Initial fill of the reactor coolant system:

Reaction (3-13) also consumes oxygen added to the reactor coolant as a natural result of air dissolved in makeup water. For initial fill of the reactor coolant system, the conditions which need the largest amounts of makeup water are feed and bleed operations to correct an abnormal chemistry parameter or cooldown after a few period of reactor operation. In that case, gamma radiation from the decay of fission products within the reactor core continues to induce the H₂ - O₂ reaction for a few periods after shutdown. In During initial fill and long shutdown periods, chemicals other than hydrogen (example for hydrazine) might be added to reactor coolant to erase any dissolved oxygen.

After essentially all of the oxygen has been consumed by reaction with hydrogen, a nitrogen hold within air will remain.  Other than small air additions, few hydrogen will also remain; therefore, the reactor coolant will hold both dissolved hydrogen and dissolved nitrogen.  Those two gases do not react in an unirradiated solution at low pressure and temperature.  While exposed to radiation, however, the gases do react by the following reaction.

          Radiation

3H₂ +N₂ ↔ 2NH₃ (ammonia)

Again, that is an equilibrium reaction, and radiation induces a reaction within both directions. Ammonia (NH₃) produced through this reaction combines along with water to form ammonium hydroxide (NH₄OH).

NH₃ + H₂O ↔ NH⁺₄ + OH^-                                                                       

Under the operating conditions of reactor coolant, the given Reaction (3-14) is far from finished.  In several cases, less than about 10 % of the nitrogen will be transformed to ammonia.  Reaction (3-14) would be enough to cause the coolant to be mildly basic, pH 9 if no additional base were added to reactor coolant. Within the presence of added base, therefore, the reaction has just an extremely slight and negligible effect on pH.

If the base NH₃ were used to control reactor coolant pH, the reverse of Reaction (3-14) would be more significant.  A reverse step of this reaction needs in which a few of the ammonia added to the coolant decompose into N₂ and H₂.  Since operating conditions favor this step of the equilibrium, rather than formation of NH₃, it would be expected in which most of the ammonia added would decompose. Therefore, the rate of the ammonia decomposition reaction is slow, and the pH of reactor coolant could be managed in the needed range. It should also be remembered in which the decomposition of NH₃ would generate hydrogen gas in important concentrations in reactor coolant (enough to satisfy normal H₂ requirements).