Total gas concentration in the reactor coolant system is another parameter of concern. Total gas is the sum of all gases contained in the coolant system and is made up primarily of hydrogen (H₂), nitrogen (N₂), argon (Ar), and oxygen (O₂). The small amounts of fission gases (Kr and Xe) normally present in the system may also contribute to the total gas concentration; however, under normal conditions these are essentially undetectable. Total gas is of concern because high concentrations can result in the formation of gas pockets in areas that are high points of the system where low or stagnant flow conditions exist. Of particular concern in PWR facilities are the erosion and corrosion that may occur on the impellers of the primary coolant pumps. As the concentration of gas is increased, the probability of the gas coming out of solution in significant amounts in areas of low pressure is also increased. This low pressure condition exists at the inlet to the primary coolant pump impeller (where centrifugal pumps are utilized). As these gas bubbles are forced back into solution on the high pressure side of the impeller, erosion can occur as a result of the gas bubble impinging on the impeller. In extreme concentrations of total gas, loss of pump priming and cavitation can occur with resultant mechanical pump damage.

Figure 3 Pressurizer

Reduction of total gas concentrations in PWRs is normally accomplished by the venting of a steam space. In those facilities utilizing a pressurizer, the steam space in the top of the pressurizer is designed to accomplish this venting operation either continuously or intermittently. This process of reducing the total gas concentration is generally referred to as degassification. A typical PWR pressurizer with degassification piping is shown in Figure 3.

Degassification is not normally required in a BWR because of its design. As discussed previously, the boiling action in the reactor vessel strips dissolved gases from the water, and they are continuously removed in the condensing phase of the energy cycle.