Effects of Radiation on Water Chemistry (Synthesis)

2H₂ O₂ 2H₂O H₂ OH H₂O H Reactor Water Chemistry DOE-HDBK-1015/2-93 EFFECTS OF RADIATION ON WATER CHEMISTRY (SYNTHESIS) Rev. 0 CH-03 Page 5 Radiation has an effect on the equilibrium in the case of water. In the absence of radiation, water does not spontaneously decompose at 500 F and the equilibrium lies far to the right. When irradiated, however, water does decompose, as shown above. Also, H and O do not 2 2 normally react at 500 F because a large activation energy is required to make the reaction occur. Radiation, in effect, supplies this activation energy, and the reaction takes place readily. Thus, radiation increases the rates of both forward and reverse reactions, although not by the same factor. In general, the effect of radiation on the equilibrium for a given reaction cannot be predicted quantitatively. The situation is further complicated by the observation that the effect on the equilibrium may vary with the intensity of the radiation. In nuclear facilities, the effect may vary with the power level of the facility. In most cases, this complication is not a severe problem because the direction of the effect is the same; only the degree or magnitude of the effect varies with the intensity of the radiation. As noted several times previously, reactor coolant is maintained at a basic pH (in facilities other than those with aluminum components or those that use chemical shim reactivity control) to reduce corrosion processes. It is also important to exclude dissolved oxygen from reactor coolant for the same reason. As shown in the preceding section, however, a natural conse- quence of exposing pure water to ionizing radiation is production of both hydrogen and oxygen. The addition of a base to control pH has essentially no effect on this feature. To prevent the formation of oxygen in reactor coolant, hydrogen is added. Hydrogen suppresses the formation of oxygen primarily by its effect on the reactions that OH radicals, produced by Reaction (3-3), undergo. In the presence of excess hydrogen, hydroxyl radicals react predominantly by Reaction (3-10) rather than as in Reactions (3-6) through (3-8). (3-10) Hydrogen atoms from this equation subsequently react to form H and H O by Reactions (3-7), 2 2 (3-8), and (3-9). None of these reactions leads to O , or H O , which decomposes to form O 2 2 2 2 and H O at high temperatures. Thus, the addition of H to reactor coolant largely eliminates 2 2 production of free oxygen.