Reactor Water Chemistry
Hydrogen gas (including tritium gas) dissolves to some extent in most materials. Tritium
and deuterium behave like protium, except for small isotopic effects. Hydrogen atoms
or diatomic hydrogen molecules and some larger hydrogen-bearing molecules dissolve
interstitially; that is, they diffuse into a structure and locate between atoms or molecular
frameworks. As it dissolves, the hydrogen slightly disrupts the structural networks of the
material and causes expansion. The extent of the disruption, along with the extent of
chemical attractive/repulsive forces between the hydrogen and the material, determines
the ultimate equilibrium state.
Solubility in Polymers, Glasses, and Ceramics
Hydrogen gas dissolves as the molecular hydrogen species in many materials. For these
systems, solubility is generally endothermic and is directly proportional to gaseous
overpressure. The materials in which hydrogen dissolves as the molecular species include
organic polymers and glasses at less than 150 C. Organic polymers generally have
atomic-scale voids in the twisted polymer chain, and the hydrogen molecules seek these
sites. Hydrogen dissolution in glasses is assumed to be similar.
Abnormal Chemistry Conditions
As indicated earlier in this module, there are times when actions taken regarding one specific
chemistry parameter may affect more than that parameter (such as feed and bleed operations).
This may also be the case during many abnormal conditions that the reactor coolant system may
be subjected to.
In this section, we will consider the conditions resulting from injection of air, fuel element
failure, and overheating of resin and discuss the probable changes to other chemistry parameters
that may occur.
Injection of Air
Injection of small and large amounts of air into the reactor coolant system was previously
analyzed. When sufficient oxygen is added to deplete the hydrogen inventory, acidic pH results
in those facilities where high (basic) pH is maintained. This reduction in pH is accompanied by
secondary effects to the coolant. The reduced pH causes a change in solubility of the corrosion
film on facility materials and results in part of this film being released for transport throughout
the coolant system. This release, or crud burst, is detected by an increase in radiation levels of
the coolant, increased conductivity, and lowering of the ion exchanger efficiency. Radiation
levels increase because corrosion products that had been attached to core surfaces and were
highly activated by the neutron flux are transported to other parts of the system.