Magnetic sensor for measurement of stainless steel cladding thickness

Nondestructive evaluation of ferritic nuclear reactor pressure vessels clad with austenitic stainless steel is typically performed using ultrasonic testing (LT) from the inside surface of the vessel Evaluation of the acceptability of detected flaws according to ASME Section XI requires a determination of the depth of the flaw into the base metal The total flaw depth from the examination surface can be determined using UT. but the depth into the base metal cannot be determined precisely unless the cladding thickness is known Because UT is not effective at measuring the cladding thickness from the inside, a magnetic sensor was developed The sensor is based on application of a DC magnetic field using a C-shaped permanent magnet and measurement of the associated magnetic leakage flux across the gap between the pole pieces using a Hall-effect sensor. The probe essentially measures the spacing between the magnet and the ferromagnetic carbonsteel base metal, because the carbon steel alters the magnetic flux path and thus the magnetic field strength measured by the sensor. Welded cladding has some magnetic phase content which affects the measurements, but accurate measurements can still be made with suitable calibration This paper describes the probe, results on test specimens, and initial field tests on a commercial nuclear reactor.