Radiation Shielding

(protecting the body against ionizing radiation). Work with any source of ionizing radiation (radioactive preparations, nuclear reactors, X-ray apparatus, accelerators, atomic and thermonuclear weapons) necessarily presupposes the protection of personnel and the surrounding population from that radiation; this is often called biological shielding. Maximum permissible levels of radiation are regulated by radiation safety norms which are periodically being made more accurate and reexamined.

Biological shielding against radiation became a subject of research soon after the discovery of X rays (in 1895) and radioactivity (in 1896). The creation of nuclear reactors that increased radiation flux to magnitudes (10–100) x 10⁹ times the maximum permissible level required the building of large protective structures (for example, with concrete as thick as 250–350 cm), whose cost in present-day nuclear installations amounts to as much as 20–30 percent of the total cost of the complex.

The problem of protecting the body from radiation has two aspects: protection from the external flux of “closed” sources of radiation (radioactive preparations, reactors, X-ray apparatus, and accelerators), which is based on attenuation of the radiation through its interaction with matter, and protection of the biosphere against contamination by “open” radioactive sources (products of nuclear arms tests, byproducts of the nuclear industry, “open” radioactive preparations), which may enter the human body either directly or with water and food (vegetable or animal).

Devices that protect against external flux are divided into continuous (totally surrounding the radiation source or, more rarely, the region to be shielded), partial (attenuated for areas of limited access to personnel), shadow (circumscribing the region to be shielded by a “shadow” thrown off by the shield), and divided (partially surrounding the radiation source or the area to be shielded).

Protective structures usually must be of minimum weight and overall size; these are economically the most profitable and ensure the required attenuation of radiation. In working with radioactive preparations that are not highly active, the necessity for special protection does not always arise. Since the radiation intensity from an isotropic point source is directly proportional to its activity and irradiation time and inversely proportional to the square of the distance from the source, it is possible in a number of cases to limit oneself to a source with the lowest possible activity for the given task and to use it for the shortest possible time at a maximal distance from it, without protection.

Shielding from the external flux of ± and ²-particles presents no difficulty, since in interacting with the medium these particles quickly lose energy. The path in matter of the ±-particles of radioactive isotopes with energy ℰ (in MeV) is

where ρ is the density in g per cu m and A is the atomic weight of the substance. The path of ²-particles of maximum energy ℰ₀ in aluminum R ≈ 2ℰ₀mm; in air, R ≈ 4ℰ₀m. Sheets of paper, rubber gloves, or 8–9 cm of air are usually sufficient for complete absorption of the crparticles emitted by radioactive isotopes; several mm of aluminum are sufficient for ²-particles. In the case of ²-particles it is necessary to use shields against bremsstrahlung, the escape of which may be diminished by ²-shielding made of light-weight materials (plexiglass, aluminum, ordinary glass).

Gamma quanta and neutrons are the most penetrating. The attenuation of unscattered ³-quanta and neutrons in a shield (“narrow beam”) is described by the exponential function

I_d = I₀e^-d/λ

where I_d and I₀ are radiation intensities behind a shield (of thickness d) and without it and λ is the thickness of the material that attentuates the radiation e times, depending on the radiation energy and the shielding material. In order to calculate attenuation while taking into account unscattered and scattered rays in the shield (“broad beam”), a cofactor is introduced into formula (1) called the build-up factor (the ratio of the sum of the intensities of the unscattered and scattered radiation to the unscattered radiation); this factor is a function of the radiation energy, the geometry and angular distribution of the radiation source, the arrangement, composition, and dimensions of the shield, and the mutual orientation of the source, the irradiated objects, and the shield.

Gamma radiation is more intensely absorbed by materials that contain elements of high atomic weight (tungsten, lead, iron, cast iron); neutrons, by materials that contain elements of low atomic weight (water, paraffin, some metallic hydrides, concrete). Therefore, shielding from mixed ³- and neutron radiation in nuclear installations is provided by materials that are a mixture of substances with low and high atomic weights (for example, iron-water and iron-lead mixtures). Because of structural and economic considerations, stationary installations are often shielded with concrete.

In calculating radiation intensities beyond the protective structure, one must take account of the geometric divergence of the beam, the absorption and repeated scatter in the shield, and the absorption and scatter of radiation in the source it-self. Shielding calculations in present-day nuclear installations are a complex problem and are usually performed by an electronic computer. All types of primary and secondary radiations are taken into account in the computation. For example, the capture of neutrons moderated to low energies is usually accompanied by the formation of a hard capture radiation, and the absorption of ²-particles, by the generation of bremsstrahlung. The penetrating capacity of secondary radiation often determines the total thickness of the shield; hence, appropriate measures must be taken in order to reduce the secondary radiation. For example, lithium or boron is added to reduce capture ³radiation in shielding materials.

In designing protective devices one must take into account the passage of radiation through inhomogeneities in the shield (for example, in the case of a nuclear reactor, safety, control, and shim rods, conduits for coolants and moderators, charging tubes, fuel channels, test holes, contraction cavities, joints between the shielding units); in certain areas behind the shield this determines the radiation intensity. Protective containers are used for transport and storage of radioactive materials.

Protection against the entry of radioactive substances into the human body is no less important. Protection of the biosphere requires special means of lowering the concentration of radioactive substances in water and air to the maximum permissible limits. In organizing operations with open sources of radiation, it is necessary to select properly the distribution and design of work areas and auxiliary premises, to conduct operations in specially equipped premises, to provide service personnel with means of individual protection (overalls, pneumatic suits, respirators, special boots, jackets, gloves), strictly to control observance by personnel of personal-hygiene measures, and properly to organize the collection, storage, processing, and disposal into the environment of solid, liquid, and gaseous radioactive wastes.

Dosimetric and radiometric control are practiced in all enterprises where operations are conducted with sources of ionizing radiation in order to prevent the overexposure of personnel. In operations with closed sources measurements are taken of the exposure of individuals to all types of irradiation; dose magnitudes in work areas and adjacent premises are checked periodically; and instruments with automatic warning devices are installed in operations using large radiation sources. In operations with open sources the content of radioactive substances in the air of work areas, the contamination of work surfaces, equipment, hands, and clothing, and the radioactivity of sewage and air discharged into the atmo-sphere are all monitored.

The individual may be protected against radiation by various chemical agents introduced into the body before or durng exposure to ionizing radiation; these agents are directed toward increasing the radioresistance—that is, the resistance to the effects of radiation—of those irradiated. Radioprotective agents may conventionally be divided into two groups: agents that increase the general resistance of the body and specific radioprotective substances called radioprotectors. Agents of general biological effect increase the natural radioresistance of the body. They are injected several days or weeks before irradiation in amounts that, as a rule, produce no harmful or toxic phenomena. The protective effect of such compounds is most pronounced with irradiation in amounts that would cause the deaths of 20–70 percent of the animals exposed. Among the most effective agents of this group are lipopolysaccharides, combinations of amino acids and vita-mins, hormones, and vaccines. Injection of such compounds in experimental animals before irradiation alleviates the course of radiation sickness, increases the survival rate, and diminishes the degree of disturbance of the metabolic processes and blood circulation. The protective action of these agents is apparently caused by an increase in the activity of the pituitary-adrenocortical system, an increase in the ability of the hemopoietic cells to reproduce, stimulation of the reticuloendothelial system, and an increase in the immunological reactivity of the body. These agents accelerate the processes of synthesis of proteins and nucleic acids in the cells and foster the restoration of unique genetic structures. There are facts that indicate the ability of these agents to increase body resistance not only to the effects of radiation but also to other pathogenic influences.

Radioprotectors are preparations that create a state of artificial radioresistance. Classified as such are compounds that exert an antiradiation effect when injected a few minutes or a few hours before irradiation. The most pronounced protective effect is observed with a total irradiation that would cause the death of 80–100 percent of the animals exposed and with the use of the radioprotector in the maximum tolerable doses (producing development of a number of toxic reactions). Among the most effective radioprotectors are mercaptoamines, indolylalkylamines, synthetic polymers, polynucleotides, mucopolysaccharides, cyanides, and nitriles. Mixtures of several radioprotectors belonging to different groups of chemical compounds prove most effective. In the general irradiation of dogs with a minimum lethal dose, the most effective chemical radioprotectors can increase the survival rate of the animals by 60–80 percent.

The basis of the antiradiation effect of these compounds is their ability to prevent changes in radiosensitive organs and tissues and to preserve the capacity of some of the cells to reproduce. Radioprotectors protect the stem cells of hemopoietic tissues better than do agents of general biological effect; in the hemopoietic organs and intestinal tract under their influence, necrobiotic processes decline, the number of cells with chromosomal restructuring decreases, and a more rapid restoration of mitotic activity occurs. This may be a result of interference by the radioprotectors with the initial physicochemical processes of radiation injury (interception of the chemically active free radicals H and OH, changes in the physicochemical properties of molecules of the biological substrates by means of their adsorption of the radioprotectors, and interaction of the protectors with labile initial products of the radiolysis of vitally important molecules which, in their absence, undergo decomposition), as well as with an alteration in the course of the radiation reaction in its later stages (for example, mobilization of repair systems of the body, which suppress chromosomal restructuring). It has been shown that the basis of the active mechanism of certain radioprotectors is the capacity to lower oxygen tension in the body. These radioprotectors hinder the formation of certain radicals and molecular products of radiolysis, resulting in the creation of conditions that exclude oxidation by oxygen of radiation-damaged, vitally important molecules. The degree of the protective effect of radioprotectors depends to a considerable degree on the type, total dose, intensity, and method of irradiation. The effectiveness of antiradiation agents is evaluated according to the dose reduction factor, that is, according to the relationship between doses that cause an equally expressed effect in the presence and in the absence of the protective agent. The greatest protection in mammals corresponds to a dose reduction factor of 2. Higher coefficients have been obtained by combining protection before irradiation with subsequent treatment.

Under conditions of prolonged irradiation of animals with exposure dose rates of less than 1 roentgen per min (4.30 x 10”6 amperes per kg), even the most effective radioprotectors exert no prophylactic effect. It is precisely because of this fact that new data on the effectiveness under these conditions of agents capable of repairing unique genetic structures (for example, adenosine triphosphate) merit special attention. Consequently the principal method of protecting the body from radiation under conditions of the peaceful use of atomic energy may be not only physical protection with dosimetric control directed at those conditions under which the level of irradiation of work areas does not exceed the maximum permissible limits, but also medicinal prophylaxis. Specifically, one may consider the possibility of using agents that will increase the natural radioresistance of the human body while exerting no toxic effects upon it.