Investigation of nuclear reactor steel by magneto-acoustic emission method

At KFKI Atomic Energy Research Institute, Budapest, investigations have been done since many years for studying how steel material of nuclear reactors degrade during operation. Part of these studies is to investigate the applicability of novel destructive and non-destructive testing methods as well as to introduce them in the measurement of reactor steel properties such as neutron-induced embrittlement, fatigue and ageing. In addition to acoustic emission and magnetic Barkhausen noise testing, experimental investigations have been commenced to apply magneto-acoustic (acoustic Barkhausen) noise testing technology in the research project. At this method high-frequency mechanical stress waves are detected and analysed which erase in the material specimen being exposed to alternating magnetic excitation. For these experiments a novel, multi-purpose micromagnetic analyser has been designed, built and put into production permitting to compare different magnetic parameters and response functions to magnetic excitation. The paper gives a short overview on the magneto-acoustic emission effect and its potential to assess steel properties by presenting some results of calibration experiments executed by the authors. Several measurements have been performed on two large-size specimens cut from nuclear pressure vessel material blocks (Soviet type 15CH2MFA and IAEA reference steel JRQ). The profiles of the transition temperature along the wall thickness are known from previous measurements: these profiles are compared to that of the measured magnetoacoustic emission curve. BY comparison correspondence has been found between these and other magnetic parameters, which may lead to promising non-invasive methods for assessing the grade of embrittlement caused by neutron irradiation. A generally employed testing method of reactor steel materials is the Charpy test. The paper presents magneto-acoustic experiment results both on broken and unbroken Charpy specimens as well as compares the micromagnetic response and other magnetic parameters with the measured mechanical properties