Iran plans to upgrade their nuclear weapons

Iran has told the International Atomic Energy Agency (IAEA) that they will be upgrading their nuclear missles. They are going from the IR-1 model to the IR2m model. This will also increase their productivity in uranium, and increase their capacity for uranium. This story interests me because nuclear weapons are already destructive enough so I don't see the point in upgrading them. Plus if anyone were to use nuclear weapons, it would probably set off a chain reaction of nuclear missiles being launched from other countries until this whole planet blows up. Then it'll be like the video game fallout, were everything is a waste land, and humanity will barely survive (humans are stubborn, I don't think we will ever truely die off). I think this is also a very strong possibility, and probably the #1 most realistic apocalypse. I'm not saying it is going to happen, or that it's going to happen soon, i'm just saying it's a possibility, and maybe we should prepare. Here is the link for more info about the upgraded nuclear weapons: http://news.xinhuanet.com/english/world/2013-01/31/c_124307791.htm