Radiation-Chemical Processes

technological processes in which ionizing radiation is used to change the chemical or physical properties of a system. The effects observed in radiation-chemical processes are a consequence of the formation and subsequent reactions of intermediate particles (ions, excited molecules, and radicals) arising from the irradiation of the original system. The efficiency of radiation-chemical processes is quantitatively expressed through the radiation-chemical yield G. In radiation-chemical processes involving chain reactions, where the value of G varies from 10³ to 10⁶, the radiation plays the role of an initiator. In many cases, this initiation offers significant technological and economic advantages, for example, better control of a process, the possibility of carrying out the process at lower temperatures, and the prospect of obtaining especially pure products. In radiation-chemical processes not involving chain reactions, the energy of the radiation is directly consumed in the act of chemical conversion. These processes require large expenditures of radiant energy and have limited use.

Among the widely studied and practical radiation-chemical processes involving chain reactions are those of polymerization, telomerization, and the synthesis of a series of low-molecular compounds. Radiation-chemical processes for the polymerization of such monomers as ethylene, trioxane, fluoroolefins, acrylamide, and styrene were developed in the early 1970’s to the point where pilot plants were set up. Radiation methods for hardening binders (polyesters) have acquired great practical importance in the production of fiber-glass reinforced plastic and paint and varnish coatings for metal, wood, and plastic items.

Radiation-chemical processes involving the formation of graft polymers have attracted considerable attention. In these processes, the original polymers or various inorganic materials are irradiated in the presence of the corresponding monomers, as a result of which the surfaces of these materials acquire new, sometimes unique, properties. Processes of this type have a practical application in the modification of threads, fabrics, films, and mineral wool. Radiation-chemical processes for the modification of porous materials (wood, concrete, and tufa) are of great interest. Here, the modification is carried out by the impregnation of the materials with such monomers as methyl me-thacrylate and styrene and the subsequent polymerization of the monomers through gamma radiation. Such treatment significantly improves the performance characteristics of the original porous materials and permits the production of a wide range of new structural materials. One example is the modified wood used in parquetry, which is now produced on a large scale. Radiation-chemical processes involving chain reactions are also carried out with the aim of synthesizing low-molecular products. High efficiency has been achieved in the radiation-chemical processes of oxidation, halogenation, sulfochlorina-tion, and sulfoxidation.

Of the processes in which reactions of the nonchain type are initiated by radiation, radiation-chemical processes for the cross-linking of separate macromolecules have become common. These processes require the irradiation of a macromolecu-lar compound. As a result of cross-linking (for example, of polyethylene), there is an improvement in the compound’s heat resistance and strength. In the case of rubbers, cross-linking induced by radiation provides for the rubber’s vulcanization. These effects of radiation have led to the development of radiation-chemical processes for the production of reinforced and heat-resistant polymer films, cable insulation, and tubes and the vulcanization of industrial rubber items. The “memory effect” of irradiated polyethylene is of especial interest. If an irradiated object made of polyethylene is deformed at temperatures above the melting point of the low-density polymer, the object will retain its deformed shape upon subsequent cooling. Reheating, however, restores the original form. This effect permits the production of thermoshrinking packaging films and electrical insulation tubes.

The use of ²³⁵U nuclear fission fragments, which form in a nuclear-reactor core, was proposed in 1956 for carrying out chemical synthesis. The processes involving the use of these fragments were called radiochemical processes. Studies and calculations showed that in principle there were no obstacles to these processes. However, the technical difficulties in creating systems for removing the inevitable radioactive impurities from the products have so far prevented the construction of even a pilot plant.

The development of industrial radiation-chemical processes has led to the creation of radiation-chemical technology, which is mainly concerned with developing methods and equipment for the economical realization of radiation-chemical processes on an industrial scale. A major division of radiation-chemical technology is the construction of apparatus, and the theoretical foundation of this division is in large part due to the work of Soviet scientists.

Isotopic sources of gamma radiation, electron accelerators with energies of 0.3 to 10 megaelectron volts, and nuclear reactors are used in carrying out radiation-chemical processes. Of the isotopic sources in current use, ⁶⁰Co is the most common. Radiation loops in nuclear reactors are also considered to be promising sources of gamma radiation. These loops consist of an activity generator, an irradiator, and connections and devices for moving the working substance about the loop. As a result of the capture of neutrons in the generator located in or near the core of the nuclear reactor, the working substance is activated, and the gamma radiation of the isotopes thus formed is then used in the irradiator for carrying out radiation-chemical processes. The experience so far gained in the USSR permits the construction of industrial radiation loops with a power of several hundred kilowatts.

The use of accelerated electrons for the irradiation of relatively thin layers of material is especially effective. Among other advantages, accelerated electrons provide a high-powered input to the medium, are less hazardous to service personnel, and do not entail a loss of energy when the accelerator is switched off.