Application of nondestructive examination methods to NPP reinforced concrete structures presents chal­lenges: wall thicknesses can be in excess of 1 m; struc­tures often have increased steel reinforcement density with complex detailing; there can be a number of penetrations or cast-in-place items; accessibility may be limited because of the presence of liners or other components, harsh environments, or structures located below ground; experience with nondestruc­tive examinations of NPP concrete structures is somewhat limited; and methods utilized for the NPP structures are often based on equipment developed for other materials or technologies. Available methods are relatively good at identifying cracking, voids, and delaminations as well as indicating the relative quality ofconcrete. Methods for determining concrete prop­erties, however, generally are somewhat more quali­tative than quantitative because they tend to be indirect in that they often require the development ofcorrelation curves for relating a measured parame­ter (e. g., ultrasonic velocity or rebound number) to a property (e. g., concrete compressive strength). Infor­mation on identification and description of methods for determining the strength of concrete and evalua­tion of concrete structures is available.97-100 A practical guide related to nondestructive examination of con­crete, which not only identifies and describes the cap­abilities, limitations, and applications of the various methods that are available but also presents results from a number of examples, has been developed.101

The status of nondestructive examination meth­ods and priorities for its development with respect to examination and instrumentation and monitoring of concrete structures in nuclear plants was addressed by prior NEA/CSNIIAGE workshops.41,45,51 It was noted that although nondestructive examination techniques have been successfully used on a variety of reinforced and post-tensioned concrete structures, there has been somewhat limited experience in their use to evaluate typical NPP safety-related structures. With respect to these structures, three conditions exist where performing inspections or conduct of nondestructive examinations is not straightforward and requires development — inspection of thick — walled, heavily reinforced concrete sections, base — mats or foundations, and inaccessible portions of a containment metallic pressure boundary. Information summarizing the activities conducted addressing these conditions has been documented.1

Noninvasive techniques for characterization, inspection, and monitoring of thick-walled, heavily reinforced concrete sections to provide additional assurances of their continued structural integrity are desirable (e. g., as-built or current structural features determination, flaw detection and characterization, identification of honeycomb areas and embedded items, and location of voids adjacent to the liner). Methods that can be used to inspect the basemat without the requirement for removal of material and techniques that can detect and assess corrosion are of particular interest. Acoustic (e. g., ultrasonic pulse velocity, spectral analysis of surface waves, impact echo, and acoustic tomography), radar, and radiography appear to have potential for application to thick-walled, heavily reinforced concrete struc­tures in NPPs; however, additional development is required. The most commonly used type of founda­tion for both concrete and steel NPP containments is a mat foundation, which is a flat, thick slab support­ing the containment, its interior structures, and any shield building surrounding the containment.10 As such, the concrete foundation elements of NPPs are typically either partially or totally inaccessible for inspection unless adjacent soil, coatings, waterproof materials, or portions of neighboring components or structures are removed. As a result, indirect methods related to environmental qualification are often utilized to indicate the potential for degradation of the NPP concrete foundations.20 This is generally done through an evaluation of the surrounding medium (e. g., air, soil, humidity, groundwater, or cool­ing water). Methods employed are based primarily on chemical evaluations to assess the presence and con­centration of potentially aggressive ions (e. g., sulfates or chlorides). In addition to an assessment of the aggres­siveness of the surrounding environment, the CFR requires a complete description of the effects ofground — water levels and other hydrodynamic effects on the design bases of the plant foundations and other struc­tures, systems, and components important to safety.104

Inspection of inaccessible portions of metallic pressure boundary components ofNPP containments (e. g., fully embedded or inaccessible containment shell or liner portions, the sand pocket region in Mark I and II drywells, and portions of the shell obscured by obstacles such as platforms or floors) requires special attention. Embedded metallic por­tions of the containment pressure boundary may be subjected to corrosion resulting from groundwater permeation through the concrete; a breakdown of the sealant at the concrete-containment shell inter­face that permits entry of corrosive fluids from spills, leakage, or condensation; or in areas adjacent to floors where the gap contains a filler material that can retain fluids. NPP inspections have identified corrosion of the steel containment shell in the dry — well sand cushion region, shell corrosion in ice con­denser plants, corrosion of the torus of the steel containment shell, and concrete containment liner corrosion. Corrosion incidences such as these may challenge the containment integrity and, if through — wall, can provide a leak path to the outside environ­ment. Several techniques have been investigated that exhibit potential for performing inspections of inaccessible portions of NPP metallic pressure boundaries (i. e., ultrasonics, electromagnetic acoustic transducers, half-cell potential measurements, mag— netostrictive sensors, and multimode guide waves).1 However, these techniques tend to be time consum­ing and costly because they tend to examine only a small area at a time. A technique that can be applied remotely to perform global inspections and determines the overall condition of the contain­ment metallic pressure boundary in a cost — and performance-effective manner is desired.