NNSA has a very helpful press release explaining those studies showing plutonium ages better than Sophia Loren.

Unfortunately, it doesn’t seem to be online … until now:

Recent studies by Lawrence Livermore and Los Alamos National Laboratories show that the aging of plutonium in U.S. nuclear weapons will not affect reliability over the next several decades. The classified studies were done for the Department of Energy’s National Nuclear Security Administration (NNSA) and have taken five years to complete.

[snip]

The classified studies looked at pits in each nuclear weapon type and gave specific information on plutonium properties, aging and other information. Overall, the weapons laboratories studies assessed that the majority of plutonium pits for most nuclear weapons have minimum lifetimes of at least 85 years.

[snip]

The scientific process used in the assessment of plutonium aging on pit lifetimes was peer reviewed by the JASON panel, an independent scientific panel of academics with experience in nuclear physics and the nuclear weapons program. The JASON study concludes that most plutonium pit types have credible lifetimes of at least 100 years, while other pit types with less than 100 years of projected stability have mitigations either proposed or being implemented. The JASON review was congressionally mandated by the Defense Authorization Bill for fiscal year 2005, and was submitted to Congress today.

So three studies: Los Alamos, Livermore and JASON.

Late Update: The JASON report is online.