That depends on concentrations of the isotope in the metal and the amount of metal we will be using. I have no idea what the concentrations of iron isotopes in

That depends on concentrations of the isotope in the metal and the amount of metal we will be using. I have no idea what the concentrations of iron isotopes in meteors are and which iron isotopes are there, but I do not think it is correct to automatically say that radioactive isotopes with long half-lives are completely safe.