Measuring lance for boiling-water nuclear reactors

Claim 1

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 a plurality of radiation detectors respectively located at different height levels thereof, and further containing a travelling chamber system movable from height level to height level of the lance for calibrating the detectors and continuously determining neutron flux distribution, the improvement therein comprising thimble tubes wherein the detectors are received, said thimble tubes extending through the wall of the pressure vessel and being sealed from the interior of the pressure vessel, means for individually displacing and exchanging said detectors during reactor operation, means for cooling the detectors, said cooling means being formed of material facilitating heat removal from said thimble tubes, and a lance enclosure tube formed with lateral bores, said large enclosure tube receiving said thimble tubes therewithin, said travelling chamber system comprising a travelling chamber tube, and said lance enclosure tube having a cross section matching the shape of the water gap between four of the fuel assemblies of the core disposed symmetrically to one another, four of said thimble tubes and said travelling chamber tube disposed centrally thereof being received in said lance enclosure tube and forming a structural unit therewith, said structural unit being releasably connected pressure-tightly to the base of the pressure vessel, said structural unit being withdrawable upwardly for replacement during reactor shutdown and when the pressure-vessel cover is opened.