Environmental conditions, high pressure chemical reactions

With SC-CO2 high solubilities can be attained by increasing the pressure, and reactions can be carried out over a wide range of temperatures, pressures and densities. SC-CO2 is readily available, nontoxic, nonflammable, chemically inert under many conditions, inexpensive, environmentally acceptable and easy to remove and recycle. It has received considerable attention as a reaction medium for organic synthesis [77d, 80] as well as in some large-scale extraction processes in food chemistry [81], The Diels-Alder reaction in SC-CO2 has been investigated quite thoroughly. 

The ratio of these two rate constants after conversion to the same time unit is about 1 X an enormously large number. Furthermore, the rate constants are unaffected by changes in environmental conditions such as temperature and pressure. These highly unusual features are not seen in ordinary chemical reactions (see Table 23.1). 

Cesium and iodine atoms which are released from fuel specimens into a high-temperature steam-hydrogen environment are thermodynamically unstable and will be rapidly converted into species that are stable under these conditions. Since the chemical form of iodine in particular will considerably influence its transport and retention behavior within the reactor pressure vessel and the primary system, it is important to know the kinetics of these conversion reactions. A kinetics assessment of the most essential reactions (Cronenberg and Osetek, 1988) has shown that for extremely low concentrations of iodine and cesium in steam (e. g. mole ratio I H2O < 10" ), the predominant form of iodine is HI and that of cesium is CsOH. This is due to the fact that because the concentrations of iodine and cesium are so dilute, the elements are much more likely to collide and react with H2O and H2 than with each other. Low concentrations of iodine and cesium increase the time for thermochemical equilibrium to be established for their reaction products. For mixtures which are so dilute in fission products, the reaction times may approach tens of seconds or longer, so that for high effluent rates the environmental conditions may change (e. g. by transport into the next control volume showing other conditions) before thermochemical equilibrium has been achieved. Under such conditions, certain limitations caused by reaction kinetics may exist. 

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