Nuclear reactor with shutdown device reacting to neutron flux

The shutdown device has at least one absorber medium, which can be introduced into the reactor core, for example absorber rods, liquids or gases, and a control device for initiating the introduction of the absorber medium into the core, depending on the neutron flux. The control device has at least one heat sensitive setting element and a heater associated with this. If this heater contains a material generating heat depending on the neutron flux because of the nuclear reaction, or consists of this material, then this material should have an effective crossection for neutrons, so that neutron reactions only occur in layers. If the heater contains at least one fission material, the fission material or materials should be in an equilibrium mixture with at least one breeder material. In both cases, the functioning of the shut-down device is maintained for a very long period by this design. (orig./HP)