Getting Near the Core
Japanese-built robot, SwRI sensors simplify inspections at nuclear power plants
Three decades ago, nuclear power generation in the United States began as an exciting new growth industry that promised relatively cheap and abundant power. Over time, costs and regulatory burdens have grown, but it continues to provide a significant share of our country's power mix. However, nuclear power plants now are fighting to stay competitive amid pending legislation that would allow unregulated marketing of electrical power from low-cost producers.
But while the long-term future of the industry is uncertain in this country, the need to ensure continued operational safety and component integrity remains for the approximately 100 operational U.S. plants. Meanwhile, nuclear power remains an important source of electricity for other nations including Mexico, several European nations, Japan, Korea, Russia, and China.
Southwest Research Institute's Nondestructive Evaluation Science and Technology Division has been a major player in nuclear power plant inspection since the inception of the division in the late 1960s. Having performed inspections at over 200 plants worldwide, SwRI is well respected, particularly in the specific area of weld inspection on the most critical component, the reactor pressure vessel (RPV).
Methodology of Inspections
The RPV is a large cylindrical tank, 15-20 feet in diameter and 30-40 feet tall, made of welded steel sections of 7-inch to 11-inch thickness. This vessel contains the internal structures that support the uranium fuel rods and, as a result, it receives the highest amount of radiation exposure. As steel is exposed to high levels of radiation over long periods, its material properties degrade and the steel, particularly weld material, can become brittle and more susceptible to failure. This was taken into account during the design of all U.S. reactors, and safeguards were integrated into the construction and operational processes to ensure that the materials would not degrade significantly during the lifetime of the plant.
One of these safeguards was to inspect the welds and adjacent material using ultrasonic testing methods prior to plant startup, and periodically during operation, to detect small material imperfections that could initiate cracking as the plant grew older. The U.S. Nuclear Regulatory Commission (NRC) mandates regular testing and inspection of plant components in accordance with American Society of Mechanical Engineers codes that require regular ultrasonic testing of the welds used to fabricate the reactor vessels.
To perform ultrasonic testing, it is necessary to place an ultrasonic transducer, or sensor, in contact with one of the surfaces of the steel vessel wall. Inspection planners have two choices when contemplating reactor vessel weld inspection: the vessel's inside wall, or its exterior wall.
If the examination is on the inside surface, the work must be performed under water, since the vessels must be filled with water during plant outages so that radiation levels are moderated. If the examination is on the outer surface, which is dry, personnel access becomes difficult. Reactor vessels are shielded inside a concrete wall, with less than 24 inches of clearance between the outside of the vessel and the inside of the wall. Access is reduced even more by the permanent placement of insulation panels into that already tight space.
For about two-thirds of the U.S. commercial reactors (the Pressurized Water Reactor type, or PWRs), the decision is an easy one. Access for these examinations is very good from the inside surface of the vessel because the structure that contains the uranium fuel is designed to be removed, leaving the vessel welds readily accessible. Although a tool is required that can operate under water, PWR vessels are large enough to accommodate fairly simple tool designs and size is not a major limitation.
However, for the other third of the plants (the Boiling Water Reactor type, or BWRs), the choice is much more difficult. Those reactor vessels were not designed with easy inside-surface inspection access in mind. The internal fuel support structure and other internal components are permanently welded in place, leaving less than 10 inches of clearance between the vessel's inside wall and these internal structures. Until recently, underwater tooling simply was unavailable to manipulate the ultrasonic transducers in these very tightly restricted areas.
As a result, early BWR reactor vessel inspection efforts were directed to the outside surface using robotic machinery that moved on metal tracks or magnetic wheels. These devices would have to be installed through small openings in the concrete shield wall and driven remotely to the inspection area. Prior to about 1990, the standard approach was to perform whatever limited testing could be accomplished through these small openings and then obtain relief from the NRC from the remainder.
Another problem was occupational radiation exposure received by personnel while installing and operating this equipment. Since the NRC mandates limits on cumulative occupational exposures, it became increasingly difficult to retain qualified personnel to perform this work.
However, concerns about the aging of these plants -- many were 20-25 years old -- and the lack of typical inspection histories for BWR vessels caused the NRC to take a harder look at inspection enforcement. In 1992, the NRC formally announced that it would no longer grant relief for these inspections and that BWR plants would need to increase the amount of inspection coverage of these welds.
Transition to Inside Surface Examinations
With that NRC decision, the industry began to reassess the feasibility of inside surface inspection of BWR vessel welds. Interior inspection in the upper part of the vessel is not particularly difficult, since the welds are reasonably accessible. However, the lower part is a different story. The uranium core assembly is surrounded by a core shroud whose outside surface is approximately 20-24 inches from the vessel's interior wall. Within this annular area, there are typically 8 to 10 jet pump assemblies which take up the majority of the available space, leaving an annular clearance of less than 10 inches. It is into this 10-inch clearance, about 50 feet under water, that the ultrasonic transducers must be placed for inspection. The transducers must be placed precisely over the weld and must be moved, or scanned, in a specific motion while the transducer's precise position is recorded and the ultrasonic signals are integrated with the position data.
To meet the NRC's requirement, several companies in the nuclear service industry designed robotic tooling to access these welds from the inside surface. While the devices represented a significant improvement, they also required large tool-assembly areas and heavy equipment-handling support. The industry seemed pleased that inspections could be performed and NRC requirements met, but concern persisted over the inspections' impact on efforts to reduce costs and minimize plant outage intervals.
About five years ago, Ishikawajima-Harima Heavy Industries Co., Ltd. (IHI) recognized those concerns and initiated work toward a new and unique scanner for conducting inside surface examinations on BWR shell welds. Initial design and development was completed about two years ago with the introduction of the AIRIS 21 scanner.
With 30 years of joint research and development programs behind them, IHI and SwRI agreed to work together to bring the new technology to the industry. Over the past two years, work has been under way to integrate a high-performance ultrasonic examination capability with the AIRIS 21 scanner to develop inspection procedures and to prepare the system for field use. IHI's equipment design and fabrication capability fit nicely with SwRI's ultrasonic examination expertise and field experience. The effort culminated with the first application of the scanner for reactor vessel inspection work during the spring of 1998 in Mexico, at Comision Federal de Electricidad's (CFE) Laguna Verde Nuclear Plant, Unit 1 (Laguna Verde 1).
SwRI's High-Performance Ultrasonic Examination System
To increase reliability and confidence in ultrasonic inspections, the ASME Code requires that companies' ultrasonic inspection procedures be tested using full-scale reactor vessel mockups with actual welding defects. This effort is known as "Performance Demonstration." To efficiently implement this on an industry-wide basis, nuclear power plant owners formed an organization called the Performance Demonstration Initiative (PDI). While IHI was designing and developing the AIRIS 21 system, SwRI was working to improve the quality and efficiency of its reactor vessel ultrasonic testing procedures and preparing for the PDI qualification tests. This included developing new transducers and new procedures, and evaluating them on SwRI-owned mockups. SwRI successfully completed the PDI reactor vessel qualification tests in April 1995.
Testing, Integration, and Qualification
As SwRI operators began accumulating experience on the AIRIS 21, performance testing was carried out in SwRI's wet tank facilities. Compatibility with SwRI's Enhanced Data Acquisition System (EDAS) was accomplished by integrating the positional readouts from the AIRIS 21 control system into EDAS. Scanner performance was evaluated in several configurations and under various operational conditions. SwRI verified that the device could meet the scanner requirements specified in the PDI-qualified procedure.
In December 1997, planning began for the first use of the robot, which had been approved by CFE at Laguna Verde 1, located about 30 miles north of Veracruz on the Gulf of Mexico. Drawings were acquired and a composite vessel drawing showing all weld locations and known internal components was prepared. Scanning limitations were determined and reviewed. Procedures were prepared to cover device checkout and operation. Several planning meetings were held to facilitate outage coordination efforts.
Summary and Conclusions
In the first field application of AIRIS 21, inspections were accomplished without impact on outage duration. With the cost of nuclear plant down time measured in tens of thousands of dollars per hour, this represents a significant savings to the utility. The robot scanner proved able to maneuver on the vessel wall as anticipated. Also as anticipated, the UT data obtained during the examinations were acceptable and similar in quality to data typically obtained during PWR examinations.
Radiation exposure received during the effort was about 500 mR, a six-fold reduction from an estimated 3,000 mR resulting if the same work were done from the vessel's exterior. The reactor building crane was needed only twice: to bring up the AIRIS 21 control console to the refuel floor, and to lower it to the building exit point. In summary, the system was proven operational and able to conduct examinations while other activities progress. Its small, lightweight, and unique design makes these examinations possible with much less impact on outage operations than ever before.
Published in the Fall 1998 issue of Technology Today®, published by Southwest Research Institute. For more information, contact Joe Fohn.