The work presents a review devoted to the metabolism and the mechanism of toxicity of seleniumcontaining supplements: elemental selenium, sodium selenite, diacetophenonyl selenide, selenopyrane, ebselen, dimethyl dipyrasolyl selenide and selenium-containing amino acids used for correction of selenium deficiency. Elemental selenium penetrating through cell walls, but not through transport channels demonstrates poorly predicted and difficultly regulated bioavailability. Sodium selenate is known to be the most toxic form of selenium in food. The metabolism of xenobiotic diacetophenonyl selenide resembles that of sodium selenide. The xenobiotic reacts with thiols, for instance, with the reduced form of glutathione leading to the formation of hydrogen selenide. Ebselen is not considered to be a well bioavailable form of selenium and thus possesses low toxicity. Xenobiotic selenopyrane eliminates selenium only in processes of xenobiotic liver exchange, and in our investigations - partially in acid-catalyzed hydrolysis. The metabolism of xenobiotic dimethyl dipyrasolyl selenide having low toxicity is poorly investigated. The toxicity of high doses of selenomethionine is determined by the possibility of incorporation in proteins and vitally important enzymes with dramatic changes of protein quaternary structure. The toxicity of high doses of methylselenocysteine seems to be caused by the lack of an exchange pool in the body and quick regeneration of hydrogen selenide from methylselenol which is formed as a result of enzymatic destruction of this amino acid. Also the issue of the most prospect selenium donor is discussed. The physiological compatibility, the low toxicity, the presence of an exchangeable pool in the organism, the antioxidantal properties and the simplicity of production indicate selenocystine as an optimal selenium donor.
The results of calculations of reactivity effects and analysis of how the computational error affects the course of accidents which involve coolant-flow disturbances and reactivity increases are presented for an IAEA test model -the BN-600 reactor with a hybrid core. It is shown that the solution of the neutron transport equation in the diffusion approximation gives satisfactory accuracy in determining the main reactivity effects for the BN-600 reactor. The reactivity of sodium and steel calculated in the two-dimensional R, Z and three-dimensional HEX, Z geometries and analysis of its effect on the course of a loss-of-coolant accident show that a three-dimensional model gives a lower computational error and, correspondingly, a more accurate description of an accident. Comparative analysis of the results obtained in different countries demonstrated that the methods and programs developed in our country for safety substantiation of fast reactors are reliable.The reliability of any safety substantiation of fast reactors is determined by, among other things, the accuracy of the predictions of reactivity effects, which play a key role in transient and accident processes. At present, there are no specific requirements, in the volume needed, for computational accuracy and for improving the computational techniques [1]. The uncertainty in the range of the admissable error makes it difficult to unify the computational codes and make recommendations for using these codes when licensing a reactor. The absence of a standard experimental base of neutron-physical safety parameters for fast reactors makes it necessary to verify the computational codes on test models. To this end, a simplified model of a hybrid (use of uranium and mixed fuel in combination)
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