The unit is strictly: Kilogram Separative Work Unit, and it measures the quantity of separative work (indicative of energy used in enrichment) when feed and product quantities are expressed in kilograms. The effort expended in separating a mass "F" of feed of assay "xf" into a mass "P" of product assay xp and waste of mass "W" and assay "xw" is expressed in terms of the number of separative work units needed, given by the expression SWU = "WV"("xw") + "PV"("xp") - "FV"("xf"), where "V"("x") is the "value function," defined as "V"("x") = (1 - 2"x") ln ((1 - "x") /"x").