A nuclear core (20) of a pressurized nuclear reactor (10) has its older detector assembly designs (32) "recharged" by a mini-detector (50) consisting of a small axially spaced cluster of rhodium detectors in a solid outer sheath sized to permit both insertion in bottom entry dry-thimbles (30) which accommodate the traditional movable fission chamber or the central "calibration" tube (42) inside of a fixed in-core instrument (ICI) (32) which is typically provided so that a movable fission chamber could be used as a backup in case the fixed detectors failed.
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