Design concept for maximized use of recycled scrap in the production of storage packages

In the decommissioning of nuclear plants large quantities of radioactively contaminated waste metal have to be disposed of. An economic alternative to final storage is the recycling of the scrap metal in the production of transport and storage containers for low and medium active waste made of nodular graphite ductile cast iron. In the particular case of the CARLA plant operated by Siempelkamp, scrap metal with an activity of up to 200 Bq/g is accepted for processing. This covers the vast majority of the metals of a plant to be decommissioned. The composition of the waste metals varies greatly, depending on the different origins like structural or stainless steels After solidification of the high-carbon, high-silicon cast iron melt, the carbon has formed nodular graphite particles embedded in the metal matrix. Nodular cast iron has high strength and elongation. A further advantage of this material are its good radiation shielding properties. Fracture toughness is an important material property in the design of containers for final storage. In the particular case of containers that have to meet the specifications for final storage these must withstand accident loadings from a height of up 5 m at temperatures of -20°C without crack initiation. Containers for final storage do not have the benefit of impact limiters. The fracture toughness of cast iron depends primarily on the microstructure of the metal matrix. A ferritic microstructure has a higher fracture toughness than a pearlitic microstructure. Carbides in the matrix lead to further embrittlement. The metals to be recycled in the decommissioning of a nuclear installation have marked contents of elements like manganese (Mn) in structural steels, chromium (Cr), nickel (Ni) and molybdenum (Mo) in stainless steels and copper (Cu) in special steels. These elements lead to a pearlitic microstructure and to carbides, even at low contents in the melt. With a rising content of pearlite and carbides, the tensile and yield strength increase while elongation to rupture and fracture toughness decrease. In order to meet the requirements of sufficient ductility and fracture toughness, the specifications for the presently licensed containers limits the embrittling content of pearlite in the microstructure to 20% of the cross section of a metallographic specimen. In order to meet this limit on pearlite content, type and quantity of waste metal that can presently be recycled is very limited.