In actual real life the briefcase that a light bulb which gave that yellow effect whenever it was opened. Now lets talk about the film.
Tarantino was a lover of film noir and has mentioned that one of his favourite films was the 1950s Kiss Me Deadly which features a private eye who was looking for a mysterious box that is later found to have radioactive materials inside it, many of you might be saying well no. You're crazy isn't uranium radioactive, well this is true but uranium found in yellowcake actually has a very low
radioactivity with an extremely long half-life of over four billion years meaning that the radiation is slowly being emitted across this time this would make it reasonable for people to be in close proximity to it without falling ill. But if this is true, what did Marcellus want to do with yellowcake uranium? I leave that up to you.