I saw this para while reading my physics textbook which said that radioactive substances were kept in thick lead containers with a very narrow opening. I understood the purpose of the lead but what is the purpose of the opening? Won't this lead to leakage of radiations?
I think it probably isn't saying that the container must be open (albeit narrowly), but rather that such opening as it obviously must have for accessing the contents ought to be narrow. Radiation particles travel in straight lines, without even any significant diffraction at edges (the wavelength of γ-radiation is extremely small even in comparison to the sharpest possible edge in a macroscopic substance); and having a narrow opening minimises the chance of the contents lying in a line-of-sight to whomever is opening it.