Detection of Magnetic Phase in the Steam Generator Tubes of NPP

Abstract

Steam generator tube (SGT) in nuclear power plant is a boundary between primary side and secondary side, whose integrity is one of the most critical factors to nuclear safety (Roberts, 1981). The SGT is made of nickel based Inconel alloy, which is composed of 75% Ni, 16.5%Cr and 8.15%Fe. Inconel alloy has very little magnetic response due to its austenitic nickel base, and the domain walls move relatively unimpeded through the material. The SGT are continuously exposed to harsh environmental conditions including high temperatures, pressures, fluid flow rates and material interactions resulting in various types of degradation mechanism such as corrosion, pitting, denting and inter granular attack. Multifrequency eddy current inspection techniques are currently among the most widespread techniques for the rapid inspection of SGT in nuclear power industry. Although the eddy current test (ECT) technique is adopted widespread in the nuclear industry (Bakhtiari, 1999), it has the limitation to size the flaw accurately because the eddy current measure the impedance signified by the conductivity change associated with the volumetric change of flaws, where the permeability of flaw is considered unity. The EC test currently applied in the nonferrous materials having relative permeability 1 such as Inconel alloy because the magnetic permeability of magnetic materials severely limits the depth of penetration of induced eddy currents. Furthermore, the small magnetic phase having permeability variation inherent in SGT can cause spurious EC test results (Park et al., 2010). It is well known that some part of SGT change as a magnetic phase known as permeability variation clusters (PVC) under the conditions of high pressure and temperature which is the operating environments of nuclear power plant. The relative permeability of the magnetic phase is greater than 1, and with a number of ferromagnetic metals, a value of several thousand can be reached. Internal stresses caused by drawing, straightening, or similar work on the material, can give rise to severe fluctuations in the permeability (Takahashi, 2004). These fluctuations would always cause interference with the test signals. In order to eliminate this interference effect during testing, the ferromagnetic test piece is magnetized by a suitable device such as magnetized ECT probe. Recently, to eliminate ECT signal fluctuation, the magnetized probe with the built-in permanent magnet is being used in the SG tube inspection, because a strong magnetic field of this probe reduces the variation of magnetic permeability, which gains S/N ratio. If we can separate magnetic phase (MP) selectively from the flaws using magnetic sensor, the reliability of EC in SGT inspection will