Reactor Water Chemistry
The use of hydrogen gas at temperatures above 200 F precludes the generation of the
compounds formed by Reactions (3-16) and (3-22). In addition, hydrogen is compatible with
the high flux levels present in the reactor core. Accordingly, advantage may be taken of the
reversibility of the radiolytic decomposition of water. The following reaction illustrates the
scavenging process utilizing hydrogen.
As indicated, the reaction is an equilibrium process and will therefore depend on the relative
concentrations of the reactants and the products. By maintaining an excess of hydrogen (H ),
the reaction is forced to shift to the right and theoretically eliminates any dissolved oxygen that
may be present. As long as an inventory of H is present in the coolant, dissolved oxygen will
be eliminated or forced to recombine immediately after radiolytic decomposition, thereby being
unavailable for corrosion reactions.
A boiling water reactor (BWR) facility is susceptible to corrosion, resulting from dissolved
oxygen, in the same reactions as are present in a pressurized water reactor (PWR). However,
because of the design of these facilities the use of chemical additives is prohibited because
continuous concentration would occur in the reactor vessel due to boiling. Boiling would result
in a plating out process, and the irradiation of these concentrated additives or impurities would
create an extreme environment of radiation levels as well as adverse corrosion locations.
By the very nature of operation of a BWR facility, the buildup of high concentrations of
dissolved oxygen is prevented. Because boiling is occurring in the reactor vessel and the steam
generated is used in various processes and subsequently condensed, removal of dissolved gases
is a continual process. As stated, boiling is an effective means of removing gases from a
solution. If we were to compare the oxygen content of the steam and the water in a BWR, we
would find typical concentrations of 100 ppb to 300 ppb in the water and 10,000 ppb to
30,000 ppb in the steam. This concentration process is continuous during operation, and the
dissolved oxygen remains in the gaseous state and is subsequently removed in the condensing
units along with other noncondensible gases. As with PWR facilities, BWR facilities minimize
the introduction of dissolved oxygen by pretreating makeup water by some method. The large
oxygen concentrations measured in the steam system result primarily from the radiolysis of
water according to Reaction (3-12), and as operation is continued, the equilibrium
concentration of 100 ppb to 300 ppb is established. This concentration of oxygen is consistent
with the objective of minimizing corrosion.