Abstract: A measuring lance forming part of the core instrumentation of a boiling-water nuclear reactor and extending from the bottom of a reactor pressure vessel through the core in a water gap between the fuel assemblies of the core, the lance having radiation detectors respectively located at different height levels thereof, and further containing a travelling chamber system for calibrating the detectors and continuously determining neutron flux distribution, the improvement therein including thimble tubes wherein the detectors are received, the thimble tubes extending through the wall of the pressure vessel and being sealed from the interior of the pressure vessel, and means for individually displacing and exchanging the detectors during reactor operation.