Reliable Replacement Warhead Program: Background and Current Developments [Updated September 12, 2008] [open pdf - 307KB]
"Most current U.S. nuclear warheads were built in the 1970s and 1980s and are being retained longer than was planned. Yet they deteriorate and must be maintained. To correct problems, a Life Extension Program (LEP), part of a larger Stockpile Stewardship Program (SSP), replaces components. Modifying some components would require a nuclear test, but the United States has observed a test moratorium since 1992. Congress and the Administration prefer to avoid a return to testing, so LEP rebuilds these components as closely as possible to original specifications. With this approach, the Secretaries of Defense and Energy have certified stockpile safety and reliability for the past 12 years without nuclear testing."
CRS Report for Congress, RL32929
U.S. Department of State, Foreign Press Center: http://www.fpc.state.gov/