## 1 Introduction  
Scattered radiation poses the risk of long-term exposure among medical professionals working in hospital radiography facilities [1, 2, 3]. To mitigate this risk, when operating radiation-emitting equipment, medical personnel wear protective aprons made of shielding sheets comprising a mixture of lead and rubber among other materials [4, 5, 6, 7]. An apron with a lead equivalent of 0.25 mmPb weighs 3.25 kg, and an apron with a lead equivalent of 0.5 mmPb weighs approximately 4.95 kg [8, 9]. These weights, however, limit their activity. To address this problem, the development of new radiation-shielding materials, with lighter, more eco-friendly materials including tungsten, bismuth, barium, boron, and tin among the proposed alternatives is extensively studied [10, 11, 12, 13, 14]. These lead substitutes are not susceptible to the risks associated with heavy metals; however, to be used for radiation-shielding sheets, they must be compatible with polymeric materials [15, 16]. In addition, the cost effectiveness of new shielding materials is an important factor with respect to their commercial prospects. Therefore, researchers face the challenge of providing shielding materials that are economical and that meet weight and human safety requirements.  
This study explores the viability of developing radiation-shielding materials by processing oyster shells that would otherwise be discarded as general waste. Various shellfish, such as oysters, clams, and mussels, which live in coastal waters, can be collected for processing. These shellfish are characterized by a large volume of shell compared to the flesh it houses. Further, the large number of shells discarded from aquaculture are a major source of waste that contributes to environmental pollution along coastlines and in landfills [17, 18].  
Currently, the shell recycling industry is very limited, and in certain cases, shells are recycled as soil fertilizer. However, this industry does not have significant economic value [19]. The main component of these shells is calcium, with an atomic number of 20 and a density of 1.55 g/cm\({}^{3}\)[20]. It also reacts easily with oxygen and water, and exists in the form of a calcium carbonate compound in nature. Compared to lead with an atomic number of 82 and a density of 11.34 g/cm\({}^{3}\), calcium exhibits inferior radiation shielding [21].  
Oyster shell is easy to process because it contains a large amount of salt and has a porous structure. In addition, it shows excellent affinity for mixing with other materials, and is therefore being studied as an aggregate for construction materials. Additionally, oyster shell has been mixed with concrete during research on gamma ray shielding [22; 23].  
As oyster shells exhibit excellent compatibility with materials such as concrete [24], this study predicted that the particle structure would be dense, and its density could be increased even after mixing with polymer materials. Therefore, this study attempted to improve shielding efficiency by manufacturing a radiation shielding sheet using oyster shells. To this end, the characteristics of powdered oyster shell were analyzed by conducting plastic working at different temperatures, and evaluating the particle distribution according to the cross-sectional structure of a polymer-based shielding sheet. Furthermore, the effect of plastic working technology on the shielding efficiency of oyster shell was analyzed, and the degree of dispersion within the polymer material was evaluated. In addition to powdered oyster shell, shielding sheets based on barium sulfate, tungsten, and bismuth oxide were manufactured using the same process, and their shielding efficiencies compared. Therefore, the relationship between plastic working and shielding efficiency was analyzed for radiation shields that are based on powdered oyster shell, and this relationship was evaluated against existing shielding sheets that are based on safer lead alternatives.