Heavy ions irradiation was conducted at JANNuS facility of CEA Saclay (France) using 10 MeV Fe5+. Higher dose rates (2.7 – 3.4 × 10-4 dpa/s) imposed to conduct the irradiation at 450 °C (see § 2.3.1). Three different irradiation campaigns were performed within this framework.
For all campaigns, the samples were mounted on a 304 L SS sample holder which was subsequently placed on the irradiation stage (Figure 2-21a). The position of samples was monitored on a screen with the help of a camera throughout the experiment. Starting from 23 °C, the samples were heated to 450° C ± 20 °C. Temperature was monitored by a two – dimensional infrared thermal imager (FLIR Type SC325) that monitored the surface temperature of the samples during irradiation (Figure 2-21b) [16]. In addition, four thermocouples were used to ascertain the temperature during irradiations, one of which was touching the sample in the irradiated region. The Fe5+ beam of proper shape (both in x and y plane) was obtained using different controls of Epiméthée accelerator. Beam centering was done using Niobium doped alumina plate. Like proton irradiation, a raster beam was used to ensure the homogeneous spread of the beam on the sample. The beam was incident on samples at an angle of 15°. Two different geometries of samples (tensile
and bars) were used for the irradiation. Details of these samples are given in appendix A.1.1. For each sample, a section of 10 mm x 2mm corresponded to irradiated zone while the rest of the sample was unirradiated.
Figure 2-21: a) Samples placed on the irradiation stage during Fe5+ irradiation at JANNuS Saclay. Irradiation zone is indicated by yellow dashed square and thermocouples are indicated by red arrows. b) Infra-red heat map indicating the temperature profile during the Fe irradiation
First campaign was conducted on mechanically polished samples. Fe5+ flux of 2.6 x 1012 ions/cm2/s for a duration of 8 hours was used which corresponds to a dose of 10 dpa KP at the surface. Samples irradiated in this campaign will be addressed as 10 dpa Fe (mech) irradiated samples. In second campaign, vibro polished samples were irradiated using Fe5+ flux of 2.24 x 1012 ions/cm2/s for 5 hours. The dose at the surface of these samples was 5 dpa KP and hence, these samples will be addressed as 5 dpa Fe. In last campaign, like first campaign, Fe flux of 2.6 x 1012 ions/cm2/s was used for a duration of 8 hours. The dose implanted was 10 dpa KP and samples will be referred as 10 dpa Fe samples. Note that the ion flux and time of irradiation used for Fe irradiation was smaller compared to proton irradiation, yet the damage induced was higher. This is due to the higher damage rate of the former.
Using the displacement threshold energy of 40 eV for Fe, Cr and Ni [15], SRIM calculation predicted a penetration depth of ~2.5 µm for 10 MeV Fe ions in austenitic stainless steel. This means that for all these irradiation campaigns the irradiated region in the material extended upto a maximum of 2.5 µm. The irradiated region consisted of continuously varying damage region and an irradiation peak at ~2 µm. Unlike proton irradiation, no region of constant damage exists for Fe irradiation (Figure 2-22).
Material Investigation
Figure 2-22: Damage profile for 10 dpa (mech), 10 dpa Fe5+ irradiation (in blue) and for 5 dpa Fe5+ irradiation (in green) in SS 304L obtained using SRIM-2011 under K-P approximation and using threshold displacement energy of 40 eV [15].
The summary of the irradiation campaigns is given in Table 2-5.
Sample Energy (MeV) Irradiation time (hrs) Flux (x 1012 ions/cm2/s) Dose rate (at surface damage) (x 10-4 dpa K-P/s) Dose (at surface) (dpa K-P) Dose (at Peak) (dpa K-P) Peak position (µm) 10 dpa – Fe (meh) 10 8 2.6 3.2 10 75 2 5 dpa – Fe 5 2.24 2.7 5 35 10 dpa – Fe 8 2.6 3.2 10 75
Table 2-5: Different parameters for all the iron irradiation campaigns conducted at 450 ± 20 °C.
In addition to these single beam irradiations, one double beam irradiation was conducted on vibro polished samples using 10 MeV Fe5+ and 1 MeV He+. The purpose of this irradiation was to facilitate the formation of cavities and bubbles in the material which will help to study their role in the SCC of the irradiated austenitic stainless steel. However, it will not be discussed in this study and the samples of this irradiation will only be used in oxidation studies. He/dpa ratio of 15 appm/dpa was used as it is the upper bound to what the core internals experience in PWRs. The Fe5+ and He+ flux used were 2.24 x 1012 ions/cm2/s and 2.09 x 1011 ions/cm2/s respectively. The irradiation was conducted for 5 hours. Helium beam was incident on samples with an angle of 15°.
Aluminium degrader of different thicknesses between 0.0 – 2.0 µm were used to homogeneously distribute the Helium in the samples. While travelling through degrader, He+ ions loses energy. Dependent on the thickness of the degrader, some of the ions are lost due to divergence after scattering in the foils and few others due to straggling in the thick foils. These phenomena were taken in account during the damage profile calculations. Damage implanted in the samples due to He+ beam was 0.019 dpa which is very low compared to damage implanted by Fe5+ (5 dpa). Hence, these samples were considered to have damage of 5 dpa KP solely from iron irradiation and will be addressed as 5 dpa FeHe samples. The damage profile is shown in figure (Figure 2-23).
Figure 2-23: Appm/dpa (KP) profile along with Damage profile for 10 MeV Fe ions and 1 MeV He ions in SS 304L obtained using SRIM-2011 under K-P approximation and using threshold displacement energy of 40 eV [15].
Both the ion irradiations were conducted with great precautions. As there was no hot spot or any other issue, it was assumed that all the irradiation campaigns were successful. To verify the assumptions, several tests were conducted which will be described in following sections.