**1. Introduction**

In almost all major industrial countries worldwide today, nuclear power plants (NPPs) are used to generate electricity. However, the Fukushima Daiichi accident affected the nuclear energy renaissance and, since then, safety aspects have been significantly strengthened. For many existing NPPs, lifetime extensions to 40, 50, 60 or even 80 years have been requested [1]. The long-term operation of existing NPPs has already been accepted in many countries as a strategic objective to ensure adequate supply of electricity over the coming decades. Operating conditions that affect the design lifetime include neutron exposure (fluence), as well as the number and magnitude of temperature and/or pressure cycles in both normal conditions and hypothetical accidental conditions [2,3]. License renewal and periodic safety reviews (PSRs) are the two basic regulatory approaches that are required for an authorization of the long-term operation of NPPs [1]. Evaluation of these parameters during the PSRs allows for an estimation of the operational lifetime of NPPs [2].

As a result of the above arguments, the regular inspection of nuclear power plants is an extremely important task, because the mechanical properties of the reactor pressure vessel (RPV) wall are modified during its operation mainly due to the long-term and high-energy neutron irradiation [4]. In boiling water pressurized reactors, the most critical and most important part is the reactor pressure vessel, because it is not changeable during the whole period of operation. Apart from the standard destructive tests (mechanical Charpy impact testing [5]) several nondestructive electromagnetic methods have been suggested recently for determination of neutron irradiation-generated embrittlement of the pressure vessel steel material. The precise measurement of the Seebeck coefficient makes it possible to derive the neutron irradiation-induced embrittlement of RPV material [6,7]. An ultrasonic technique is also widely used in the inspection of NPPs [8–10].

**Citation:** Vértesy, G.; Gasparics, A.; Szenthe, I.; Bilicz, S. Magnetic Investigation of Cladded Nuclear Reactor Blocks. *Materials* **2022**, *15*, 1425. https://doi.org/10.3390/ ma15041425

Academic Editor: Giovanni Bruno

Received: 19 January 2022 Accepted: 8 February 2022 Published: 15 February 2022

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The RPV material is ferromagnetic steel, so magnetic methods are useful for this inspection. An overview of nondestructive magnetic methods are given in References [11,12]. The magnetic Barkhausen noise (MBN) technique was developed for inspection of residual stresses, surface defects and microstructure changes [13–16]. Another method, magnetoacoustic emission, can also be frequently used for the monitoring of residual stresses [17].

A correlation exists between the modification of the microstructure of the material, generated by different effects, and the observed magnetic behavior if the material is influenced by a magnetic field. This phenomenon can be used to characterize the ferromagnetic materials via magnetic hysteresis measurement. Dislocation movement and domain wall motion are both affected by the microstructure of the material. In ferromagnetic materials, the correlation between mechanical and magnetic hardness is well-known and understood [18,19]. Magnetic methods are advantageous because they are not expensive, are technically simple, and they can be used easily, even on active materials. One of them, the so-called 3MA approach (3MA = micromagnetic, multiparameter, microstructure and stress analysis) applies several methods [20–22], and it was found suitable for the characterization of the damage in ferromagnetic materials like RPV steels and for monitoring the progress of materials.

Another method of magnetic nondestructive testing is the measurement of the magnetic hysteresis loops. For instance, the so-called magnetic minor loops power scaling laws (PSL) were developed, whereby different parameters of minor hysteresis loops are used for material characterization [23]. A similar method, magnetic adaptive testing, also measures systematically minor magnetic hysteresis loops. This is also a multi-parametric, powerful and sensitive method of magnetic inspection [24]. As a conclusion of these works, a reasonable correlation could be found between the destructively measured parameters and nondestructively measured magnetic characteristics through application of these methods. This fact makes possible the future potential use of magnetic methods in the inspection of RPVs' structural integrity.

In several previous works [16,20,25–27] irradiated Charpy samples were measured using magnetic methods, and results of magnetic measurements were compared with the destructively measured ductile to brittle transition temperature (DBTT). However, for the full inspection of nuclear reactors, blocks cut from RPV steel should be also measured, not only samples of Charpy geometry. This aspect of the inspection of a nuclear reactor's integrity has still not been extensively investigated. If the base material itself is directly measured, magnetic measurement is easy. However, in RPV, the base material (ferromagnetic steel) is covered by a cladding. This cladding, which is made of austenitic steel, is the integral component of a WWER 440-type nuclear reactor pressure vessel. Its role is to warrant an anticorrosive protection for the vessel material. Cladding is about a 10 mm thick stainless steel weld-overlay, which is deposited on the pressure vessel's inner surface. It shields the base metal of the pressure vessel from the corrosive environment produced by primary light water coolant [28]. Cladding in WWER 440 reactors is made by submerged arc welding technology by using strip electrodes. The surface of the cladding is either ground or machined roughly to ensure ultrasonic testing coupling.

For future inspection of nuclear reactors, a tool should be developed that is suitable for non-destructive evaluation of the embrittlement of the vessel wall. The final system should be capable of inspecting the degradation of the microstructure through the cladding. The first step has been made in this direction: cladded blocks were successfully measured even through the cladding [29] via the magnetic adaptive testing method [24]. In this work, cladded RPV blocks were investigated, which had been treated thermally by a step cooling procedure, which caused embrittlement of the material. It was demonstrated that the base material degradation could be followed by magnetic measurements even through the cladding. It was shown that a reliable, nearly linear correlation existed between magnetic parameters and DBTT, as expected.

In this type of investigation, when base material is measured through the cladding, the main problem for the magnetic measurement is that cladding means a thick, almost

nonmagnetic layer between the magnetizing yoke and base material, which resulted in an extremely low and noisy probe response, as presented in Reference [29]. In other words, cladding causes serious difficulty for base material investigation. Nevertheless, through the proper choice of measuring parameters and suitable software, the measured signal could be successfully evaluated.

On the other hand, the impact of cladding on the reactor pressure vessel wall integrity has been investigated in a very limited way in spite of the fact that it can potentially be of grea<sup>t</sup> significance to RPV integrity. The reason is that plasticity and the elevated fracture toughness of cladding can provide additional strength to the pressure wall and this process can justify an extended reactor lifetime [30]. In addition, in thermal expansion coefficients, significant differences can be found with respect to pressure vessel base metals, which can cause a stress peak [31]. This is the so-called pressurized thermal shock and it is a potential risk of interfacial crack initiation and propagation. Safety analysis of this phenomenon has lately become a subject of interest for operators of nuclear power plants [32]. In a very recent work, the results of an experimental investigation were presented, aimed at the evaluation of microstructure and failure mechanisms of WWER 440 reactor pressure vessel austenitic cladding (made of stainless steel Sv 08Kh19N10G2B) [33].

The question is arising, as to whether the base material and cladding could be investigated independently of each other by magnetic measurements. The purpose of this paper was to study this problem and to give an answer to this question. Material of cladding is basically austenitic, but it also contains several percentages (2–8%) of ferrite (magnetic) phase. The existence of this ferrite phase gives a chance for successfully applying magnetic measurements to study the properties of cladding. However, the huge volume of highly ferromagnetic base metal, close to the weakly ferromagnetic cladding material, causes difficulty for cladding material investigation. In this work, we will show that the characterization of base and cladding material can be separated from each other by a suitable technique of measurement.

The idea is to choose the proper size of magnetizing yoke—different sizes of yokes can be used for the characterization of base metal and for the characterization of cladding. To make a qualitative interpretation of the experimental results, the effect of the yoke dimension is calculated by numerical simulation of the magnetic flux distribution in the sample.

### **2. Materials and Methods**
