Dissolved Oxygen:

Control of the dissolved oxygen content in the reactor facility system is of paramount importance because of its contribution to increased corrosion.  A base reaction of concern regarding high concentrations of dissolved oxygen is the following.

3 Fe + 2 O₂      → Fe₃O₄

4 Fe + 3 O₂     →2 Fe₂O₃

 That is dependent on both the concentration of temperature and oxygen. Reaction (3-19) is predominant at high temperatures (>400 F) within the presence of lower oxygen concentrations. This ferrous oxide, corrosion film, is also called as magnetite and is a black, commonly tightly-adherent film which gives a protective function to surfaces inside the facility. Reaction (3-20) occurs at temperatures below about 400º F in the presence of higher oxygen concentrations.  Ferric oxide (Fe₂O₃) is more generally known as rust and is commonly a reddish color. That corrosion product adheres loosely to surfaces and is thus simply erased and transported by the system for following deposition and probable irradiation. In  either  of  the  reactions and  the  corrosion  rate  is  accelerated  through  increased concentrations of dissolved O₂ and could be aggravated additional through the presence of other substances which might be present in the system.

Further to the direct contribution to corrosion, oxygen reacts along with nitrogen to lower the pH of the reactor water that also gives output in an increased rate of corrosion.  Oxygen and nitrogen react to form nitric acid through the following reaction.

With all the reactions presented, it could be seen that oxygen concentrations promote corrosion.  It follows then which if corrosion is to be minimized; oxygen concentrations must be managed as low as probable. In most nuclear facility reactor coolant systems, a limit for dissolved oxygen concentrations is expressed within ppb (parts per billion).  Concentration might be monitored on a continuous basis through using an in-line analyzing system or periodically through withdrawing a sample volume and analyzing which sample.  Monitoring oxygen levels is completed not just to ensure which no oxygen is available for corrosion, other than also to denote the introduction of air within the system.