Load up the level DEAD CITY
. Keep playing trough the level until you get separated form your comrade Bourbon. Now you will have to enter a big building. When inside you can jump trough a broken window into an irradiated spot. Now just stand here for 60 seconds. You will take damage over time so it is advised to bring some medkits. Alternatively you can restart checkpoint and run back to the irradiated spot as resetting checkpoint does not reset your progress.
Here is a video which illustrates the guide: