Complex sentence: Inside the cylindrical case was a smaller cylinder of lithium deuteride fusion fuel (the secondary) with a fission atomic bomb (the primary) at one end, the latter employed to create the conditions needed to start the fusion reaction.
Split into simpler sentences: Inside the cylindrical case was a smaller cylinder of lithium deuteride fusion fuel (the secondary) with a fission atomic bomb (the primary) at one end. The latter employed to create the conditions needed to start the fusion reaction.