## 1 Introduction  
Nuclear power plants play a very important role in energy production in many industrially developed countries of the world, and their importance has been recently enhanced due to the energy crisis and climate problems. Several new nuclear power plants are being planned or being built. The safety aspects of these power plants are crucial. It means that any time, safe operation should be guaranteed. A great majority of nuclear power plants belong to the family of the so-called light water reactors, where pressurized and boiling water is used as a cooling material. In the case of light water reactors, the most important, irreplaceable component of reactors is the reactor pressure vessel (RPV). The material of the RPV is under permanent, long-term, and high-energy neutron irradiation within the whole nuclear power plant's lifetime. The mechanical properties of the RPV are modified by this neutron irradiation. Hardness, tensile strength, and yield stress are increased, and toughness is decreased, which is generated by the influence of fast neutrons on material structure interaction processes [1]. The neutron-irradiation-caused material embrittlement is the most important ageing phenomenon. Because of the above arguments, it cannot be overemphasized that the regular inspection of nuclear reactors is an extremely important task. During the operation of the nuclear reactor, the permanent neutron irradiation on the pressure vessel material structure generates modification of the mechanical properties (decrease in toughness; increase in yield stress, hardness, and tensile strength) [1]. The radiation-induced embrittlement is measured by examining the shift in the energy-temperature relation by impact testing of Charpy V-notch specimens, which increases the brittle-to-ductile transition temperature (DBTT) and is followed by hardening. Evidently, all methods that are suitable for the monitoring of neutron-irradiation-generated degradation of nuclear pressure vessel steel are a very important recent duty. Surveillance specimens are used in the second generation of nuclear power plants. They are placed inside the RPV, and after certain periods, they are withdrawn and destructively tested. Mechanical testing methods, i.e., Charpy impact, tensile, fracture toughness, and hardness testing, are used as a traditional way of evaluating the embrittlement [2]. In nuclear industry, the DBTT measured by destructive Charpy tests is the standardized parameter that characterizes the embrittlement. One of the drawbacks of this technique is that many samples are necessary for this inspection, and the measurement error is high. Because of this, several nondestructive techniques have also been recommended for monitoring material degradation. For instance, the Seebeck coefficient measurement [3] or the well-known ultrasonic technique [4] can be applied. Nondestructive tests, in contrast to destructive tests, do not measure directly the mechanical properties of the measured material. Evidently, before practical application of any nondestructive test, the results must be rigorously compared to the standardized destructive ones. Taking into account that the majority of the presently used nuclear reactors are pressurized water reactors (PWRs) with a pressure vessel made of ferromagnetic steel, magnetic methods can be successfully applied for this purpose. It is known experimentally and theoretically that some magnetic properties are sensitive to the material microstructure [5]. These magnetic methods can be Barkhausen noise measurement, hysteresis measurement, or magnetoacoustics emission measurement. A summary of the possible applications of magnetic techniques in nondestructive testing can be found in Ref. [6]. Several attempts have been made to solve this problem, and it has been shown that a good correlation exists between nondestructively measured magnetic parameters and the ductile-to-brittle transition temperature (DBTT) of pressure vessel steel. In Ref.