“Nuclear weapons have been a cornerstone of U.S. national security since they were developed during World War II. During the Cold War, nuclear forces were central to U.S. defense policy, resulting in the buildup of a large arsenal. Since that time, they 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 current strategic nuclear forces—consisting of submarines that launch ballistic missiles (SSBNs), land-based intercontinental ballistic missiles (ICBMs), long-range bombers, and the nuclear weapons they carry—are reaching the end of their service lifetimes. Over the next two decades, the Congress will need to make decisions about the extent to which essentially all of the U.S. nuclear delivery systems and weapons will be modernized or replaced with new systems. To help the Congress make those decisions, the National Defense Authorization Act of 2013 (Public Law 112-239) required CBO to estimate the 10-year costs of the Administration’s plans to operate, maintain, and modernize U.S. nuclear forces. In response, CBO published Projected Costs of U.S. Nuclear Forces, 2014 to 2023. The National Defense Authorization Act of 2015 (P.L. 113-291) requires CBO to periodically update its estimate of the cost of nuclear forces. This report constitutes the first such update. CBO estimates that over the 2015–2024 period, the Administration’s plans for nuclear forces would cost $348 billion, an average of about $35 billion a year, and an amount that is close to CBO’s December 2013 estimate of $355 billion for the 2014–2023 period. (Both estimates are given in nominal dollars; that is, they include the effects of inflation.) Although the two estimates of total costs are similar, projected costs for nuclear programs of both the Department of Defense (DoD) and the Department of Energy (DOE) have changed. Over the next 10 years, CBO estimates, DoD’s costs would total $227 billion, which is about $6 billion (or 3 percent) more than the 10-year estimate published in 2013, and DOE’s would total $121 billion, which is about $13 billion (or 9 percent) less than CBO’s 2013 estimate.”