a neutron-absorbing material, as boron or cadmium, in the shape of a rod or other configuration, that can be moved into or out of the core of a nuclear reactor to regulate the rate of fission.
one of a number of rods or tubes containing a neutron absorber, such as boron, that can be inserted into or retracted from the core of a nuclear reactor in order to control its rate of reaction
Collins English Dictionary - Complete & Unabridged 10th Edition
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