Abstract: Coolant flow (900) in the core (103) of a natural circulation boiling water reactor (100) is monitored by making use of a detector (208) for nuclear radiation and a turbine device (204) which comprises a rotor (304) which can modulate the flux (206) of the nuclear radiation on the detector. The turbine rotor can be installed in a fuel bundle (104), while the detector can be placed in an adjoining instrumentation tube (320). Included in the turbine rotor is material (308) which modulates the radiation field (neutron field and/or gamma field). The radiation detector detects (602) variations in the radiation field when the turbine rotor device is set into rotation by the coolant flow (900). The coolant flow rate is then calculated (603) from the speed of these variations.