The Collaborative International Dictionary of English v.0.48:

Protactinium \Protactinium\
   The radioactive chemical element, of atomic number 91; Atomic
   symbol Pa; at. wt. of longest-lived isotope, 231 (T1/2 =
   32,500 yrs.) Also called brevium, Uranium X2 and UX2.
   [PJC]

WordNet (r) 3.0 (2006):

protactinium
    n 1: a short-lived radioactive metallic element formed from
         uranium and disintegrating into actinium and then into lead
         [syn: protactinium, protoactinium, Pa, atomic number
         91]