Command and Control (by Eric Schlosser) is a great book if you're interested in the history of US nuclear weapons.
According to the author, most warheads in the US arsenal are (or were) equipped with single-use thermal batteries that are activated during the firing sequence.
Thermal batteries were originally developed by German scientists, but the concept was taken further in the 1950s when the US armed forces were looking for a more reliable and less maintenance intensive power source for their nuclear weapons once fired.
