Questions of choosing a thermodynamic cycle and the working parameters for a high-temperature reactor with solid coolant are examined. The results of investigations showing that such a reactor can be used in a power-generating unit of a nuclear power plant with high unit capacity with well-understood turbine facilities operating on superheated steam are presented.The thermodynamic cycles and working parameters of a reactor facility for electricity generation based on an innovative design of a high-temperature nuclear reactor with solid coolant (HRSC) are examined in the present paper. A block diagram of a system with small, spherical, heat-carrying particles based on graphite and pyrolytic carbon and the technical aspects of such a scheme -proof that the coolant velocity can be acceptable under gravity only, experimental determination of the heat emission coefficients, preliminary studies of the durability and evaluation of the physical characteristics of the reactor facility -are presented in [1,2]. Microfuel and fuel compacts based on microfuel and graphite with dimensions adopted in the GT-MGR design (12.5 mm diameter, 50 mm height) were chosen as fuel at the first stage of the investigations. The conceptual questions concerning the engineering arrangement of the reactor setup, which make it possible to implement the advantages of solid coolant in the form of small heat-carrying particles moving through the core without special orientation, including passive cooldown, are presented in [2,3].It should be noted that using a solid coolant in reactors with coarse-elemental graphite coolant was studied in 1974-1988. The conceptual ideas presented in [4][5][6][7] proposed reactors constructed with massive elements which move along a circle in a horizontal plane on special rollers and cyclically heated in the core followed by heat transferred to a secondary coolant by means of radiation.The thermal diffusivity of a heat-carrying element with finite dimensions determines the characteristic heat transfer time and is inversely proportional to a squared characteristic linear dimension. It is difficult to make heat transfer by large elements cost-effective because of the high thermal inertia. The reactor construction examined employs small particles with thermal relaxation time ~0 .15 sec, moving through the core under gravity with direct contact with fuel elements, which fundamentally distinguishes it from previous designs [4][5][6][7]. We recall [3] that in accidents with complete loss of electrical power the reactor is cooled down using a standard scheme within ~100 sec as a result of excess coolant in the top hopper. The temperature of the fuel elements in an accident with protection actuated in 6 sec increases slightly above the nominal values. The concept provides for bottom hoppers for the coolant, which are designed to hold the entire coolant volume. If the protection is not actuated, the reactor is suppressed as coolant flows down into the bottom hoppers. Since the coolant is the main moderator, this occurs befor...

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Computational studies of specific questions arising when solids are used in nuclear reactors as the cooling medium are described. The concept of using approximately 1 mm in diameter spherical heat-carrying elements made of pyrolytic carbon coated graphite as a medium for transmitting heat from a fuel element to a steam generator is examined. A computational analysis of the internal stresses arising in 1-10 mm in diameter spherical elements transferring heat and the temperature lag of the heat-carrying elements relative to the temperature of the medium under cyclic heating and cooling is performed. The results of experimental studies were used to determine the boundary conditions of the problem. Supplying heat uniformly over the surface and through a finite number of contact points, which is characteristic for a fill consisting of spherical particles, was modeled. The transmission of heat through a finite number of contacts results in a complicated stress state of the heat-transmitting elements and a higher thermal inertia. It is shown that the internal stresses are weak in small heat-carrying particles, but when the diameter is increased to 10 mm the stresses from thermal cycling become comparable to the ultimate strength.This article presents the results of a computational study of specific particulars of using a solid substance as a coolant in nuclear reactors. The internal stresses in heat-transmitting elements of the coolant during heat transfer and, correspondingly, thermal cycling and thermal inertia during the propagation of heat inside the elements fundamentally distinguish a solid coolant from liquids and gases. To ensure that heat is transferred from the core to a secondary heat exchanger various designs were proposed for large-size heat-transferring elements which are caused to move in special directions by driving mechanisms.The concept of using small heat-carrying particles moving through the core and heat exchangers without orientation and only under gravity with direct contact with the surface of the fuel elements was recently proposed and validated. To realize this variant, the optimal nuclear and thermophysical properties of the coolant must be combined, and properties securing the ability to move under gravity through a core with constant porosity must be imparted to the particles. In [1] it is proposed that the technology for fabricating microfuel elements be used to make heat-carrying particles similar in shape and size to microfuel. The heat-carrying particles do not contain nuclear fuel and consist of a graphite kernel and a pyrolytic carbon coating. These particles combine a small neutron-absorption cross section, high heat capacity, and the ability to withstand high

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