Rethinking doomsday summarizes a number of doomsday scenarios, their likelihoods, and their likely consequences. The good news is that the media have been overblowing the threats.
This anecdote about nuclear reactor security didn’t put me at ease though:
Could intruders force their way into a reactor to wreak havoc? Past incidents have demonstrated that one need not have sophisticated plans or skills in order to gain some level of access to a nuclear plant. In 1993, a mentally ill man drove his mother’s station wagon past the guarded entrance at Three Mile Island (TMI). Although he was driving at about 35 miles per hour, the surveillance cameras couldn’t swivel fast enough to keep up with his car. The intruder drove through a fence, then a roll-up door, and into the turbine building, where he got out of his car and hid before he was arrested four hours later. Fortunately, his intentions were not malicious.