Apparatus for expanding a tube into a bore formed in tube sheet in which a primary explosive containing a relatively high number of grains of explosive per unit length extends within the tube coextensive with that portion of the tube to be expanded. An energy transfer cord extends between a detonator and the primary explosive and includes a relatively low number of grains of explosive per unit length which are insufficient to detonate the primary explosive but which be covered by a sheath to contain the debris and gases associated with the explosion of the latter explosive. A booster extends between the energy transfer cord and the primary explosive and contains an explosive which is detonatable by the energy transfer cord and, upon exploding, detonates the primary explosive.