The strategic path toward the safest possible nuclear power plants is to switch to inherently safe nuclear reactors, capable of self-shielding, without interference by man, in any accident situations. Examples of such plants are modular medium and low-power plants based on fast reactors with a breeding ratio close to one. As a result, in such plants the operational reactivity margin is less than/~eff, negative temperature feedbacks are present, the void effect of reactivity is negative, and the colloant is a liquid metal whose aggregate state remains unchanged in the entire operating temperature range and at the temperatures reached during accidents. The physical properties of such reactors eliminate reactivity accidents with runaway on prompt neutrons and the chain reaction is self-quenched in any accident.Liquid-metal coolant, combined with the single-unit construction of the plant, implementing passive cooldown (when all other possibilities are precluded) through the reactor vessel to naturally circulating atmosphere air, will make it possible to avoid destructive overheating of the core. This concept is being implemented in the USA in the projects ALMAR (PRISM and others) and in Russia in lead-cooled reactors, which are being developed by the Scientific-Research and Design Institute of Energy and Fuels [1], and in designs with lead-bismuth coolant, which are being developed by the Institute of Physics and Power Engineering and the Special Design Office "Gidropress" [2]. Japan has also started the conceptual development of such lead-cooled plants.The transition to lead-bismuth liquid-metal coolant, which is inert with respect to water and air, makes the concept of an inherently safe fast reactor more complete than the ALMR concept, which is based on sodium as the coolant.Mastery of Lead-Bismuth Alloy as a Coolant. Russia was the first to use and has unique experience in assimilating and using this coolant in application to moving power plants. The scientific development was directed by the Institute of Physics and Power Engineering and the engineering development was directed by the Special Design Office "Gidropress" and the Special Design Office for Machines.Work on the application of the eutectic alloy lead-bismuth was started in this country at the initiative and under the leadership of A. I. Leipunskii in 1952. The fundamental problems arising in the development of a nuclear power plant with such a coolant, including the questions of heat transfer, hydrodynamics, coolant technology, corrosion, mass transfer, and many others, were solved. The required experimental base, equipped with unique test stands, was created. Commercial and prototype nuclear power plants, on which operation, repair, and refueling of the reactors were perfected and preliminary service-life tests of the equipment were performed, were constructed. The total service life of a nuclear power plant with lead-bismuth coolant was -80 reactor.years.During this work skilled teams of investigators, designers, and operating personnel, capable of ...
Experience in the Use of Lead-Bismuth Coolant. In the USSR, liquid-metal cooled reactors were developed for and used in nuclear submarines together with water-cooled water-moderated type reactors [1]. The eutectic alloy lead-bismuth was chosen as the liquid-metal coolant. The usefulness of this alloy (Pb 44%, Bi 56%) is due to its physicochemical and thermodynamic properties, which make it possible to satisfy best the safety requirements.The high boiling point 1670"C makes it possible to have a low pressure in the first loop, which simplifies the construction and makes the construction more reliable, to decrease the thickness of the walls of the reactor vessel, and does not introduce any restrictions on the rate of change of the temperature according to the conditions of thermocyclic strength, which ensures operation of the reactor plant in a maneuvering regime.The weak chemical activity of the coolant makes it possible to realize a two-loop scheme for removing heat. The accidents associated with a rupture of the first-loop pipelines and interloop leakages of the steam generator, occur without liberation of hydrogen. The melting point of the coolant is ~ 125"C, and for this reason it is maintained in the liquid state under different operating conditions of the reactor plant by simply solutions to technological problems. During the operation no coolant is expended and the coolant can be used repeatedly. There is no need to decontaminate the first loop during repair work and reloading of the reactor.A specific feature of the coolant is the formation of s-active 21~ with a half life of -138 days in it. The main factor of its radiation hazard is the formation of radioactive aerosols and a steam phase of polonium during contact of the hot coolant with air, which is possible under conditions of accidental rupture of the first loop and spilling of the alloy. As operating experience has shown, the yield of polonium aerosols and the radioactivity of air decrease rapidly after the temperature decreases and the spilled alloy solidifies; this limits the area of radioactive contamination and makes it easier to remove it in the form of solid radioactive wastes.The low polonium concentration in the alloy (an atomic fraction of 10-8%) and formation of a thermodynamically stable chemical compound with lead, lowering the polonium vapor pressure by another factor of 1000, have resulted in the fact that over many years of operation of nuclear power plants with lead-bismuth coolant no cases of excessive irradiation of personnel have been observed. This positive practical result agrees with the conclusions made by foreign specialists [2].Among the basic problems which were solved in the course of the development, adoption, and operation of plants of this type, one must distinguish the coolant technology --development of a complex of systems and devices which enable monitoring and maintenance of the required quality of the coolant in the process of prolonged operation, both under normal conditions and in the case of a leak in the steam...
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