A steam generator which may be mobile or stationary, which includes a fire box the outlet of which is connected to a cyclone section; the outlet of the cyclone section opens into a water tube steam boiler, with the connection between the cyclone section and the water tube steam boiler being such that the cyclone section substantially isolates the water tube steam boiler from the radiant heat of a fire in the fire box; a structural shell contains the fire box, cyclone section and water tube steam boiler and is arranged to both provide a containment vessel if the contained components fail in use and also to bear any of the structural loads applied to the generator in use.

A method for flexibly operating a nuclear power plant with a waste heat steam generator that operates according to the forced-flow principle and that has heating surfaces of different stages of the waste heat steam generator, the heating surfaces being arranged in the flue gas channel, is provided. In order to increase the output, a mass flow of the feedwater flowing through the heating surfaces is increased while almost simultaneously activating a supplementary firing arranged in the flue gas channel of the waste heat steam generator.