**5. Conclusions**

Neutron irradiation-generated embrittlement was investigated by three different types of nondestructive magnetic methods on two different types of reactor pressure vessel steel materials and the results were compared with the destructively measured transition temperature. A reasonable correlation was found between magnetic parameters and DBTT, which can be used in future potential applications to estimate DBTT from the results of magnetic measurement. A good correlation was found, as well, between the results obtained by the different methods.

The present work is considered as a direct continuation of Reference [11]. In this, recently published paper, a possible explanation was given for the big scatter of nondestructively measured magnetic characteristics as functions of transition temperature. By applying the so-called MAT method, a possible reason has been found for the scatter. Here, two other principally rather than different magnetic methods have been applied on the same series of samples, and also, on another nuclear pressure vessel steel material having different chemical composition and different properties in order to establish a much larger context of the source of the observed scatter, which was not entirely explained in our previous paper. The results from the other methods were surprisingly similar as in the case of MAT. This means:


A common feature of different techniques—large scatter of points—was also analyzed. As an explanation, this scatter was attributed to the local material inhomogeneity. It was shown that the measurement error is not responsible for the scatter. It was clearly demonstrated in these experiments that, if the behavior of the reference (non-irradiated) samples are similar to each other, the irradiation-induced embrittlement can be determined very well, and in this case the scatter of the magnetic parameters is very low.

One of the most important conclusions of this work is, that the parameters determined by magnetic measurements seem to characterize better the neutron generated material embrittlement than the conventionally used destructive methods. The scatter of the magnetic results is lower than the scatter of the Charpy tests. In addition, the magnetic method characterized the actual, individual samples, in contrary to the transition temperature values determined by the Charpy impact testing methods, which can provide only statistical values on the set of samples.

Another important conclusion is that local material inhomogeneities have a grea<sup>t</sup> influence on the neutron irradiation-induced material embrittlement. Different parts of the reactor pressure vessel, even if they are cut from the same larger block, are hardened differently. Taking into account the measurement conditions' analysis, local material conditions can be responsible for the different neutron irradiation generated embrittlement of the pressure vessel steel material caused by the same dosage of neutron irradiation.

These facts mean a telling argumen<sup>t</sup> for the application of non-destructive magnetic measurements in the reactor industry for future operations.

**Author Contributions:** Conceptualization and original draft preparation, G.V.; investigation and methodology, G.V., I.S., M.R., M.K. and J.M.G.; project administration, A.G. and I.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was carried out in frame of the "NOMAD" project. This project (Nondestructive Evaluation System for the Inspection of Operation-Induced Material Degradation in Nuclear Power Plants) has received funding from the Euratom research and training programme 2014–2018 under gran<sup>t</sup> agreemen<sup>t</sup> No 755330.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data are contained within the article.

**Acknowledgments:** Authors are grateful to Inge Uytdenhouwen (SCK CEN) for her valuable help in sample preparation and characterization. A508Cl.2 samples were prepared in SCK CEN, and the neutron irradiation of all samples was done in SCK CEN in BR2, together with the performance of the mechanical (destructive) properties for the DBTT. Microstructure picture of the A508Cl.2 sample was made also in SCK CEN. Partners performed NDE tests in the hot cell of SCK CEN.

**Conflicts of Interest:** The authors declare no conflict of interest.
