Editing Control rod (section)

==Safety==
In most reactor designs, as a [[nuclear safety|safety measure]], control rods are attached to the lifting machinery by [[electromagnet]]s, rather than direct mechanical linkage. This means that in the event of power failure, or if manually invoked due to failure of the lifting machinery, the control rods fall automatically, under gravity, all the way into the pile to stop the reaction. A notable exception to this [[fail-safe]] mode of operation is the BWR, which requires hydraulic insertion in the event of an emergency shut-down, using water from a special tank under high pressure. Quickly shutting down a reactor in this way is called [[scram]]ming.

===Criticality accident prevention===
Mismanagement or control rod failure have often been blamed for [[nuclear accident]]s, including the [[SL-1]] explosion and the [[Chernobyl disaster]].
''Homogeneous'' neutron absorbers have often been used to manage [[criticality accident]]s which involve aqueous solutions of [[fissile]] [[metal]]s. In several such accidents, either [[borax]] ([[sodium]] [[borate]]) or a cadmium compound has been added to the system. The cadmium can be added as a metal to [[nitric acid]] solutions of fissile material; the corrosion of the cadmium in the acid will then generate cadmium [[nitrate]] ''in situ''.

In [[carbon dioxide]]-cooled reactors such as the [[Advanced Gas-cooled Reactor|AGR]], if the solid control rods fail to arrest the nuclear reaction, [[nitrogen]] gas can be injected into the primary coolant cycle. This is because nitrogen has a larger absorption cross-section for neutrons than [[carbon]] or [[oxygen]]; hence, the core then becomes less reactive.

As the neutron energy increases, the neutron cross section of most isotopes decreases. The [[boron]] [[isotope]] <sup>10</sup>B is responsible for the majority of the neutron absorption. Boron-containing materials can also be used as neutron shielding, to reduce the [[neutron activation|activation]] of material close to a reactor core.