Tritium decays by emitting a weak beta particle together with an antineutrino. The product is helium-3. Helium is a monatomic gas; therefore, the decay of 1 mole of T₂ yields 2 moles of helium. This causes a pressure buildup in sealed vessels containing diatomic tritium gas (or HT or DT gas). The following reaction is tritium disintegration.

Solubility

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 150C. 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.