## 3 Results  
### Mechanical Properties  
The BR2 material test reactor at SCK CEN, with a nominal power of 125 MW and unique, adaptable core configuration, is the most powerful material test reactor currently operating in Europe. The neutron spectrum is typical of a research reactor, with thermal fluxes between \(7\times 10^{13}\) and to \(10^{15}\) n/(cm\({}^{2}\)s) and fast fluxes (E > 0.1 MeV) between \(1\times 10^{13}\) and \(6\times 10^{14}\) n/(cm\({}^{2}\)s). Like all nuclear reactors, there is no single neutron energy. The actual fast fluence values for all Charpy specimens were measured (Tables 3 and 4) by Fe dosimeters. The \({}^{235}\)U equivalent fission neutron flux was calculated from the \({}^{54}\)Mn activity formed by the \({}^{54}\)Fe(n,p)\({}^{54}\)Mn reaction. The flux (fluence) was calculated using the \({}^{235}\)U fission spectrum averaged cross-section <\(\sigma>\) = 81.7 mb, adopted from [31]. The equivalent fission fluence was converted to a fast fluence (E > 1 MeV) in Fe to provide the material damage of the reactor pressure vessel specimens.  
First, the actual fluence values for all Charpy specimens were measured (Tables 3 and 4). The results of the Charpy impact tests are shown in Figure 8 for A508 CL2 and 15 Kh2 NMFA. An irradiation-induced embrittlement shift, ranging between 100 and 200 \({}^{\circ}\)C, was achieved.  
A correlation usually exists between the yield strength increase, hardening, and embrittlement. To obtain some estimations on the correlation between yield strength increase and the embrittlement (\(\Delta\)T\({}_{41\text{J}}\)), some additional tests were performed. The first was Vickers hardness at HV5 (Figure 9). Correlations between hardness increase and neutron fluence were found for both steels. In Figure 10, the increase in yield strength \(\Delta\sigma_{\text{Y}}\) is plotted for the two grades at the three neutron fluence levels. The increase in yield strength was similar for both grades at the tested fluence levels, even though the initial yield strength was different between the two grades. All test results are summarized in Tables 3 and 4 for A508 CL2 and 15 kHNMFA steel, respectively.  
Figure 8: Embrittlement of 15 Kh2 NMFA and A508 Cl.2 after NOMAD_3 irradiation as a function of neutron fluence. The dashed lines are added to guide the eye (power law fit).  
Figure 10: Tensile strength increase (hardening), tested at room temperature for A508 Cl.2 and 15 Kh2 NMFA steels, as a function of the fast neutron fluence. The dashed lines are added to guide the eye (power law fit).  
Figure 9: Vickers hardness (5 kgf) for A508 Cl.2 and 15 Kh2 NMFA. The dashed lines are added to guide the eye.  
All mechanical properties, such as hardness, yield strength, and DBTT, increased with increasing irradiation fluence. The increase in yield strength and DBTT caused by neutron irradiation for both materials was almost the same; the increase in the hardness of the eastern RPV material was slightly higher than that of the western RPV material. Additionally, we observed that hardness, yield strength, and DBTT slowly increased at middle and high fluence for the western RPV material. The same behavior was observed in terms of hardness and yield strength for the eastern RPV material. For this material, the DBTT at high fluence was slightly lower than at medium fluence. However, no conclusions can be drawn here due to the uncertainties in these values.