Reactor coolant:

Reactor coolant makeup water commonly holds a small amount of air that is composed primarily of nitrogen and oxygen in a volume ratio of 4:1 (80 % nitrogen, 20 % oxygen). These gases undergo radiation-induced reactions. The reactions are the similar as those which occur in many accident situations and are involved in the subsequent discussion.

Further to the small amount of air generally dissolved in makeup water, there is a little possibility in which air might be accidentally injected straight within the reactor coolant system.

Whereas air enters the reactor coolant system, and the reactor is operating, the most immediate reaction includes oxygen from the hydrogen and air that is generally present within the coolant.

                                                                                (3-13)

That is, the addition of O₂ disturbs the above equilibrium and causes the equilibrium to shift to the right, intense both H₂ and O₂.  The concentration of hydrogen generally managed in reactor coolant is such in which small amounts of oxygen will be rapidly consumed before any excess oxygen could cause severe corrosion problems to occur.