control rod

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Any of a number of steel tubes, containing boron or other neutron absorber, that is inserted into the core of a nuclear reactor in order to control its rate of reaction

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a steel or aluminum rod that can be moved up or down to control the rate of the nuclear reaction