Swedish KBS-3 copper overpack
In the Swedish solution, engineers erected five barriers to delay the migration of radioactive atoms: 1) the material in the uranium oxide fuel pellets; 2) the metal cladding of the fuel rods surrounding the aforementioned pellets; 3) an iron cylinder binding the fuel rod assemblies together; 4) a thick copper cylinder acting as an outer coating around the iron cylinder; 5) bentonite, a swelling clay impermeable to water, surrounding the copper cylinder to immobilise radioelements.