## 4 Discussion  
In this study, several material properties were determined through mechanical tests and micromagnetic examinations. Changes in mechanical properties are caused by neutron-irradiation-induced microstructure defects, which, in turn, influence the magnetic properties measured using micromagnetic measurements. It is well-known that an increase in neutron fluence yields an increase in the density and diameter of microstructure defects. The nature of the microstructure defects and of the degradation mechanism caused by neutron irradiation in RPV steels is dependent on, among other parameters (irradiation fluence, irradiation temperature), the chemical composition of the RPV steel, especially the Cu, Ni, and P contents. There are three important neutron-irradiation-induced damage mechanisms caused by matrix features, Cu-rich precipitates and Mn-Ni-Si-precipitates. The first one is responsible for hardening in both RPV steels, having Cu content below and above 0.04 wt %, generally at low fluences. The second one occurs at high fluences in RPV steels with Cu content higher than 0.04 wt %. The third one is typical for low-Cu RPV steels at high fluences [31]. In this study, the Cu contents of both RPV steels were higher than 0.04 wt % (Tables 1 and 2). Thus, for both RPV steels, the neutron-irradiation-induced embrittlement could be caused by matrix features at low fluence and Cu-rich precipitates at high fluences. However, microstructural examinations (e.g., small neutron angle scattering and high-resolution transmission electron microscopy) were not performed in this study. For both materials, we observed that neutron irradiation at low fluences caused a significant modification in material properties: hardness, embrittlement (DBTT), and yield strength. A further increase in neutron fluence caused a much smaller further degradation of the mechanical properties, whereas a further increase in the neutron fluence did not cause any further change in the mechanical properties, indicating saturation that was observed (Tables 3 and 4). For 15 Kh2 NMFA steel, a small decrease in DBTT was observed at high neutron fluence, but it remained within the uncertainties of such measurements. All these material properties and their progress, induced by neutron irradiation, are strongly influenced by neutron-irradiation-induced microstructure changes. Microstructure defects impede dislocation movement and Bloch-wall movement and cause changes in mechanical and magnetic properties, respectively. Micromagnetic parameters depend on Bloch-wall movement during magnetization and characterize their interaction with microstructure defects such as precipitates, voids, dislocations, or grain boundaries. Coercivity proportionally depends on the interaction intensity between Bloch walls and microstructure defects. [PERSON]'s and [PERSON] and [PERSON]'s theories describe the \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Parameter** & \multicolumn{2}{c}{**R2**} & \multicolumn{2}{c}{**RMSE**} \\ \hline  & **Training** & **Test** & **Training** & **Test** \\ \hline DBTT & 99.6\% & 92.5\% & 5.2 \({}^{\circ}\)C & 21.4 \({}^{\circ}\)C \\ \hline Yield strength & 99.4\% & 91.5\% & 13.2 MPa & 51.6 MPa \\ \hline Hardness & 99.5\% & 92.8\% & 2.1 HV & 7.9 HV \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Correlation coefficients and root mean square errors for the estimation of DBTT, mechanical hardness, and yield strength of 15 Kh2 NMFA material. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Parameter** & \multicolumn{2}{c}{**R2**} & \multicolumn{2}{c}{**RMSE**} \\ \hline  & **Training** & **Test** & **Training** & **Test** \\ \hline DBTT & 99.8\% & 97.5\% & 2.9 \({}^{\circ}\)C & 12.5 \({}^{\circ}\)C \\ \hline Yield strength & 99.9\% & 94.5\% & 6.2 MPa & 38.0 MPa \\ \hline Hardness & 99.9\% & 96.4\% & 1.1 HV & 6.6 HV \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Correlation coefficients (R2) and root mean square errors (RMSEs) for the estimation of DBTT, mechanical hardness, and yield strength of A508 CL2 material.