For more on 239, check out the K-239 thread in the news section. Pyrophoric plutonium is likely the cause as mentioned previously.......although they'll never admit it.
http://www.abqjournal.com/401669/abqnewsseeker/photos-show-cracked-lanl-container-at-wipp.html
No need to fear the deadliest isotope ever created, they've come up with a solution....
" Among the possibilities that officials have since confirmed are being studied: a switch in the type of kitty litter used to absorb moisture before the containers are sealed and shipped to WIPP."