Formation of oxygen:

To avoid the formation of oxygen in reactor coolant, hydrogen is added. Hydrogen suppresses the configuration of oxygen primarily through its effect on the reactions which OH radicals, generates through Reaction (3-3), undergo.  Within the presence of excess hydrogen, hydroxyl radicals react predominantly through Reaction (3-10) rather than as in Reactions (3-6) by (3-8).

H₂ + OH → H₂O + H                                                                       (3-10)

Hydrogen atoms from this equation then react to form H₂ and H₂O through Reactions (3-7), (3-8), and (3-9). None of these reactions leads to O₂, or H₂O₂, that decomposes to form O₂ and H₂O at high temperatures.  Therefore, the addition of H₂ to reactor coolant hugely eliminates production of free oxygen.

The other way of viewing the effect of hydrogen on reactor coolant is by its effect on the equilibrium of the reaction.

2H₂O + radiation → 2H₂ + O₂                                                        (3-12)

Through LeChatelier's principle, the further of excess hydrogen forces the equilibrium to the left that needs that O₂ be consumed.  If the dissolved hydrogen concentration is sufficiently huge, only an extremely small amount of oxygen will be present at equilibrium. Generally, thus, reactor coolant contains excess dissolved hydrogen, and there is no important net decomposition of water through radiation.