## 2 Materials and Methods  
Oysters were collected from the coasts of Nosan-ri, Gwangdo-myeon, and Tongyeong (South Korea). First, the flesh was separated from the shells, and then the shells (Figure 1) were washed with distilled water and dried for 12 h at 80 \({}^{\circ}\)C using a hot air dryer. Next, the dried shells were pulverized, and the particles were sieved using a micromesh to ensure a uniform particle size. To perform plastic working, the refined oyster shell powder was subjected to heat treatment ranging from 600 \({}^{\circ}\)C to 1200 \({}^{\circ}\)C using muffle furnaces (Nabertherm, Co., Lilienthal, Germany; model: L5/11). Throughout this process, changes were observed in the morphology of the particles, and shielding sheets were manufactured by mixing the powdered oyster shell with a polymer. Figure 2 shows photographs of refined oyster shell powder prior to plastic working and after plastic working at 600 \({}^{\circ}\)C and 1200 \({}^{\circ}\)C. To produce the radiation shielding sheet samples, high-density polyethylene (HDPE) was selected as the polymer material and mixed with the powdered oyster shell and the base material. Owing to its excellent  
Figure 1: Oyster shells collected from the coasts of Nosan-ri, Gwangdo-myeon, and Tongyeong-si (South Korea). strength, HDPE is mainly used as a disposable plastic product [25]. The HDPE used in this study had a molecular weight of \(\geq\)4,000,000 g/mol and a density of 0.91 g/cm\({}^{3}\). To make the final composite materials, N-dimethylformamide (DMF, 99.5%) was used to make a casting solution. Next, DMF (\(\sim\) 10 wt%) and powdered oyster shell (70 wt%) were placed in a stirrer and stirred at 5000 rpm to disperse the particles. Diisononyl phthalate (DINP, 0.85-0.95 wt%) was used as a plasticizer to remove the voids inside the shielding sheet. To ensure the uniformity of the shielding efficiency of the final casting solution, contaminants were removed through filtering and defoaming, with the sheet finished using a compression molding calendar process. The final shielding sheets measured 300 mm \(\times\) 300 mm \(\times\) 0.3 mm. The appearances of the manufactured shielding sheets are shown in Figure 3. This process was repeated to fabricate shielding sheets using barium sulfate, bismuth oxide, and tungsten, and their respective shielding efficiencies were compared. Each of the manufactured shielding sheets had the same shielding material capacity and the same dimensions. The particle dispersibility of the shielding materials was observed using field-emission scanning electron microscopy (FE-SEM; S-4800, Hitachi) after thin-film sectioning using a microtome (RM2235, Leica). Radiation transmission characteristics were used to evaluate the shielding efficiency of the sheets. When radiation passes through a medium, that is, a material, the change in the incident energy due to the nuclei and electrons leads to attenuation. Therefore, to increase the attenuation effect of the incident energy, it is necessary to provide the maximum possible time for the interaction to occur within the material through which the radiation passes. As the thickness of the shielding sheet increases, its range of interaction also increases, and consequently, thickness can serve to increase the interaction time. The degree of reduction with respect to the radiation energy per unit distance traveled in the shielding material is represented by the linear attenuation coefficient [26]:  
\[ \begin{split} I&=I_{0}e^{-(\mu_{1}x_{1}+\mu_{2}x_{2 })}\\ \mu_{1}&=\frac{\left[I_{0}\left(\frac{x_{1}}{0} \right)-\mu_{2}x_{2}\right]}{x_{1}},\end{split}  \tag{1}\]  
Figure 3: Comparing appearances of shielding sheets manufactured using the same process: (**a**) powdered oyster shell (1200 \({}^{\circ}\)C), (**b**) barium sulfate, (**c**) bismuth oxide, and (**d**) tungsten.