Background
During the Cold War, the United States, in an effort to achieve and maintain an advantage in the nuclear arms race with the Soviet Union, invested large amounts of money and technical resources into nuclear weapons design, testing, and maintenance. Many of the weapons designed required high upkeep costs, justified primarily by their Cold War context and the specific and technically sophisticated applications they were created for. With the end of the Cold War, however, nuclear testing has ceased in the United States, and new warhead development has been significantly reduced. As a result, the need for high technical performance of warheads has decreased considerably, and the need for a longer lasting and reliable stockpile has taken a high priority.
Prior nuclear weapons produced by the U.S. had historically become extremely compact, low weight, highly integrated, and low-margin designs which used exotic materials. In many cases the components were either toxic and/or unstable. A number of older US designs used high explosive types which degraded over time, some of which became dangerously unstable in short lifetimes (PBX 9404 and LX-09). Some of these explosives have cracked in warheads in storage, resulting in dangerous storage and disassembly conditions.
Most experts believe that the insensitive explosives (PBX 9502, LX-17) currently in use are highly stable and may even become more stable over time.
The use of beryllium and highly toxic beryllium oxide material as neutron reflector layers was a major health hazard to bomb manufacturer and maintenance staff. The long term stability of plutonium metal, which may lose strength, crack, or otherwise degrade over time is also a concern. (See Nuclear weapons design and Teller-Ulam design for technical context.)
The question of whether the plutonium-gallium alloy used in the cores of the weapons suffered from aging has been a major topic of research at the weapons laboratories in recent decades. Though many at the labs still insist on scientific uncertainty on the question, a study commissioned by the National Nuclear Security Administration to the independent JASON group concluded in November 2006 that "most plutonium pits have a credible lifetime of at least 100 years." The oldest pits currently in the US arsenal are still less than 50 years old.
Read more about this topic: Reliable Replacement Warhead
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