Abstract: A natural circulation reactor, especially a boiling water reactor, includes a reactor pressure vessel, a reactor core and a core jacket disposed within the reactor pressure vessel, surrounding the reactor core and having at least one inlet port and one outlet port for core coolant. A flow path for the core coolant, which is formed between the core jacket and the reactor pressure vessel, leads through the inlet port into the core jacket and has a flow cross section. At least one throttle element is disposed in the flow path for varying the flow cross section in order to regulate the core coolant flow rate. The flow cross section can be both increased and reduced in size by varying the cross sectional area of the throttle element, so that a regulation of the power of the natural circulation reactor is carried out in a simple way and, in an incident, a reduction in power to approximately 40% of the instantaneous reactor power is ensured.

Abstract: A light water reactor, in particular a boiling water reactor, includes a reactor pressure vessel having an interior, a core disposed in a lower half of the pressure vessel, fuel assemblies disposed in the core, and a column of water covering the core and acting as a coolant and a moderator. The column has an initial level range during normal operation. A passively operating safety device is provided. Fluid lines are connected between the safety device and the interior of the pressure vessel. The fluid lines automatically transmit an actuation criterion to the safety device, with at least a drop of a level in the pressure vessel to a value below the initial level range serving as the actuation criterion.