[en] The invention deals with the improvement of a method to treat off-gases from nuclear plants which contain radioactive impurities, in particular krypton and xenon nuclides. The enrichment system contains at least two adsorption-desorption groups filled with activated carbon and one delay line filled with activated carbon. It is attempted to keep the volume of the system as small as possible. In the invention this is achieved by separating the gas stream to be cleaned when the first active component breaks through in non-permissible concentration. Whilst the poor gas fraction can be removed to the outside, the other part of the delay line is added; the fresh off-gas now runs through the second adsorption-desorption reactor whilst the first is regenerated. Data on advantageous cleaning gases, temperatures, pressures and suitable gas flows are given for this regeneration. The invention is described using an example of a nuclear power plant with 38 m3 off-gas per hour. (orig./PW)