postulated cases of corrosion in the steel liners and shells, as well as other forms of degradation.Copyright

DYNAMIC BEHAVIOR OF SINGLE AND DOUBLE NEAR-EDGE ANCHORS LOADED IN SHEAR

This paper describes a research effort that aimed to obtain technical information to determine how the seismic behavior and strength of anchors and their supporting concrete differ from the static behavior. The research program was conducted on the dynamic behavior of anchors in concrete and comprised 4 tasks: 1) static and dynamic behavior of single tensile anchors; 2) static and dynamic behavior of multiple tensile anchors; 3) static and dynamic behavior of near-edge anchors; and 4) static and dynamic behavior of multiple-anchor connections. Anchor types included cast-in-place headed bolts, grouted headed bolts, 2 wedge-type expansion anchors, 1 sleeve-type expansion anchor, and 2 undercut anchors. Loading conditions included tension, shear, and combined tension and shear. Test variables included different concrete types and strengths, loading rate, and the presence of cracks. In this paper, the dynamic response of single and double near-edge anchors loaded in shear is described.

Acoustic monitoring of containment tendons

Assured safety and operational reliability of post-tensioned concrete components of nuclear power plants are of great significance to the public, electric utilities and regulatory agencies. Prestressing tendons provide the principal reinforcement for 40% of the containment structures in the United States. This paper briefly examines current in-service inspection requirements for prestressed containments and also reviews the feasibility of developing a passive surveillance system for identification of ruptures in tendon wires and its application to a one-tenth scale ring model containment structure.

Phosphate Ions: Does Exposure Lead to Degradation of Cementitious Materials?

An assessment of the potential effects of phosphate ions on cementitious materials was made through a review of the literature, contacts with concrete research personnel, and conduct of a “bench-scale” laboratory investigation [1]. The objectives of this limited study were to: (1) review the potential for degradation of cementitious materials due to exposure to high concentrations of phosphate ions; (2) provide an improved understanding of any significant factors that may lead to a requirement to establish exposure limits for concrete structures exposed to soils or ground waters containing high levels of phosphate ions; (3) recommend, as appropriate, whether a limitation on phosphate ion concentration in soils or ground water is required to avoid degradation of concrete structures; and (4) provide a “primer” on factors that can affect the durability of concrete materials and structures in nuclear power plants. Results of a literature review, contacts with industry personnel, and a laboratory investigation indicate that no harmful interactions occur between phosphate ions and cememtitious materials unless phosphates are present in form of phosphoric acid. Relative to the “primer,” a separate NUREG report has been prepared that provides a review of pertinent factors that can affect the durability of nuclear power plant reinforced concrete structures.