Reactor Water Chemistry
Conductivity increases because many of the corrosion products are released in an ionic or
radical form and are capable of conducting electricity. In the early stages of the introduction
of air, conductivity may exhibit erratic behavior because of the relationship of conductivity and
pH. As pH begins to fall, conductivity tends to follow, but the presence of suspended corrosion
products offsets the theoretical conductivity. The resultant increase is dependent upon the
concentration of corrosion products. Additionally, should pH fall below about 7, conductivity
increases as pH is further reduced.
Ion exchanger efficiency decreases because the increased crud inventory consists of both
particulate and colloidal products. The ion exchanger is only about 90% effective as a filter,
as compared to approximately 100% effectiveness as an ion exchanger, so effluent radioactivity
levels increase by a greater proportion. Also, crud in the colloidal form has an even lower
probability of being removed by ion exchange or filtration than does particulate crud, and again
the effluent radioactivity is increased.
Hydrogen and total gas react as expected. Because air contains primarily N and O , the gas
inventory in the system increases. Hydrogen levels decrease because of reactions with both O2
and N . Air also contains approximately 1% of Ar by volume. When large amounts of air are
added to the reactor coolant system, the Ar becomes activated by the neutron flux in the core
region and becomes radioactive in the form of Ar, causing an increase in the gaseous activity
of the reactor coolant.
Figure 6 illustrates changes to chemistry in an experimental facility that was started up with high
oxygen and nitrogen in the system from the deliberate addition of air. As the power level and
temperature were increased, nitric acid formed and neutralized the ammonium hydroxide
present from previous operation. Conductivity and pH decreased until pH 7 was reached; when
the water became acidic the conductivity showed a corresponding increase. With the formation
of nitric acid (from the nitrogen present), a corresponding increase in chromate ion was
observed (in this case Cr concentrations are directly related to crud inventories and result from
chemical attack on facility materials). The process was reversed by adding hydrogen, which
combined with the oxygen and reduced the nitrate and chromate ions.
It should be noted, however, that in the above experiment the chromate ions were effectively
removed from the coolant by decreasing their solubility. This would not be the case for the
majority of the crud. The vast majority of crud would remain suspended in the coolant until it
was either removed mechanically (filtered) or settled and redeposited at other locations in the
system (including redeposition on the core surfaces).