[en] A nuclear power plant operating with both a thermal and a fast reactor housed in the same containment building is discussed in this invention. The fuel elements are transferred between the two reactors using a charge/discharge machine. As the enrichment of the fuel elements in the thermal reactor falls (and hence also the reactor reactivity), the fuel elements are transferred to the fast reactor to permit re-enrichment of the isotopes which are fissionable by thermal neutrons. The fuel elements are then replaced in the thermal reactor. The advantage of obtaining a higher breeding ratio in the fast reactor by using a gaseous rather than a liquid metal coolant is also discussed. (U.K.)