A quick and easy Google search will tell you that 00031944 is the code. I don't know how to enter it as I haven't played Fallout 3 on the PC, so I suggest googling it further if you don't already know how.
[Edited by yosup, 12/7/2010 3:21:55 AM]
player.additem 00031944 1 will give you 1 scrap metal. For more scrap metal just change 1 to 20 or 50 etc.