A chemical oxygen generator includes multiple parallel sequentially connected chemical core sections that extend the duration of operation of the chemical oxygen generator. An end of a first core section is contiguous with a chemical core high reactivity section, and a thermal insulation layer is disposed between an end of a second core section and the chemical core ignition plate, as well as between the bodies of the first and second core sections. A chemical core transition section is disposed between the other end of the first core section and the second core section, to insure that the reaction front propagates properly.