INSPECTION OF INACCESSIBLE AREAS OF CONTAINMENT LINERS AND SHELLS

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1 INSPECTION OF INACCESSIBLE AREAS OF CONTAINMENT LINERS AND SHELLS Thomas Esselman, Bahaa Elaidi, Brian Hohmann, and Theodore Taylor Lucius Pitkin, Inc., 304 Hudson St. New York, NY ABSTRACT The reactor containment building is an important engineering structure in a nuclear power plant complex. The containment building serves as the final barrier for fission products in the event of an accident. Ensuring that the structural capacity and leak-tight integrity of the containment has not deteriorated unacceptably due either to aging or environmental stressor effects is essential to reliable continued service evaluations and informed aging management decisions. Assuring the integrity of the steel liner in a concrete containment vessel and assuring the integrity of a steel shell containment is vital. A challenge to the assurance of integrity is at locations where the liner or shell in inaccessible. This paper will describe the locations that are inaccessible and will provide an approach to apply existing NDE techniques to the assurance of integrity. INTRODUCTION AND DEFINITIONS A primary focus for long term operation of nuclear plants is aging of plant concrete structures. The containment building is of an example of a concrete structure that is of primary importance in the operation of a nuclear plant. It is a safety related structure and must be capable of maintaining structural capability and leak tightness for the operating life of the plant. Some areas of the liner and shell are accessible and can be visually inspected for any signs of degradation. Ultrasonic thickness detection can easily be performed if there are any indications of corrosion or other degradation. There are parts of the liner and shell, though, which are inaccessible. In the broadest sense, an inaccessible area of a liner or shell is defined as a region of the liner or shell that is 1) encased in concrete, 2) normally covered with insulation or blocked by other equipment so that visual examination cannot be routinely performed, and 3) in a location where visual examination is possible, even with binoculars or other visual aid, but hands-on examination or measurement of thickness cannot be routinely performed. An example of these three categories are 1) a liner or shell in a location where it passes behind concrete, 2) a liner that is insulated inside containment, and 3) the liner on a containment dome. A project is underway that will consider methods to perform inspections of the first two of these inaccessible locations. These are specifically for liners and shells that are beneath concrete and liners and shells that are insulated or blocked from examination. The methods of performing an examination will be described. Mock-ups that will allow for the testing of NDE methods will be prepared and tested. These are also intended to provide a basis for examination methods that can be provided to operators of nuclear plants that will allow a consistent and accurate method to confirm the integrity of the liners and shells in nuclear plants. The third category of inaccessible liners, liners that can be visually seen but not readily examined, is an important category that may benefit from robotic inspections or other means that determine the condition of the liner. These operations are not included in this paper. OPERATING EXPERIENCE Two reports have been recently prepared and published that provide an extensive overview of the operating experience on liners [1, 2]. These reports describe liner corrosion that has occurred from both the concrete side and from the inside of the liners. Liner penetration due to corrosion has been noted in four plants in the United States, primarily due 1021

2 to foreign material that was embedded in the concrete and that was in close proximity to the liner [1]. Corrosion from the concrete side that was detected prior to penetration has been detected at two plants. At one plant, this was at a location of the containment where the liner was exposed for steam generator replacement. The other was near a personnel airlock sleeve [1]. Liner corrosion from the inside of the containment generally occurs at locations where water can accumulate and be in contact with the liner [1]. This is most commonly at locations near poured concrete floors that are adjacent to the liner. There is generally a water barrier that is placed at the location where the liner passes behind the concrete. The potential for water accumulation near that location exists, mostly during outages, and, if the water barrier deteriorates, water may leak into the region between the liner and concrete and corrosion can occur. The liner in this region is also generally coated, so coating deterioration also must occur prior to the occurrence of corrosion. Liner corrosion from the inside of the containment caused by coating failures or moisture barrier degradation was noted at twenty three PWRs in the United States [1]. The region of this corrosion can be behind the concrete where visual examination is not possible. Steel containment shells have also experienced corrosion. Early degradation was noted in what is referred to as the sand bed region of the BWR drywell shell [3, 4]. This corrosion was noted as being likely caused by water in the gap between the drywell and the concrete shield. Corrosion of the torus in BWRs has been recently documented in NRC Information Notice [5]. Numerous instances of localized corrosion and coating degradation of the torus shell were noted. The corrosion was noted in the water region of the suppression pool at locations where the coating had degraded and at locations of air gaps around the drywell where water could have accumulated. An Expert Panel met and prepared report on Nuclear Containment Steel Liner Corrosion Workshop: Final Summary and Recommendation Report [2]. Among other recommendations, the Expert Panel recommended that NDE techniques be developed that could be used remotely to determine the overall condition of large areas of the containment liner. The Panel recommended that the techniques be examined in mock-ups to assess the detection and sizing capability of simulated flaws representing general and pitting corrosion. This review of Operating Experience on liners and shells [1] and the recommendations of an Expert Panel [2] indicate a need to be able to locate regions of thinning and to determine the wall loss in inaccessible regions and to be able to scan a large section of containment for degradation. Locations that should be considered for scanning or for thickness measurement include: Concrete side of liner anywhere. This is expected to be at locations of foreign material or voids in the concrete. Liner/shell interior at locations of interface with concrete. Liner or shell that is embedded in concrete. Interior surfaces where coating has degraded or been damaged. Areas where water may accumulate on the inside or outside of the liner or shell. NDE TECHNIQUES There are several NDE techniques that are being considered for use in liners and shells. Some of these have been described previously [6]. Guided wave and Synthetic Aperture Focusing Technique will be described below. Other techniques will also be evaluated for use on inaccessible regions of liners and shells. Guided Waves There are several guided wave applications that may be feasible to use for the application of inspection of the containment liner. Southwest Research Institute (SwRI) has developed a magnetostrictive sensor based system. The 1022

3 SwRI system is described as being capable of being used for long range global inspection of containment metallic pressure boundaries [7]. The study describes the use of the guided-wave approach for remotely inspecting the regions that are inaccessible. One example was areas where the metallic liner is embedded in concrete. The containment metallic pressure boundaries can be backed by concrete on one or both sides. EPRI has conducted an assessment of the effect of concrete on guided wave inspection in 2000 [8, 9]. These reports concluded that when concrete is bonded to the steel liner, the bond limits the long-range inspection capability of the guided waves, except for the A0 wave mode at a fairly low frequency (below approximately 25 khz), which could be used for detection of relatively large defects within several feet of the concrete interface. When concrete is not bonded to the steel liner, there is no measurable effect on the long-range inspection capability of guided waves. Several lamb wave techniques that have been reported in the literature use a basic tomographic image reconstruction procedure. The procedure used to develop the tomogram may be outlined as follows. A sparse guided wave sensor array is embedded on the structure; actuators are designed to excite theoretically predicted guided wave modes and frequencies. Reference guided wave data is then acquired by transmitting and receiving guided waves with every possible sensor combination in the array. Guided wave data is reacquired at predetermined time intervals [8, 9]. Computed tomography (CT) images are constructed by comparing the reference data to the reacquired data. A similarity coefficient or various physically based features could be used for image reconstruction. Damage location, area, and severity are finally accurately mapped in the CT image. The lamb wave (or guided waves) techniques have been developed or used on piping. Developing a tomographic image of the inspected areas in containment will require substantial development. Synthetic Aperture Focusing Technique (SAFT) Another potential method that maybe used to examine inaccessible areas of the containment is Synthetic Aperture Focusing Technique (SAFT) [10]. This technique has been adapted for the inspection of the inaccessible knuckle region for buried tanks. The SAFT technology propagates sound from an accessible area of the tank to the area of interest. The sound propagates around the knuckle and along the bottom of the tank. The ultrasonic method which is known as Tandem-SAFT or T-SAFT utilizes two transducers in a pitch-catch mode to characterize the detected crack. T-SAFT has shown the ability to propagate sound from distances of several feet and using the SAFT algorithm, accurately locate and size defects in length and depth. The system has been used in qualifying trials and been used in the field for defect detection [10]. At Pacific Northwest National Lab (PNNL), research into the use of a Remotely Operated NDE (RONDE) technique has been investigated. The RONDE system was fabricated as a proof-of concept prototype system for using the SAFT technique to inspect the knuckle region of double shell tanks [11]. One disadvantage to using (or adapting) the RONDE system is that the method requires couplant. While the couplant may be a non-water based material such as glycerin, often times in nuclear power plants a water based couplant is used. Proper cleanliness must be performed so that the couplant is fully removed. MOCK-UPS To demonstrate the adequacy of the inspection technique described for inspection of inaccessible metallic liner and shell surfaces, a number of mock-up tests are being performed. The mock-ups will provide an accurate representation of a PWR containment liner and a BWR drywell shell. The PWR containment liner mock-up is represented by two mock-ups. One is for the transition of the external wall liner into the containment floor concrete. This is shown in Figure 1. A second mock-up is prepared for demonstrating a degradation screening process for a straight wall. The straight wall simulates an area where classical 1023

manual UT could be performed but would be too time consuming. Manual UT could be used to size flaws once a location of potential degradation is noted. This mock-up is shown in Figure 2.

4 manual UT could be performed but would be too time consuming. Manual UT could be used to size flaws once a location of potential degradation is noted. This mock-up is shown in Figure 2. The BWR drywell mock-up included the drywell shell adjacent to the sand cushion region. This is shown in Figure 3. Figure 1 Mock-Up of PWR Containment Wall Liner The mock-up in Figure 1 is a replica of a typical liner design and includes liner plate butt welds, attachments, and surrounding concrete. The liner plate is 3/8 inch thick along the wall and the curvature into the floor where it transitions to 1/4 inch thickness. Testing of this mock-up will demonstrate the extent of the coverage of the inspection system into the embedded liner and beyond the 90 degrees curvature where significant losses are anticipated due to contact with the steel embedment and concrete. There is currently no other non-destructive techniques available to inspect the embedded portions of the liner. The mock-up will be oriented in the field position with the containment wall concrete oriented vertically and the floor oriented horizontally. The potential effects of concrete bonding, concrete shrinkage, and deadweight on horizontal surfaces will be examined. 1024

Figure 2 Mock-Up of PWR Flat Wall Liner The mock-up of the straight wall shown in Figure 2 represents a typical wall where the liner is exposed on one side and is backed by

The value of this mock-up is that it will provide for the evaluation of a system to scan a large surface area of the liner in a relatively short time.

5 Figure 2 Mock-Up of PWR Flat Wall Liner The mock-up of the straight wall shown in Figure 2 represents a typical wall where the liner is exposed on one side and is backed by reinforced concrete on the other. The liner plate is 1/4 inch thick. It contains butt welds and stud attachments on the concrete side. The value of this mock-up is that it will provide for the evaluation of a system to scan a large surface area of the liner in a relatively short time. This will demonstrate the applicability of the technique to the containment wall liner above the basemat where direct visual examination would require scaffolding and/or removal of the liner insulation. Figure 3 Mock-Up of BWR Drywell Shell 1025

6 The mock-up of the BWR drywell shell demonstrates the feasibility to inspect the area embedded in the concrete and adjacent to the sand cushion. The shell is slightly over 1 inch thick. This increased thickness will necessitate the use of lower frequency range. Testing of these mock-ups will allow the investigation of multiple sizes and configurations of flaws. The ability to detect or not detect certain flaws will be examined. The ability to accurately size flaws will also be examined. FLAWS AND TESTING The degradation of the liner shell is expected to be by corrosion. The modes of corrosion are expected to be by pitting or uniform corrosion [1, 2]. Other forms of corrosion may also contribute, particularly if foreign material is present behind the liner in concrete. Artificial flaws are mechanically introduced in the liner and shell plates. The flaws are designed to represent realistic material loss and to determine the limits of the inspection technique. The flaws will be placed both in the regions embedded with concrete, in the regions that are embedded in concrete on one side, and, for the drywell shell, in the region that has no concrete. The flaw configurations include a variety of flaws that range from shallow to through-wall with diameters that range from 1/8 inch to 2 inches. Collections of pits of various depths are also provided. The flaws will simulate corrosion both from the inside of the containment and the outside of the containment. Before encasing the liner is concrete, the NDE techniques that will be used will be tested. This will allow the capability of the NDE method to detect and size the flaws without the effects of the concrete. The concrete will then be placed and allowed to cure. The tests will then be repeated. This method of testing will provide insight into the signature of butt welds, attachment studs, clips used to attach insulation, and other features that may be on the liner. CONTAINMENT INSPECTION GUIDELINE The results of the examination and other studies on the containment shell and liner will be placed into a Containment Inspection Guideline that is being prepared by EPRI and DOE to support the long term operation of nuclear plants. The inspection guidelines is expected to recommend a review of design features, the definition of environmental conditions, identification of possible degradation mechanisms, and identification of possible inspection techniques. SUMMARY AND CONCLUSIONS For the long term operation of nuclear plants, enhanced capabilities to inspect and determine the condition of liners and shells will be important. Mock-ups are required that will provide a basis to take existing inspection techniques and to quantify their capability to detect and size flaws. The mock-ups will also allow the definition of the signature that will come from normal construction features, such as welds, attachment studs, clips, the presence of concrete, and other similar design features. The plans being implemented will provide an application of NDE techniques to liners and shells. ACKNOWLEDGMENTS This work is being sponsored by EPRI as part of the Long Term Operation Program and by the Department of Energy as part of the Light Water Reactor Sustainability Program. REFERENCES 1) Dunn, D., Pulvirenti, A., and Hiser, M., Containment Liner corrosion Operating Experience Summary, Technical Letter Report, Revision 1, U. S. Nuclear Regulatory Commission, 1026

7 ML , August, ) Petti, J., Naus, D., Sagues, A., Weyers, R., Erler, B., and Berke, N., Nuclear Containment Steel Liner Corrosion Workshop: Final Summary and Recommendation Report, Sandia Report SAND , July, ) NRC Information Notice IN 86-99, Degradation of Steel Containments, United States Nuclear Regulatory Commission, December 8, 1986 and Supplement 1, February 14, ) NRC License Renewal Interim Staff Guidance (LR-ISG) , Plant-Specific Aging Management Program for Inaccessible Areas of Boiling-Water Reactor Mark I Steel Containment Drywell Shell, United States Nuclear Regulatory Commission, November 24, 2006 (ADAMS Accession No. ML ). 5) NRC Information Notice , Steel Containment Degradation and Associated License Renewal Aging Management Issues, United States Nuclear Regulatory Commission, August 1, ) Ishida, H., Kurozumi, Y., and Kaneshima Y., Development of Ultrasonic Testing Technique to Inspect Containment Liners Embedded in Concrete on Nuclear Power Plants, 16th WCNDT World Conference on NDT, Montreal, ) Kwun, H., Feasibility of Magnetostrictive Sensor Inspection of Containments, NUREG/CR 5724, Oak Ridge National Laboratory and Southwest Research Institute, March, ) Nondestructive Evaluation: Further Developments of Guided Wave Examination Application, Electric Power Research Institute, Report , ) Experimental Validation of Concrete Effects on Guided Waves in Plate, Electric Power Research Institute, Report , ) Pardini, A. F., Evaluation of SAFT/T-SAFT Technology for the Inspection of Hanford s Double Shell Waste Tank Knuckle Regions, PNNL Report 13321, ) Pardini, A. F., Annual Report: Development of a Remotely Operated NDE System for Inspection of Hanford s Double Shell Waste Tank Knuckle Regions, PNNL Report 13682,

At a nuclear power plant, the containment building is of primary importance for safe operation. The primary

At a nuclear power plant, the containment building is of primary importance for safe operation. The primary Journal of Modern Physics, 2014, 5, 1173-1185 Published Online July 2014 in SciRes. http://www.scirp.org/journal/jmp http://dx.doi.org/10.4236/jmp.2014.513119 Flaw Detection Capability and Sensitivity