Sentence:  The threshold mass is derived from the momentum threshold with the assumption of a mean impact velocity of * * f in the U.S. work and * * f in the <WSD>U.S.S.R.</WSD> work . 
Meaning of the word marked with <WSD> tags:  a former communist country in eastern Europe and northern Asia; established in 1922; included Russia and 14 other soviet socialist republics (Ukraine and Byelorussia and others); officially dissolved 31 December 1991