Key parameters affecting net volatile iodine production

The production of molecular iodine in the aqueous phase involves the relative balance between reactions that oxidise iodide ion and reactions that reduce molecular iodine. This balance may be affected the radiation dose rate, which is beyond control by accident management measures, and by the many chemicals introduced during accident progression. The balance between oxidation and reduction is also affected by temperature and by the pH of the solution. The temperature dependence of the molecular iodine concentration in solution is complicated. Reactions that oxidise 

Much more profound is the sensitivity of molecular iodine formation to the solution pH. The aqueous concentration of molecular iodine will decrease by about an order of magnitude with an increase of the pH by one unit in the pH range of five to ten. Therefore, maintaining the pH of water in the containment at a high value following an accident is thought to be a practicable method for mitigating the formation of molecular iodine. 

Nitric acid can be formed by the radiolysis of moist air. This nitric acid will rapidly dissolve in water and cause a decrease in the pH. On the other hand, intermediate products in the air radiolysis process interact rapidly with surfaces and other chemical species in the atmosphere. Furthermore, nitric acid is susceptible to decomposition in a radiation field. It has been difficult, then, to assess the overall effect nitric acid formation by radiolysis will have on solution pH. 

Experimental studies have shown that irradiation and heating of polyvinyl chloride insulation found on electrical cables in many reactor containment will yield both hydrochloric acid (HCl), sulfiirous acid (H2SO3), and sulfuric acid (H2SO4). Some investigators believe that acids formed by the combination of radiolysis and pyrolysis of electrical cable insulation can be of dominant importance to the pH of solutions in the reactor containment. Others feel that acid formation by this process is over-emphasized because acids generated within the pol5mer are not able to escape to the atmosphere except when they are formed near the polymer surface. 

Finally, mention must be made of the effects of other materials released into water in the containment over the course of an accident. Some of these materials are released to the containment during core degradation. The effects of these materials on water pH are not well understood. Hot water will leach calcium hydroxide from concrete and this can cause an increase in pH. Core debris interactions with concrete release copious quantities of aerosol that are rich in species like CaO, Na20 and K2O that will dissolve in water to form hydroxide ions and raise the pH. On the other hand, CaO can precipitate buffers intended to control the water pH. 

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