Editing Control rod (section)

===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.