It comes with the job.
http://news.yahoo.com/s/yblog_thelookout/20110405/ts_yblog_thelookout/japanese-nuclear-plant-worker-discusses-choice-to-sacrifice-his-life
I asked my cousin Rudy about this, (yeah, the same one) who is a shift captain at a nuclear power station. And everyone knows it when they sign up.
He says it's never spelled out for you when you get the job, but everyone understands it. If you're on watch when it goes down, you stay at your post until you're relieved. However long it takes.
I hope these brave men survive. Like soldiers, police and firemen, we owe them more than we can ever pay back. They say thousands died at Chernobyl.