Reactor radiation-induced reactions

For elucidation of mechanisms, rate data at very low conversions may be highly desirable. They can be obtained more easily from a batch reactor than from a CSTR or plug-flow tubular reactor. A standard CSTR would have to be operated at very high flow rates apt to cause fluid-dynamic and control problems. The same is true for a standard tubular reactor unless equipped with a sampling port near its inlet, a mechanical complication apt to perturb the flow pattern. If the problem of confining the reaction to a very small flow reactor can be solved—as is possible, for example, for radiation-induced reactions—a differential reactor operated once-through or with recycle may be the best choice. 

Differential reactors are primarily used for studies of heterogeneous catalysis, Homogeneous reactions are very difficult to confine as sharply as necessary to a very small flow reactor. An exception are radiation-induced reactions (see comment in preceding section on tubular reactors). 

Radiation-induced processes at solid-liquid interfaces are of significant importance in many applications of nuclear technology. In water-cooled nuclear reactors, ionizing radiation induces reactions in the water as well as in the interface between the coolant and various system surfaces such as the reactor vessel and the fuel cladding. These processes will directly or indirectly influence the performance as well as the safety of the reactor. In nuclear fuel reprocessing, the significance of radiation-induced interfacial processes is even more obvious. Many countries plan to store spent nuclear fuel in deep geological repositories. 

Confinement to the reactor is automatic in heterogeneous catalysis, and easily achieved for radiation-induced reactions, e.g., gas-phase chlorination of alkanes initiated by ultraviolet light. The reactor, usually a differential one, then is a cell in which the fluid passes through a beam of light. 

As already indicated, the presence of water (or water vapour) in the stored reactor vessels cannot be discounted. Direct reaction between carbons and water requires extremely high temperatures - typically >1200K - even when catalysts are present and even over the long timescale associated with safestorage is of no consequence. Radiation-induced reaction may also be discounted at the low residual dose rates. Water itself has only a small effect on the rate of graphite and carbon oxidation in air, although the available data are rather ambiguous, with some indications of an increase in rate and others of a decrease. 

The UO4 will precipitate if its solubility ( 10 M) is exceeded, (b) It (or the UO4) can decompose by a radiation-induced reaction via the free radicals H and OH produced by decomposition of the water, (c) In a reactor operating at high temperatures the H2O2 will decompose thermally at an appreciable rate according to the over-all reaction ... 

Clusters, as possible catalytic reactors, are perfectly dispersed in solutions. They are thus suitable systems for observing, under quasi-homogeneous conditions by time-resolved techniques, the kinetics of catalyzed electron transfer, which would be inaccessible on a solid catalyst. It was demonstrated that the reaction of radiation-induced free radicals COT and (CH3)2COH catalyzed by metal clusters started by the storage of electrons on clusters as charge pools and that electrons were then transferred pairwise to water-producing molecular hydrogen [22,75]. 

A different model to explain the variations in Gh, with LET is induced here. The failure of Equations 2-6 for Co60 7-radiation, Equation 12 for reactor radiation, and Equation 13 for 18.9 m.e. v. D + to represent quantitatively the dependence of Gh, on solute concentrations less than 0.01M is interpreted as evidence that H2 formation for these radiations results from two reactions intraspur and interspur reactions. In interspur reactions, intermediates from one spur react with intermediates from an adjacent spur before they escape into the bulk of the solution. 

The intense primary y radiation due to nuclear fission, the secondary y radiation emitted by the fission and activation products and the radiation from the fission products give rise to radiation-induced chemical reactions. The most important reaction is the radiation decomposition of water in water-cooled reactors, leading to the formation of H2, H2O2 and O2. Many substances dissolved in the water influence the formation of H2 (Fig. 11.18). In most closed coolant systems equipment for... 

The is one of the radionuchdes produced by neutron-induced reactions in aU tjrpes of nuclear reactors. In a nuclear power facility the production of can occm in the fuel, the moderator, the coolant, and the core construction materials mainly by the reactions 0(n, a) C, and N(n, p) C. Part of the created in reactors is continuously released as air-borne effluents in various chemical forms (such as CO2, CO, and hydrocarbons) through the ventilation system of the power plant during normal reactor operation. Another part of the C produced is released into the atmosphere from fuel reprocessing plants. The enviromnental release of this reactor-derived C leads to an increase in atmospheric specific activity and hence, to an increased radiation dose... 

These reactions are of different importance for the fractional concentrations of the individual iodine species formed from initial I2 in the aqueous solution. In the development of mechanistic models describing iodine chemistry in a severe reactor accident, these reactions also have to be considered in spite of some gaps in knowledge, the currently available thermodynamic database is considered to be sufficiently accurate. However, because of the action of third partners, in particular of radiation-induced phenomena (see the following sections), the pure I2 hydrolysis and disproportion reactions frequently are only of limited significance. 

Trace contaminants present in the solution may also intensify or reduce the extent of h production considerably by reaction with the intermediate products of water radiolysis. Such traces of metals (e. g. iron, copper) may be unintentionally present in the test solutions of laboratory experiments as weU as, at even higher concentrations, in real containment sump water. The widely unknown nature and concentration of trace ingredients in the sump water is one of the main problems in the application of the laboratory results on radiation-induced iodine reactions to the situation prevailing in a severe reactor accident. 

Radiation-induced changes in plastics can be reduced or increased by certain measures. Stabilization is desired for plastics used, e.g., in nuclear reactor construction, or intended to be sterilized by radiation. Here, irradiation doses can reach levels of several kGy at which plastic properties would already begin to change. Sensibilization, on the other hand, is desired for radiation-chemical processes to reduce the doses required for crosslinking or for other reactions, i.e., to lower the cost of irradiation. Stabilization and sensibilization thus can affect various parallel reactions either uniformly or selectively [710]. 

High power density. Because of the homogeneous nature of the reactor fuel-fluid, virtually no heat-transfer barrier exists between the fuel and coolant. Thus reactor power densities of 50 to 200 kw/liter may be possible, being limited by considerations other than heat transfer, suc h as radiation-induced corrosion and chemical reactions. 

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