From The Vault
Rad Absorption is a perk added to Fallout 3 with the Broken Steel add-on.
With this perk, your Radiation level will decrease by 1 rad every 20 seconds. Waiting one hour will heal 60 rads, meaning that waiting 17 hours will reduce your rad count to zero from any amount. The rate also applies to fast-traveling.
The actual effectiveness of this perk varies, mostly because player characters at a level this high will almost certainly have a house with a My First Infirmary that can instantly cure all rads. It's as simple as fast-traveling to the house, curing all rads, and returning to whence you came, and you won't have wasted a perk. Moreover, the player will no doubt have an abundant supply of Rad Away, and enough caps to buy as much Rad Away as necessary even if they don't. Other alternatives include purchasing or collecting supplies of the abundant Punga Fruit, should the player have the Point Lookout DLC installed, or trading Buffout and Strange Meat to the residents of Little Lamplight for Cave Fungus.