## Materials and Methods  
### Materials and Mechanical Tests  
To characterize the damage caused by neutron irradiation, two types of RPV materials were considered: western RPV material A508 CL2 and eastern RPV material 15 Kh2 NMFA. A large part of the Lemoniz reactor vessel, a Spanish reactor of the western type that was never in operation, was chosen to manufacture Charpy specimens at the Belgian Nuclear Research Centre (SCK CEN) [25; 26]. ISO-V Charpy specimens were cut out from 3/4 depth. Figure 1 shows the geometry of the Charpy specimen and the dimensions as well as the definition of the T-L orientation. The T-L specimen orientation was selected according to ASTM E23-16b (Standard Test Methods for Notched Bar Impact Testing of Metallic Materials) except if otherwise stated.  
The chemical composition was measured with a spark atomic emission spectrometer (Spectromaxx LMX06) (Table 1). The working method and manipulations were conducted according to the ASTM E415 standard. The typical heat treatment of RPV forgings consists of quenching, tempering, and postveld heat treatment; methods and conditions are described in ASME and ASTM specifications.  
A part from original eastern 1000 MW RPV steel was provided. ISO-V Charpy specimens were cut out from the \(\frac{1}{4}\) depth. The 15 Kh2 NMFA (CrNiMoV) forging steel was manufactured by the IZHORA company (Russia) for a 1000 MW WWER (Water Water Energy Reactor). The original heat number was 181,358, and the forging steel was produced according to Russian specification TU 108.765-78. The chemical composition of 15 Kh2 NMFA steel is provided in Table 2.  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline
**C** & **Mn** & **Si** & **S** & **P** & **Cr** & **Ni** & **Mo** & **Cu** \\ \hline
0.201 & 0.578 & 0.27 & 0.0085 & 0.0091 & 0.372 & 0.668 & 0.599 & 0.0472 \\ \hline \end{tabular}
\end{table}
Table 1: Chemical composition (wt %) of A508 CL2 base metal, as measured by optical emission spectroscopy at the Belgian Nuclear Research Centre (SCK CEN) on a Charpy specimen.  
Figure 1: Schematic representation of ISO-V Charpy specimens.  
The microstructure of the as-received specimens was a mixed-tempered ferrite-bainite structure. After manufacturing Charpy specimens from western RPV material A508 CL2 and from eastern RPV material 15 Kh2 NMFA, one part of them was mechanically tested and the other parts were nondestructively investigated. After the nondestructive examination (Sections 2.2 and 3.2), this set of specimens was divided into three sets of specimens for neutron irradiation. They were neutron-irradiated in the BR2 reactor at three different irradiation fluences (E > 1 MeV) in the primary water pool at an irradiation temperature of \(\sim\)100-120 \({}^{\circ}\)C (Tables 3 and 4). The neutron irradiation was performed in a specially designed rig called NOMAD\({}_{-}\)3 [27], where 24 Charpy specimens were directly irradiated. This ensured that the damage created was large enough to be detected by nondestructive analysis. The fluence achieved was between 1.55 and 7.90 \(\times\) 10\({}^{19}\) n/cm\({}^{2}\) (E > 1 MeV). Four Charpy specimens for each irradiation level (low, middle, and high fluence) were available. Subsequently, the irradiated specimens were nondestructively investigated. To correlate the results of the micromagnetic testing measurements with the mechanical properties of the same specimens, several mechanical tests were conducted on Charpy specimens after they were nondestructively characterized. They were tested using an instrumented pendulum according to ISO 148-1 and ASTM E23 for the as-received nonirradiated and neutron-irradiated materials. Uniaxial tensile tests were performed with a crosshead rate of 0.2 mm/min, according to ASTM standards E8M and E21 on a conventional static universal tensile test machine. Vickers hardness HV10 tests were performed according to ASTM 92-17 on each Charpy specimen after neutron irradiation.