Development of Surface Wave Magnetostrictive Sensors for Health Monitoring of Dry Storage Casks, 20-R9721Printer Friendly Version
Inclusive Dates: 07/01/07 07/01/09
Background - The U.S. Department of Energy and nuclear power plant operators place spent nuclear fuel assemblies in dry storage casks after initial storage in spent fuel pools. Storage casks are licensed typically for 20 years of service. Longer storage periods are expected, and license renewals are under way because of potential delays in commissioning a high-level waste repository. The storage casks are usually constructed with metallic components that are several inches thick. These metallic components could undergo localized corrosion and stress corrosion cracking under certain environmental conditions, and thereby compromise the integrity of the storage casks. A high-frequency magnetostrictive sensor operating in the frequency range of 250 to 500 kHz was proposed to detect corrosion-induced cracks and flaws that could form on the exposed metallic surfaces of storage casks. The proposed sensor would complement existing magnetostrictive sensors that are used for inspecting the integrity of pipeline systems, plates, and heat exchangers tubes with wall thicknesses less than 1 inch [2.54 cm]. In addition, the proposed sensor also could be used to monitor the structural integrity of various equipment in the nuclear power industry and pressure vessels in the chemical and petrochemical industries.
Approach - The project included four research and development activities: (1) assess corrosion-induced flaw sizes for dry storage cask system materials using models and published data; (2) design and fabricate the sensor; (3) test the sensor for various metallic plates 1-inch thick or more; and (4) develop a monitoring plan for one of the commonly used dry storage cask systems. Based on the sensor resolution for detecting crack sizes, range, and surface area coverage, SwRI developed a monitoring plan that could potentially be used for continuous inspection of dry storage cask systems, thus ensuring their structural integrity.
Accomplishments - Fracture mechanics models and localized corrosion penetration data for stainless and carbon steel were analyzed to determine the critical flaw sizes on exposed metallic surfaces of dry storage cask systems. A high-frequency magnetostrictive sensor was designed and fabricated. The sensor consisted of five components: (1) a thin ferromagnetic strip material with a large coefficient of magnetostriction, (2) a set of permanent magnets to bias the ferromagnetic strip, (3) an electronic coil to drive a time-varying magnetic field in the ferromagnetic strip, (4) a capacitor to match the input impedance of the probe to the impedance expected by the probe driver instrument, and (5) an enclosure to house the other probe elements and minimize the effect of external electronic noise. These five components were designed and assembled such that the sensor was able to generate, transmit, and detect surface waves of 500 kHz. The sensor was tested on a two inch-thick plate to determine its defect detection capabilities. The sensor detected defects as small as 0.79-inches long and 31-mils deep at different locations on the plate, demonstrating that a functioning surface wave probe had been successfully designed and built. Based on the results of these tests and the dimensions of the critical defects, a detection zone was defined and the coverage area that can reasonably be monitored using the sensor was estimated to be roughly 6.56-feet long and 6.3-inches wide. Using the results of the defect detection study, a monitoring plan was developed to detect corrosion-induced surface cracks and flaws on exposed metallic surfaces of the VSC-24 dry storage cask.