Testing for Materials Aging in Nuclear Power GeneratorsMay 1, 2006 By: Kirby Woods, Innotech Engineering Solutions LLC Sensors
An eddy current probe with frequency-independent magnetic sensors mounted on a teleoperated crawler detected stress corrosion cracks in a reactor vessel head assembly.
Although our country is young by European, and especially by Asian standards, its infrastructures are getting old. These aging structures' rising costs are becoming an epidemic that directly affects public safety and has an adverse impact on the country's competitive advantage in the international marketplace. One area of particular concern is the nuclear power industry, whose aging cost has been estimated at an annual $17.3 billion . The U.S. Nuclear Regulatory Commission considers the corrosion that has taken place in various facilities to be a public safety issue . This article describes a nondestructive examination (NDE) process, and a new technology that could offer a cure for this "aging epidemic."
The nuclear energy industry here and overseas has identified the sources of its aging problem as the result of both an aggressive operating environment and inadequate programs for predicting structure, system, and component remediation or replacement. One notable event was a leakage in France's Bugey Unit 3 nuclear power plant primary system in September 1991. The cause was a stress corrosion crack (SCC). In February 2002, serious erosion was discovered in the reactor head at the Davis-Besse Nuclear Power Station in Oak Harbor, Ohio (Figure 1). The affected area was identified as a bimetallic connection between a carbon steel vessel and a SS-clad surface welded to an iron-nickel-chromium pipe penetration. This material failure in an inherently corrosion resistant alloy surprised the industry. Similar material failures in U.S. nuclear facilities have led to a new approach to managing material reliability, but it has met with obstacles in its acceptance and implementation.
Figure 1. A football-sized eroded carbon steel section in the reactor vessel head exposed the nonpressure boundary cladding
A new nondestructive examination technique has been developed from earlier NDE work and from destructive evaluations of components and environments that have shown elevated permeability signatures in affected areas. The data thus acquired indicate a classic environmentally assisted stress-corrosion cracking model. This model simply suggests that material aging is due to property changes that are a function of grain boundary structure and strength deterioration .
These stages of deterioration can be detected by the correct NDE process and sensor technology. This new approach will provide the means to greatly enhance material reliability through effective remediation techniques, improved fabrication processes, and new corrosion-resistant materials.
The development and application of the new NDE process presented challenges. The inspection area was characterized by limited access, complex geometry, interferences, and a high-radiation harsh environment. The sensor had to evaluate microscopic material changes (on the order of the Earth's magnetic field strength) to a depth of 20 mil. The severity of the environment also required the use of a remote examination system capable of navigating a congested area. This called for integration with a teleoperated robotic crawler with three degrees of freedom, a power supply, data collection and processing systems, and an umbilical cable to a remote control and analysis center. This remote scanner also needed axial, circumferential, radial, and tip-angle adjustment capabilities for an elliptical weld pool surface with a contour that goes from concave to convex (Figure 2).
Figure 2. A mockup of a reactor vessel head penetration nozzle shows its J-groove weld
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