## 2 Materials and Methods
### Methods  
The objective of this method was to establish a robust and reproducible protocol for the quantitative determination of trace levels of \({}^{55}\)Fe and \({}^{63}\)Ni in steel samples via LSC. A preliminary validation study was conducted using spiked, non-irradiated AISI 316L steel to evaluate chemical yield, overall detection efficiency, and the resulting MDC. This methodology was then applied to four steel samples (0.4-0.5 g each) from the decommissioned ENEA FTU (Frascati Tokamak Upgrade) facility. These samples were potentially activated due to neutron irradiation during the facility's operation. Based on theoretical activation modeling, the expected activity concentrations were below the clearance MDC of 0.3 Bq/g established by the Italian regulatory authorities. The radiological characterization aimed to confirm these theoretical estimations. As LSC measurements are highly sensitive to color quenching, which affects the photon transmission from scintillation events to the photomultiplier, steel samples must undergo a rigorous chemical separation prior to counting [13; 14]. Following microwave digestion, the resulting solutions were intensely colored due to high concentrations of non-radioactive Fe and Ni isotopes, making them unsuitable for direct measurement (Figure 1). To purify the target radionuclides, the digested solution was evaporated to dryness and redissolved in 6 mL of 6 M HCl. The solution was passed through a column containing 10 mL of AG 1x8--200-400 mesh (anion exchange resin), previously pre-cleaned with 10 mL \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Step** & **Time (min)** & **T1 (\({}^{\circ}\)C)** & **T2 (\({}^{\circ}\)C)** & **P (bar)** & **Power (W)** \\ \hline
1 & 20 & 220 & 120 & 45 & 1500 \\
2 & 15 & 220 & 120 & 45 & 1500 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Microwave program from application notes HPR-ME-02. of 3 M HNO\({}_{3}\), rinsed with 15 mL of H\({}_{2}\)O, and conditioned with 15 mL of 6 M HCl. Fe was retained and then eluted with 0.5 M HCl. The Ni-containing fraction was collected for further purification. Ni purification was carried out on a second column packed with AG 50 Wx8--200-400 mesh (cation exchange resin), pre-conditioned with 15 mL of 6 M HCl, rinsed with 32 mL of H\({}_{2}\)O, and eluted with 15 mL of 1.8 M HCl (Figure 2). To assess overall efficiency, three non-irradiated AISI 316L samples were processed: two were respectively spiked with \({}^{55}\)Fe (20 \(\pm\) 1 Bq) and \({}^{63}\)Ni (70 \(\pm\) 1 Bq), and the third served as a blank for background subtraction (sample weights are listed in Table 2). The blank sample contained all the components of a regular sample except for the radioactive material being measured. It served to subtract background noise or contributions from other sources that were not attributable to the analyte of interest (e.g., solvent and/or resin residues). The overall workflow applied to the three samples is shown in Figure 3. The radioactive standard for \({}^{55}\)Fe is a calibrated solution of 0.1 M HNO\({}_{3}\) containing 100 micrograms of iron per mL, while for \({}^{63}\)Ni it is a 0.1 M HCl solution containing 100 micrograms of nickel per mL. The volume of calibrated solution added to the vial to prepare the spiked samples corresponds to only fractions of a microgram of iron and nickel. The impact of these small amounts on the overall quench was considered negligible compared to the quenching effect caused by the iron and nickel already present as steel components, which were present in quantities more than four orders of magnitude higher. Figure 1: Visual evidence of color quenching in mineralized steel solutions. Figure 2: Flow diagram of Fe and Ni isolation procedure. Each Fe and Ni fraction was dried and redissolved in 4 mL of 2 M H\({}_{3}\)PO\({}_{4}\), which was found to significantly reduce color quenching [15]. As previously mentioned, the solutions eluted from the chromatographic columns remain visibly colored.