"Nuclear weapons have been a cornerstone of U.S. national security since they were developed during World War II. In the Cold War, nuclear forces were central to U.S. defense policy, resulting in the buildup of a large arsenal. Since that time, nuclear forces have figured less prominently than conventional forces, and the United States has not built any new nuclear weapons or delivery systems for many years. The nation's current nuclear forces are reaching the end of their service life. Those forces consist of submarines that launch ballistic missiles (SSBNs), land-based inter- continental ballistic missiles (ICBMs), long-range bomber aircraft, shorter-range tactical aircraft, and the nuclear weapons that those delivery systems carry. Over the next two decades, essentially all of those nuclear delivery systems and weapons would have to be refurbished or replaced with new systems to continue operating. Consequently, the Congress will need to make decisions about what nuclear forces the United States should field in the future and thus about the extent to which the nation will pursue nuclear modernization plans. [...] Besides presenting an estimate of those costs, this report also describes the major differences between CBO's current estimate and its most recent previous estimate, which was published in January 2015."
Congressional Budget Office: http://www.cbo.gov/