http://en.wikipedia.org/wiki/Nuclear_weapon_design#Plutonium_pit

“The core of an implosion weapon — the fissile material and any reflector or tamper bonded to it — is known as the pit. Some weapons tested during the 1950s used pits made with U-235 alone, or in composite with plutonium,[8] but all-plutonium pits are the smallest in diameter and have been the standard since the early 1960s.”

As well as the definition cited in the AP report that I cited:

http://ap.google.com/article/ALeqM5hLlCrUKKwQnr5aj2NZS-hzUqzkOgD91KUJ002

You can not have a “plutonium pit” as PIT is an acronym for Plutonium Implosion Trigger. See http://en.wikipedia.org/wiki/Boosted_fission_weapon for more details.

,dave